Patent Publication Number: US-11649451-B2

Title: Evolution of bioactive sequence-defined synthetic polymers using DNA-templated polymerization

Description:
RELATED APPLICATIONS 
     This application is a national stage filing under 35 U.S.C. § 371 of international PCT application, PCT/US2018/041127, filed Jul. 6, 2018, which claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Application, U.S. Ser. No. 62/638,901, filed on Mar. 5, 2018, and to U.S. Provisional Application, U.S. Ser. No. 62/529,787, filed on Jul. 7, 2017, each of which is incorporated herein by reference. 
    
    
     GOVERNMENT SUPPORT 
     This invention was made with government support under grant numbers N66001-14-2-4053 awarded by the Department of Defense, and GM118062 and GM065865 awarded by the National Institutes of Health. The government has certain rights in the invention. 
    
    
     REFERENCE TO A SEQUENCE LISTING SUBMITTED AS A TEXT FILE VIA EFS-WEB 
     This application contains a Sequence Listing which has been submitted in ASCII format via EFS-Web and is hereby incorporated by reference in its entirety. Said ASCII copy, created on Jan. 6, 2020, is named H082470267US02-SEQ-WWZ and is 36,010 bytes in size. 
     BACKGROUND OF INVENTION 
     Polymerases and ribosomes impose structural requirements on the building blocks that can be polymerized and thereby limit the diversity of synthetic polymers that are accessible to directed evolution. Accordingly, there remains a need for efficient and effective methodologies that allow for the generation of modified nucleic acid based polymers to create chemically diverse sequence-defined highly functionalized nucleic acid based polymers. 
     BRIEF SUMMARY OF INVENTION 
     Using a test-tube translation and Darwinian selection system, sequence-defined synthetic polymers containing many chosen side-chains were evolved that bind proteins of biomedical interest. The evolution of sequence-defined synthetic polymers made of building blocks beyond those compatible with polymerase enzymes or the ribosome has the potential to generate new classes of receptors, catalysts, and materials. A ligase-mediated DNA-templated polymerization system and in vitro selection was used to evolve highly functionalized nucleic acid polymers (HFNAPs) made from 32 building blocks containing eight chemically diverse side-chains on a DNA backbone. Through iterated cycles of polymer translation, selection, and reverse translation, HFNAPs that bind PCSK9 and IL-6, two protein targets implicated in human diseases were discovered. Mutation and reselection of an active PCSK9-binding polymer yielded evolved polymers with high affinity (K D =3 nM). This evolved polymer potently inhibited binding between PCSK9 and the LDL receptor. Structure-activity relationship studies revealed that specific side-chains at defined positions in the polymers are required for binding to their respective targets. The findings expand the chemical space of evolvable polymers to include densely functionalized nucleic acids with diverse, researcher-defined chemical repertoires. It should be appreciated that the disclosure provides modified nucleic acid bases, for example any of the modified cytosine and thymine bases provided herein. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1   . Shows design and construction of the sequence-defined polymer library. (A) Reaction scheme for DNA ligase-mediated translation of DNA templates into sequence-defined highly functionalized nucleic acid polymers (HFNAPs). (B) Structures of 5′-phosphorylated trinucleotide building blocks for HFNAP library synthesis. (C) Translation of libraries of randomized DNA templates into HFNAPs that incorporate up to 15 consecutive functionalized trinucleotide building blocks. (D) A complete cycle of HFNAP translation, HFNAP strand isolation, and reverse translation back into DNA faithfully recovered sequence information from the original DNA templates. In control experiments in which the trinucleotide building blocks were omitted from the polymerization reactions, the PCR step did not generate any amplicons of the correct size. 
         FIG.  2   . Shows selection of PCSK9-binding polymers from a random HFNAP library. (A) Overview of translation, selection, and reverse translation scheme. (B) PCSK9 binding selection progress. The HFNAP pool&#39;s bulk affinity to PCSK9-coated beads was assessed by quantifying the amount of HFNAP in the flow-through versus the elution at each round of selection by quantitative PCR. Higher values in the graph indicate higher ratios of polymers that bound to immobilized PCSK9 and were eluted relative to polymers that flowed through the immobilized PCSK9. (C) Sequence and side-chain structure of selected polymer PCSK9-A5. Side-chains essential for binding activity are boxed. (D) SPR sensogram characterizing binding kinetics between surface-immobilized PCSK9-A5 polymer and the target PCSK9 protein. The concentrations of injected PCSK9 were 10, 30, 100, and 300 nM. The observed sensogram is shown in red and the fitted curve with the kinetic parameters listed is shown in black. (E) Kinetic parameters for binding of PCSK9-A5 or its side-chain-deficient variants to PCSK9 protein, as measured by SPR. For the variants “TTT ΔSide chain”, “TTT Linker only”, and “CTT ΔSide chain”, no SPR signal was observed at highest analyte concentration tested (300 nM PCSK9). 
         FIG.  3   . Shows evolution of an improved PCSK9-binding polymer. (A) Evolution scheme and DNA sequencing results of the diversification and iterated selection of PCSK9-A5 variants with increased PCSK9 binding activity. (B) Affinity maturation of the diversified PCSK9-A5 pool. The evolving polymer pool&#39;s bulk affinity to immobilized PCSK9 was assessed by quantifying the amount of HFNAP in the flow-through and the elution at each round of selection by quantitative PCR. (C) Sequence and side-chain structure of the resulting PCSK9-Evo5 polymer. Side chains essential for binding activity are boxed. (D) Kinetic parameters for binding of PCSK9-Evo5 or its side-chain-deficient variants to PCSK9 protein, as measured by SPR. For the variants “TGT ΔSide chain”, “TGT Linker only”, and “CTT ΔSide chain”, no SPR signal was observed at the highest analyte concentration tested (60 nM PCSK9). For the variant “CAC ΔSide chain”, the binding interaction fits a two-state reaction kinetic model with KD≈420 nM. Representative sensograms are provided in  FIG.  8   . (E) SPR sensogram characterizing binding kinetics between PCSK9-Evo5-syn and surface-immobilized PCSK9 protein. The concentrations of injected PCSK9-Evo5-syn were 1.8, 6, 18, 60, and 180 nM. The observed sensogram is shown in red and the fitted curve with the kinetic parameters listed is shown in black. (F) SPR response on an LDLR-coated surface produced by flowing PCSK9 in the presence of either PCSK9-Evo5-syn, unfunctionalized DNA of identical sequence to PCSK9-Evo5-syn, unlabeled LDLR, or a known PCSK9-neutralizing monoclonal antibody. The SPR response shown is normalized to the response in experiments without any competitor (defined as an SPR response of 1). Raw sensograms are provided in  FIG.  12   . 
         FIG.  4   . Shows characterization of IL-6-binding HFNAPs selected from a random library. (A) Retention of individual selection-enriched HFNAPs on immobilized IL-6 (target; bottom bars) or immobilized PCSK9 (non-target; top bars). The percentages of each sequence in the pool after seven rounds of selection are listed to the left. (B) Sequence and side-chain structure of IL6-A7. Side-chains essential for binding activity are boxed. (C) SPR sensogram characterizing binding kinetics between biotinylated IL6-A7 and its target IL-6 protein. The concentrations of injected IL-6 were 10, 30, 100, and 300 nM. The observed sensogram is shown in red and the fitted curve with the kinetic parameters listed is shown in black. 
         FIG.  5   . Shows sequence homology and characterization of selection-enriched PCSK9-binding HFNAPs. (A) Relative abundances and sequence logos of the dominant seven sequence clusters after nine rounds of PCSK9-binding selection from a naïve library. (B) Retention of selection-enriched HFNAPs (only the variable, functionalized parts of the sequences are shown) on immobilized PCSK9 (target; top bars) and immobilized thrombin (non-target; middle bars) and of sequence-matched unfunctionalized DNA on immobilized PCSK9 (bottom bars), as quantified by qPCR. n.d.=not determined. 
         FIG.  6   . Shows structures of bases in side-chain mutants characterized during structure-activity relationship studies of PCSK9-A5 and PCSK9-Evo5. 
         FIG.  7   . Shows cheater suppression strategy. (A) In addition to the desired polymerization reaction (top), incorporation of a polymerization primer in the coding region can generate undesired cheaters (bottom) that rapidly amplify during PCR due to their small size and eventually dominate the sequence pool over rounds of selection. (B) Using a non-extendable 2′,3′-dideoxyribose-terminated 3′ polymerization primer addresses this problem, as truncated cheaters can be removed during PAGE purification. 
         FIG.  8   . Shows SPR sensograms characterizing binding kinetics between PCSK9 protein and (A) surface-immobilized biotinylated PCSK9-Evo5 or (B) surface-immobilized biotinylated PCSK9-Evo5-“CACΔSide chain” or (C) surface-immobilized biotinylated PCSK9-“3 side chains.” The concentrations of injected PCSK9 were 2, 6, 20, and 60 nM. The raw sensograms are shown in red and the fitted curves with the kinetic parameters listed are shown in black. 
         FIG.  9   . Shows effects of truncating (A) PCSK9-A5 and (B) PCSK9-Evo5. No binding of up to 300 nM solution-phase PCSK9 to surface-immobilized PCSK9-EvoTrunc was observed by SPR. 
         FIG.  10   . Shows multi-milligram-scale synthesis scheme for PCSK9-Evo5-syn. 
         FIG.  11   . Shows ESI-MS spectrum of PCSK9-Evo5-syn (molecular formula: C536H683N177O305P48). (A) Full spectrum. (B) Spectrum detailing the “7-” group of peaks, showing the peak clusters for [M-7H]7- (around m/z 2280.5777), as well as the sodium adduct (around m/z 2284.0039), the Tris adduct (around m/z 2297.8727), and other multiple adducts. (C) Top: observed spectrum of the [M-7H]7- peak cluster. Bottom: predicted spectrum based on the molecular formula and isotopic abundances. 
         FIG.  12   . Shows SPR sensograms characterizing the interaction between surface-immobilized LDLR and PCSK9 pre-incubated with varying concentrations of (A) PCSK9-Evo5-syn, (B) sequence-matched unfunctionalized DNA, (C) a known PCSK9-neutralizing antibody, or (D) unlabeled LDLR. 
         FIG.  13   . Shows SPR sensograms characterizing binding kinetics between surface-immobilized biotinylated PCSK9-Evo5 and either full-length PCSK9 protein or a truncated PCSK9 protein missing the prodomain. The concentrations of injected protein were 2, 6, 20, and 60 nM. 
         FIG.  14   . Shows SPR sensograms characterizing binding kinetics between biotinylated IL6-A7 (1) or two side-chain variants (3 and 4) and  E. coli -expressed IL-6 protein, and between biotinylated IL6-A7 and HEK293 cell-expressed IL-6 protein (2). For all experiments, comparable amounts of the biotinylated HFNAPs (all between 90 and 120 RU) were immobilized on the active flow cell. The concentrations of injected IL-6 were 10, 30, 100, and 300 nM. 
         FIG.  15   . Screening of ligases for polymerization. The translation reactions, as well as control reactions from which the trinucleotide building blocks were omitted, were analyzed by polyacrylamide gel electrophoresis on a non-denaturing 10% TBE gel and imaged by SYBR Gold staining. T3 DNA ligase mediated higher translation efficiency than T4 and T7 DNA ligases. Increasing the temperature ramp-down time from 2 hours to 8 hours had little effect on translation yield. 
     
    
    
     EXAMPLES 
     Some aspects of the disclosure are based at least in part on the surprising discovery that modified tri-nucleotide polymers may be assembled using nucleic acid chemistry to evolve sequence-defined highly functionalized nucleic acid polymers that are capable of binding proteins (e.g., PCSK9 and IL-6) that are implicated in human diseases. 
     The gene-encoded synthesis and Darwinian selection of sequence-defined biopolymers are fundamental features of all known forms of life. These processes have been harnessed in the laboratory to evolve RNA (1-3), DNA (4-6), and polypeptides (7-10) with a variety of binding and catalytic properties through iterated cycles of biopolymer translation, selection, replication, and mutation. The speed and effectiveness of the evolutionary process has inspired efforts to apply these principles to the much larger chemical space of synthetic polymers (11). To date, however, the evolution of sequence-defined non-natural polymers in the laboratory has been limited to analogs of nucleic acids (12-15) and polypeptides (16) that can be synthesized by polymerases and ribosomes. Polymerases and ribosomes impose structural requirements on the building blocks that can be polymerized and thereby limit the diversity of synthetic polymers that are accessible to directed evolution. For example, polymerases use only mononucleotides as substrates, precluding the ability to encode a diverse set of codons and side chains. Known classes of polymerase-synthesized functional non-natural nucleic acid polymers, including those derived from non-natural sugar backbones (17-20), uniform installation of hydrophobic (21-28) or positively charged (29-35) side-chains on nucleobases, or introduction of novel nucleobases among the four possibilities (36-38), therefore have chemical diversities that are only modestly expanded beyond those of natural DNA and RNA, and fall short of the much more diverse chemical functionality present in proteins. 
     Previously an in vitro system was developed that uses DNA ligase to translate DNA sequences into sequence-defined highly functionalized nucleic acid polymers (HFNAPs) containing a wide range of side-chains chosen by the researcher (39). In a previous report, it was shown that DNA sequences can be translated into HFNAPs using DNA ligase to catalyze the polymerization of up to 50 consecutive short, chemically functionalized oligonucleotide building blocks along a DNA template (39) ( FIG.  1 A ). It was discovered that T4 DNA ligase accepts trinucleotide building blocks with a wide range of side-chains on the 5′ nucleobase, including side-chains at the C5 position of pyrimidine nucleobases that are both synthetically accessible and unlikely to disrupt Watson-Crick base pairing (39). 
     This artificial translation system allows researchers, in principle, to mimic and even expand the chemical repertoire of protein building blocks in an evolvable synthetic polymer system. The broad chemical scope of HFNAPs gives them the potential to adopt unique folding and functional properties distinct from those of known natural or non-natural nucleic acid polymers. The original HFNAP system proved unable to support the evolution of functional polymers, however, likely because of the limited diversity provided by its eight-codon genetic code and the long, flexible linkers present in the building blocks. In this study a new HFNAP “genetic code”, translation system, and in vitro selection system was designed that overcomes these challenges, then applied the resulting HFNAP evolution system to generate sequence-defined synthetic polymers that binds two protein targets of biomedical interest. 
     Results 
     The new genetic code was designed to offer a high degree of both codon and side-chain diversity to evolving polymers ( FIG.  1 B ). The maximum number of different trinucleotides containing a 5′ pyrimidine (all 32 possible YNN combinations, where Y=C or T and N=A, C, G, or T) as codons was included. The corresponding 32 building blocks were each linked to one of eight side-chains (four codons per side-chain) that include hydrophobic, aliphatic, aromatic, halogenated, polar, and charged groups, several of which are not found among proteinogenic amino acid side-chains. The linkers between side-chains and nucleobases were redesigned compared with our original genetic code with limited conformational flexibility in order to increase the likelihood that the polymer backbone, nucleobases, and side-chains would cooperatively adopt defined folded structures. Each side-chain was assigned to a set of four codons that collectively contained the same balance of A/T versus C/G bases following the side-chain-functionalized 5′ pyrimidine base. 
     Translation DNA-templated polymerization (artificial “translation”) reactions were improved by screening ligase enzymes and adjusting polymerization conditions. It was found that subjecting translation reactions to a slow (0.01° C./s) temperature ramp to 4° C. before initiating ligation with T3 DNA ligase substantially improved yields of full-length HFNAP from libraries of DNA templates containing random coding regions of 45 nt, which encoded the incorporation of 15 consecutive side-chain-functionalized trinucleotide building blocks of mixed sequence ( FIG.  1 C ). To test the ability of translated polymers to be “reverse-translated” back into DNA, thereby enabling iterated cycles of translation, selection, reverse translation, and PCR amplification, polymerization was performed on four templates that each encoded the incorporation of eight different building blocks and collectively covered all 32 building blocks, and then subjected the resulting HFNAP products, separated from template DNA, to reverse translation in a PCR reaction using Q5 DNA polymerase (39) ( FIG.  1 D ). One of the PCR primers binds a 3′-overhang ( FIG.  1 D ) present in the HFNAP products but absent in template DNA, precluding the amplification of any contaminating template DNA. DNA sequencing of the resulting PCR products showed that the original sequence information in the templates was faithfully recovered ( FIG.  1 D ), indicating that both translation from DNA to HFNAPs and reverse translation from HFNAPs to their encoding DNA occur with high sequence fidelity using this set of building blocks. 
     These observations are qualitatively consistent with results from Hili and coworkers, who reported fidelities ranging from 95.1% to 98.4% per codon for ligase-mediated DNA-templated polymerization of functionalized pentanucleotides (40). Perfect fidelity is not expected for a ligase-mediated polymerization, which lacks proofreading mechanisms, but we reasoned that the level of fidelity in our system may be sufficient to support iterated selection for functional polymers, consistent with our previous mock selection results (39). Modest levels of mutations may also confer a benefit to the selection, as reported by Benner and coworkers for selections of aptamers containing novel nucleobases (37). 
     Encouraged by these developments, a library of HFNAPs was generated containing 15 consecutive building blocks drawn from the set of 32 (theoretical polymer library space=3×10 22 ; average HFNAP molecular weight=28 kDa) and subjected the resulting library (starting quantity=3×10 12  molecules) to iterated rounds of in vitro selection for binding to PCSK9 protein, a target implicated in low-density lipoprotein (LDL) metabolism and cardiovascular disease (41-43) ( FIG.  2 A ). The HFNAP library was incubated with PCSK9 protein immobilized on agarose beads. After washing the beads with buffer, the bound HFNAPs were eluted by boiling the beads in buffer containing detergent. The surviving HFNAPs were reverse translated in a PCR reaction using Q5 DNA polymerase, and the resulting double-stranded DNAs were purified by PAGE. The non-template strands were removed by alkaline denaturation, and the streptavidin-bound template strands were translated back into HFNAPs by ligase-catalyzed DNA-templated polymerization for the next round of PCSK9-binding selection. It is noted that the ability of HFNAPs to be directly reverse-translated by a DNA polymerase, though not an absolute requirement for iterated selection as demonstrated by the use of display methods to evolve nucleic acids (17, 44), provides a practical and high-fidelity way to complete a selection cycle. 
     As the iterated rounds of selection progressed, the fraction of HFNAP that was retained on PCSK9-linked beads generally increased, consistent with enrichment of PCSK9-binding polymers, even though selection stringency was steadily elevated by decreasing the amount of PCSK9 protein ( FIG.  2 B ). At the eighth round of selection, the polymer population was retained by PCSK9 protein-conjugated agarose beads with an efficiency of approximately 10%. In contrast, less than 0.1% of the same polymer population was retained on agarose beads not conjugated to any protein ( FIG.  2 B ), suggesting that the ability of the selected polymers to bind PCSK9-linked beads arose from their ability to bind PCSK9, rather than agarose beads. 
     Results from high-throughput DNA sequencing after nine rounds of selection indicated that the HFNAP pool had strongly converged to just seven sequence families containing conserved sub-sequences suggestive of common binding motifs ( FIG.  5 A ). Sequences within the same family were likely descendants of a single parental polymer derived through mutations that accumulated through the selection process, as evidenced by high levels of homology. The seven most highly enriched HFNAPs were individually synthesized by ligase-mediated DNA-templated polymerization and tested their retention on immobilized PCSK9 or immobilized thrombin, an unrelated protein. Five of the seven tested polymers exhibited substantial apparent binding activity to immobilized PCSK9 beyond any apparent binding to immobilized thrombin. Unfunctionalized DNA sequences (lacking any side-chains) corresponding to these seven HFNAPs were also assayed and no evident PCSK9-binding activity was observed ( FIG.  5 B ). Together, these results suggest that the HFNAP populations emerging from nine rounds of in vitro selection converged on a small number of polymer families that bind immobilized PCSK9 in a manner dependent on the polymers&#39; side-chains. 
     PCSK9-A5, the polymer with the highest apparent PCSK9 binding activity ( FIG.  2 C ) was characterized. Biotinylated PCSK9-A5 was synthesized by templated translation and confirmed its binding affinity for PCSK9 (dissociation constant K D =98 nM) by surface plasmon resonance (SPR) ( FIG.  2 D ). To probe the role of the side-chain functional groups, side-chain mutants of PCSK9-A5 were synthesized in which all instances of each building block were replaced by the corresponding trinucleotide either lacking any side-chain or containing a linker but missing the side-chain&#39;s functional group ( FIG.  6   ). The removal of a single phenol side-chain in codon 9 or a single cyclopentyl side-chain in codon 14 completely abolished the binding between PCSK9-A5 and its target ( FIGS.  2 C and  2 E ). Furthermore, the removal of the isopentyl side-chain at codon 13 resulted in an approximately two-fold reduction in affinity ( FIGS.  2 C and  2 E ). The individual removal of other side-chains had less significant effects on binding affinity. However, they may play important roles in PCSK9 binding in the context of the entire polymer. 
     Given the vast sequence space of the HFNAP library (3×10 22  possible polymers), evolution would likely generate polymer variants with improved activity in the initial population of 3×10 12  HFNAP molecules. To evolve the PCSK9-A5 polymer into variants with improved PCSK9 affinity, a library of mutated PCSK9-A5 templates was synthesized containing 79% identity and 21% diversity (79:21 at the pyrimidine-only first position and 79:7:7:7 at the second and third positions of each codon) for each nucleotide in the variable region ( FIG.  3 A ). The resulting mutated PCSK9-A5 library was subjected to six additional iterated cycles of translation and selection for PCSK9 binding ( FIG.  7   ). After just one round of enrichment at a stringency level comparable to that of the last round of initial selection, the mutated PCSK9-A5 library exhibited bulk affinity for PCSK9-conjugated beads ( FIG.  3 B ). High-throughput DNA sequencing revealed that the mutant population after a second round of translation and selection began to converge toward the sequence of PCSK9-A5 at many positions ( FIG.  3 A ). In subsequent rounds, the amount of immobilized PCSK9 was reduced to further increase selection stringency. Four additional rounds of translation and selection resulted in steadily improved retention of the polymer population on immobilized PCSK9 ( FIG.  3 B ). 
     High-throughput sequencing revealed new consensus codons at four out of 15 positions within the population of evolved polymers ( FIG.  3 A ). Among these four positions, codons 6, 10, and 12 evolved a different side-chain compared with that of PCSK9-A5, while codon 9 evolved a different codon (TGT instead of TTT) encoding the same phenol side-chain at this position. In addition, five other codons converged to the original sequence in PCSK9-A5, and six other codon positions, mostly near the 5′ end, retained sequence heterogeneity introduced during mutation, suggesting that these positions do not strongly contribute to binding activity ( FIG.  3 A ). The three side-chain functional groups that were shown to be crucial to the PCSK9-binding activity of PCSK9-A5 were all maintained in the consensus sequence of the evolved polymer population. 
     A biotinylated, truncated HFNAP (designated PCSK9-Evo5;  FIG.  3 C ) was synthesized retaining only the evolved consensus sequence and the 3′ constant region, and measured its affinity for PCSK9 to be K D =3.0 nM ( FIG.  3 D  and  FIG.  8 A ), representing a 33-fold increase over that of PCSK9-A5. In contrast, a similarly truncated version of PCSK9-A5 (designated PCSK9-A5Trunc) did not substantially improve affinity (K D =68 nM) over that of full-length PCSK9-A5 (K D =98 nM) ( FIG.  9 A ). Additional truncation of PCSK9-Evo5 from the 3′-end resulted in complete loss of affinity ( FIG.  9 B ). Similar to the structure-activity relationships observed for PCSK9-A5, the removal of a single phenol or cyclopentyl side-chain from PCSK9-Evo5 abolished its affinity to PCSK9 protein ( FIG.  3 D ), and the removal of the isopentyl side-chain severely impaired target binding (the binding interaction approximately fits a two-state reaction kinetic model with K D ≈420 nM;  FIGS.  3 D and  8 B ). The individual removal of other side-chains had less significant effects on binding affinity; indeed an HFNAP containing only three side chains (phenol at position 9, isopentyl at position 21, and cyclopentyl at position 24) and the rest of the Evo5 sequence as unfunctionalized DNA maintains strong binding to PCSK9 (K D =3.7 nM;  FIGS.  3 D and  8 B ). Together, these results establish the evolution (iterated selection with intervening mutation and replication) of a sequence-defined synthetic polymer with improved target affinity from mutation followed by iterated translation, selection, and reverse translation. 
     To further characterize PCSK9-Evo5, the multi-milligram-scale total synthesis of a variant of PCSK9-Evo5 (PCSK9-Evo5-syn) was executed, which has an additional 3′ inverted dT base for exonuclease resistance (46), using standard phosphoramidite chemistry on solid support ( FIG.  10   ). It was planned to install all side-chain-functionalized building blocks entirely through corresponding side-chain-functionalized phosphoramidite reagents, with the nucleophilic functional groups protected with base-labile protecting groups that would be removed under standard oligonucleotide deprotection conditions. Although most of the side-chain-functionalized nucleoside phosphoramidites were readily synthesized (Methods), the imidazole-bearing thymidine reagent proved difficult to prepare. PCSK9-Evo5-syn was therefore synthesized on solid phase with an activated ester (NHS-carboxy-dT) in place of the imidazole-functionalized thymidine, and coupled histamine to that position in the bead-bound polymer chain to install the imidazole side-chain before global deprotection and cleavage from solid support with ammonium hydroxide ( FIG.  10   ). The identity of PCSK9-Evo5-syn (approximately 2 mg from a one-micromole solid-phase synthesis, ˜13% overall yield) was confirmed by high-resolution mass spectrometry ( FIG.  11   ). High-affinity binding (K D =6.8 nM) between totally synthetic PCSK9-Evo5-syn and biotinylated Avi-tagged PCSK9 was confirmed by SPR ( FIG.  3 E ). 
     PCSK9 regulates cholesterol metabolism by binding the LDL receptor (LDLR) and promoting the lysosomal degradation of LDLR (42). The ability of PCSK9-Evo5-syn to disrupt PCSK9-LDLR binding in an SPR assay was tested. PCSK9-Evo5-syn dose-dependently reduced binding of PCSK9 to surface-immobilized LDLR ( FIG.  3 F  and  FIG.  12 A ) The potency of PCSK9-Evo5-syn inhibition of PCSK9-LDLR binding (IC 50 =˜9 nM) was similar to that of a known PCSK9-neutralizing monoclonal antibody ( FIG.  3 F  and  FIG.  12 C ). In contrast, unfunctionalized DNA of the same sequence as PCSK9-Evo5-syn produced no inhibitory effect ( FIG.  3 F  and  FIG.  12 B ), consistent with the necessity of the side chains implicated in PCSK9 binding ( FIGS.  2 C-E  and  FIGS.  3 C-D ). The affinity of surface-immobilized PCSK9-Evo5 to different PCSK9 protein constructs was also tested and it was found that a truncated PCSK9 variant lacking the prodomain exhibited no apparent binding to PCSK9-Evo5 ( FIG.  13   ), implicating the PCSK9 prodomain, known to be involved in a secondary binding interface between PCSK9 and LDLR (47), in mediating the interaction of PCSK9 with PCSK9-Evo5. 
     To test the generality of our polymer evolution system and to investigate the potential of this new class of polymers to evolve receptors to different proteins, a separate selection for HFNAPs that bind a protein unrelated to PCSK9 was performed. Human interleukin-6 (IL-6), a key cytokine involved in inflammation and the target of many drugs and drug candidates (48), including modified DNA aptamers (24, 25) was chosen. After seven iterated cycles of translation, selection for binding to immobilized IL-6 protein, reverse translation, and amplification, the most abundant sequence accounted for 3.6% of the population ( FIG.  4 A ). The top seven HFNAPs were individually synthesized and assayed for binding to immobilized IL-6. 
     Based on its high apparent binding activity to immobilized IL-6, but not to immobilized PCSK9 ( FIG.  4 A ), the HFNAP IL6-A7 ( FIG.  4 B ) was chosen for further characterization. Binding of biotinylated IL6-A7 to the target IL-6 protein was confirmed by SPR. Although the binding kinetics of IL6-A7 to IL-6 protein did not conform to a classical one-to-one binding model, a phenomenon often observed in aptamer-protein binding (18, 24) fitting to a heterogeneous ligand model resulted in an apparent affinity of K D =12 nM for the major component and K D =22 nM for the minor component ( FIG.  4 C ). 
     The binding kinetics and affinity of IL6-A7 to  E. coli -expressed IL-6 (used in the selection) and to human HEK293 cell-expressed IL-6 protein were comparable ( FIG.  14   ). SPR was used to measure the IL-6 affinity of IL6-A7 side-chain mutants in which all instances of a trimer building block were replaced by the corresponding trinucleotide lacking any side-chain. The mutants missing isopentyl side-chain in either codon 14 or codon 15 exhibited severely impaired binding ( FIG.  14   ), implicating these two side chains as key determinants of IL-6 binding activity in IL6-A7. 
     Discussion 
     We used a ligase-mediated DNA-templated polymerization system and in vitro selection to evolve HFNAPs, nucleic acid polymers that are densely functionalized with chemically diverse side-chains. HFNAPs that bind PCSK9 and IL-6 were selected from random polymer libraries. Through diversification and reselection, we evolved an improved PCSK9-binding HFNAP (Evo5) with KD=3 nM. We characterized structure-activity relationships within this polymer, revealing side chains at specific positions that are critical to target-binding activity. Evo5 potently inhibits binding between PCSK9 and the LDL receptor. 
     Collectively, these findings represent the first laboratory evolution of functional, genetically encoded sequence-defined synthetic polymers without the constraints imposed by polymerases or ribosomes. The DNA-templated, ligase-based translation system developed here supports many rounds of iterated selection of polymers with diverse side-chains, including side-chains that mimic and extend beyond the repertoire of amino acid side-chains found in proteins. Both the PCSK9-binding and IL-6-binding polymers generated in this system exhibit position-dependent and side-chain dependent structure-activity relationships resembling those of proteins. Finally, it is noted that the PCSK9-binding polymers generated in this work depend on the presence of multiple side-chains with different physical properties, consistent with the importance of chemical diversity to the functional potential of these polymers. 
     Recently, Gawande and coworkers performed selections for PCSK9 aptamers from modified DNA libraries in which all instances of one or both pyrimidines (C and/or T) were replaced by side-chain-functionalized variants (27). High-affinity aptamers with dissociation constants similar to those of FDA-approved anti-PCSK9 monoclonal antibodies (evolocumab, KD=8.0 pM49, and alirocumab, KD=0.58 nM50) were enriched from doubly modified libraries in which hydrophobic or phenolic side chains were present on 50% of the nucleobases on average. Aptamers enriched from singly modified libraries (25% hydrophobic side chains on average) were less potent (KD≥100 pM), while libraries containing hydrophilic side chains or consisting of unmodified DNA did not produce aptamers with KD≤30 nM. Consistent with their findings, the highest affinity binders from our HFNAP library, which contains a roughly equal mix of hydrophilic and hydrophobic side chains installed at 33% total frequency, has KD=3 nM to PCSK9. We note, though, that different modifications may be suitable for other applications, as demonstrated by DNA-based catalysts functionalized with nitrogen nucleophiles as side-chains (29-33, 35). Therefore, the diverse, balanced set of side-chains in HFNAPs, similar to the natural repertoire of proteins, may be more versatile in other settings. 
     The ligase-based polymerization method allows straightforward redesign of the genetic code of the polymer, as it was exploited to expand the sequence and structural diversity of the polymers used in this work compared with those of another system (39). This feature also enables researchers to generate and select HFNAPs with side-chains tailored toward specific applications, as recently demonstrated by Hili and coworkers for scaffolding peptides on a DNA template(51). Moreover, the side-chain flexibility of this polymer evolution system raises the possibility of performing parallel evolution experiments with libraries of different side-chain compositions to shed light on the fundamental relationship between the structure of the building blocks in a genetic code and the evolutionary potential of the resulting polymers. 
     Methods 
     Additional experimental procedures and characterization data are provided herein. 
     Synthesis of HFNAP by Templated Translation Via DNA Ligase-Mediated Polymerization 
     DNA template [up to 10 pmol, either in solution or immobilized on MyOne Streptavidin C1 magnetic beads (ThermoFisher Scientific)], polymerization initiation and termination primers (1.5 equivalents each relative to template), functionalized trinucleotide building blocks (10 equivalents relative to template for each occurrence of the corresponding codon) and 10× T4 RNA ligase reaction buffer (New England Biolabs; 1 μL) were mixed in a total volume of 8 μL in a PCR tube. The mixture was subjected to the following temperature program on a thermocycler: 95° C. for 10 sec; 65° C. for 4 min; a ramp from 65° C. to 4° C. at 0.1° C. per 10 s. To the PCR tube were added 1 μL of 10 mM ATP and 1 μL of T3 DNA ligase (New England Biolabs). The reaction was incubated at 4° C. for 12 h and then at 16° C. for 2 h. 
     Selections of HFNAP that Bind Protein Targets 
     Selection bait was prepared by immobilizing recombinant protein onto AminoLink Plus aldehyde-functionalized agarose resin via reductive amination with a MicroLink Protein Coupling Kit (ThermoFisher Scientific). Loading was 1 mg PCSK9 protein (ACROBiosystems) per mL resin for the initial PCSK9 binder selection and the first two rounds of PCSK9 binder re-selection; 150 μg PCSK9 per mL resin for rounds 3-5 of the re-selection; 40 μg protein per mL resin for round 6 of the re-selection; and 250 μg IL-6 protein (PeproTech) per mL resin throughout the IL-6 binder selection. 
     To initiate the selection, primer extension was performed with a biotinylated primer on 5 pmol of the sense strand randomized DNA library (Integrated DNA Technologies or TriLink BioTechnologies) with Klenow (exo-) polymerase (New England Biolabs). Biotinylated species was captured on streptavidin magnetic beads, which were then washed three times with 20 mM NaOH and then twice with 1× T4 RNA ligase reaction buffer. The bead-immobilized template strand library was then translated in a ligase-mediated polymerization to produce HFNAPs. The beads were suspended in 20 mM NaOH to denature the HFNAP-template hybrids. HFNAP strands in the supernatant were cleaned up with a MinElute column (Qiagen). 
     The HFNAP library was added to DPBS (with calcium and magnesium; Lonza) supplemented with BSA (0.1 mg/ml final) and Tween-20 (0.01% final), and then incubated with PCSK9 resin in a micro-spin filtration column (Pierce) at room temperature for 1 h on a rotor. (The amounts of resin-bound protein used in each round of the PCSK9 selection are indicated in  FIG.  2   b   . Throughout the IL-6 selection, 240 pmol of immobilized IL-6 protein was used in each round.) The flow-through was collected by centrifugation at 1000 g into an Eppendorf tube. The beads were washed three times with 50 μL each of DPBS. The column was cut open, and the beads were collected by centrifugation into an Eppendorf tube and then incubated in 50 μL of lithium dodecyl sulfate (LDS) loading buffer (Life Technologies) at 95° C. for 15 min. After cooling, HFNAP strands were isolated from the mixture by cleaning up with a QiaQuick column (Qiagen), eluting the HFNAP into 50 μL of water. 
     Samples of 1 μL each from the flow-through, the three washes, and the elution were quantified by qPCR (20 μL reaction volume) using the iTaq Supermix (Bio-rad). The number of cycles for the qPCR curve of the elution sample to reach the end of exponential growth was used as the number of cycles for the preparative PCR (400 μL reaction volume split into 8×50 μL) of the selection elution pool (20 μL) with Q5 Hot Start High-Fidelity 2× Master Mix (New England Biolabs), using a biotinylated primer for the strand that will serve as translation template. The PCR product was cleaned up with a MinElute column and PAGE purified on a non-denaturing 10% TBE gel. A portion (indicated in  FIG.  2   b   ) of the dsDNA product was captured on streptavidin magnetic beads to initiate the next round of selection. 
     Surface Plasmon Resonance (SPR) Assays 
     All SPR assays were performed at 25° C. on a Biacore X100 or Biacore T200 (GE Healthcare Life Sciences). Binding kinetics between enzymatically synthesized biotinylated HFNAPs and unlabeled recombinant proteins were measured using single-cycle kinetics with the Biotin CAPture kit (GE Life Sciences) using 0.9×HBS-EP buffer (GE Life Sciences) at a flow rate of 30 μL/min. The injected PCSK9 concentration ranged from 10 to 300 nM for PCSK9-A5 and its variants, or from 2 to 60 nM for PCSK9-Evo5 and its variants. The injected IL-6 concentration ranged from 10 to 300 nM. 
     Binding kinetics between chemically synthesized PCSK9-Evo5-syn and biotinylated Avi-tagged PCSK9 (ACROBiosystems) were measured using single-cycle kinetics on a Series S SA chip (GE Life Sciences) using 0.9×HBS-EP buffer at a flow rate of 30 μL/min. The injected PCSK9-Evo5-syn concentration ranged from 1.8 to 180 nM. 
     Binding of PCSK9 on surface-immobilized LDLR in the presence of various competing agents was measured on a Series S SA chip using 10 mM HEPES, 150 mM NaCl, 0.1 mM CaCl 2 ), 0.005% Tween-20, pH 7.5 as bulk buffer at a flow rate of 10 μL/min. The injected solutions contained 20 nM PCSK9 and various competing agents ranging from 2 to 200 nM. 
     Data Availability 
     The principal data supporting the findings of this work are available within the figures and information provided herein. Additional data that support the findings of this study are available from the authors on request. 
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Materials and Methods
 
General Information
   

     Unless otherwise specified, all materials and compounds were prepared using commercially available reagents from Sigma-Aldrich, and used without further purification. Water was purified with a Milli-Q purification system. DNA oligonucleotides without nucleobase side chain functional groups were purchased from Integrated DNA Technologies (IDT) unless noted otherwise. In-house synthesis of side-chain-functionalized DNA was performed on a PerSeptive Biosystems Expedite 8909 DNA synthesizer and purified by reverse-phase high-pressure liquid chromatography (HPLC, Agilent 1200) using a C18 stationary phase (Waters XBridge Prep C18, 5 m, 10×250 mm) and an acetonitrile/100 mM triethylammonium acetate gradient. All materials and reagents used for oligonucleotide synthesis were purchased from Glen Research, Berry &amp; Associates, or ChemGenes, or custom synthesized by WuXi AppTec. Oligonucleotide and protein concentrations were quantified by UV spectroscopy using a Nanodrop ND1000 spectrophotometer, using extinction coefficients calculated with the IDT Oligo Analyzer and Expasy ProtParam web servers, respectively. Non-commercial oligonucleotides were characterized at the Harvard FAS Small Molecule Mass Spectrometry Facility by ESI-MS on a Bruker Impact II q-TOF mass spectrometer equipped with an Agilent 1290 uHPLC using flow injection analysis. Polyacrylamide gels were purchased from Bio-Rad. Sanger sequencing was performed by Eton BioSciences and analyzed with ApE—A plasmid Editor. Quantitative polymerase chain reactions (qPCRs) were performed on a Bio-rad CFX96 system. Deep sequencing was performed on an Illumina MiSeq. Surface plasmon resonance (SPR) analysis was carried out on a Biacore X100 or Biacore T200 (GE Healthcare Life Sciences). Time-resolved FRET assays were performed on a Tecan Infinite M1000 PRO microplate reader. 
     Oligonucleotide Sequences 
     All occurrences of U below are 2′-deoxy-U. Commercially available oligonucleotide modifiers are denoted by shorthand notations used by IDT. 
     Evaluation of Translation Yield on Template Libraries 
                                 Name   Sequence                  Template-11codon   CGTACGGTCGACGCTAGCNNRNNRNNRNNRNNR           NNRNNRNNRNNRNNRNNRCACGTGGAGCTCGGA           TCC (SEQ ID NO: 1)               Template-13codon   CGTACGGTCGACGCTAGCNNRNNRNNRNNRNNR           NNRNNRNNRNNRNNRNNRNNRNNRCACGTGGAG           CTCGGATCC (SEQ ID NO: 2)               Template-15codon   CGTACGGTCGACGCTAGCNNRNNRNNRNNRNNR           NNRNNRNNRNNRNNRNNRNNRNNRNNRNNRCAC           GTGGAGCTCGGATCC (SEQ ID NO: 3)               pp1-library   /5Phos/GCTAGCGTCGACCGTACG           (SEQ ID NO: 4)               pp2-library   GGATCCGAGCTCCACGTG (SEQ ID NO: 5)                    
Validation of Sequence Specificity of Translation and Amplification
 
                                 Name   Sequence                  Template-Bt-CATA   /52-Bio//iSp18/CGTACGGTCGACGCTAG           CTTGAAAGTGCAAGAGACACCGCGACACGTGG           AGCTCGGATCC (SEQ ID NO: 6)               Template-Bt-CBTB   /52-Bio//iSp18/CGTACGGTCGACGCTAG           CATGTTACTGGTATCGTGAGCGGGACACGTGG           AGCTCGGATCC (SEQ ID NO: 7)               Template-Bt-CCTC   /52-Bio//iSp18/CGTACGGTCGACGCTAG           CTAGATATGGCTAGGGTCAAGGGCACACGTGG           AGCTCGGATCC (SEQ ID NO: 8)               Template-Bt-CDTD   /52-Bio//iSp18/CGTACGGTCGACGCTAG           CAAGTAACAGGAAACGAGACGGCCACACGTGG           AGCTCGGAUCC (SEQ ID NO: 9)               pp1-library-T7   /5Phos/GCTAGCGTCGACCGTACGAGCGTCG           CTACGCGTGAC (SEQ ID NO: 10)               pp2-library   GGATCCGAGCTCCACGTG (SEQ ID NO:           11)               T7-out-PCR2   TAATACGACTCACTATAGGGCTCGATTTAATT           TCGCCGACGTGATGACATTCCAGGCAGTGTCA           CGCGTAGCGACGCT (SEQ ID NO: 12)                    
PCSK9 Binder Selection and Evolution
 
                                 Name   Sequence                  Naïve library AZ15   CGA ATC AGA TTG GAC CAG YNN YNN           YNN YNN YNN YNN YNN YNN YNN YNN           YNN YNN YNN YNN YNN GAG TCC AGA           TGT AGG TAG (SEQ ID NO: 13)               BtBt-ExtA   /52-Bio//iSp18/CTA CCT ACA TCT           GGA CTC (SEQ ID NO: 14)               ExtA   CTA CCT ACA TCT GGA CTC           (SEQ ID NO: 15)               pp1A   /5Phos/GAG TCC AGA TGT AGG TAG           (SEQ ID NO: 16)               pp2Z   CGA ATC AGA TTG GAC CAG           (SEQ ID NO: 17)               MiSeqA   ACA CTC TTT CCC TAC ACG ACG CTC           TTC CGA TCT NNNN CTA CCT ACA           TCT GGA CTC (SEQ ID NO: 18)               MiSeqZ   TGG AGT TCA GAC GTG TGC TCT TCC           GAT CT NNNN CGA ATC AGA TTG GAC           CAG (SEQ ID NO: 19)               IlluminaAdapterFwd   AATGATACGGCGACCACCGAGATCTACAC           [8-base barcode]ACACTCTTTCCCTA           CACGAC (SEQ ID NO: 20)               IlluminaAdapterRev   CAAGCAGAAGACGGCATACGAGAT[8-base           barcode]GTGACTGGAGTTCAGACGTGTGC           T (SEQ ID NO: 21)               Rediv library AZ15   CGA ATC AGA TTG GAC CAG XZP XFO           JPZ JFP JOZ JOZ JPZ XFO JOO JFZ           XZZ XFF XPZ XOO XOF GAG TCC AGA           TGT AGG TAG (SEQ ID NO: 22)           X = 79% dC, 21% T           Z = 7% dA, 79% dC, 7% dG, 7% T           P = 79% dA, 7% dC, 7% dG, 7% T           F = 7% dA, 7% dC, 79% dG, 7% T           O = 7% dA, 7% dC, 7% dG, 79% T           J = 21% dC, 79% T               pp1A-3ddC   /5Phos/GAG TCC AGA TGT AGG TAG/           3ddC/ (SEQ ID NO: 23)                    
Synthesis of Putative PCSK9 Binders for Bead Retention Assay
 
                                 Name   Sequence                  pp1A   /5Phos/GAG TCC AGA TGT AGG TAG (SEQ ID NO: 16)               pp2Z   CGA ATC AGA TTG GAC CAG (SEQ ID NO: 17)               PCSK9A1-   /52-Bio//iSp18/CTA CCT ACA TCT GGA CTC CAG AAG GTG ATG CAA       BtTempl   AGG CAA ACG GTA GGA GAG AGA ATA TAG TGG CTG GTC CAA TCT           GAT TCG (SEQ ID NO: 24)               PCSK9A1-DNA   CGA ATC AGA TTG GAC CAG CCA CTA TAT TCT CTC TCC TAC CGT           TTG CCT TTG CAT CAC CTT CTG GAG TCC AGA TGT AGG TAG           (SEQ ID NO: 25)               PCSK9A2-   /52-Bio//iSp18/CTA CCT ACA TCT GGA CTC TCA GCG CAG AAG GTG       BtTempl   AGG GCA AAA ACG GTA GAA GAA TCA TTG TTG CTG GTC CAA TCT           GAT TCG (SEQ ID NO: 26)               PCSK9A2-DNA   CGA ATC AGA TTG GAC CAG CAA CAA TGA TTC TTC TAC CGT TTT           TGC CCT CAC CTT CTG CGC TGA GAG TCC AGA TGT AGG TAG           (SEQ ID NO: 27)               PCSK9A3-   /52-Bio//iSp18/CTA CCT ACA TCT GGA CTC TGA TCA GGA TAA GTA       BtTempl   GTG CTA AAG ACA TGA AAG AGG TTG TAG AGA CTG GTC CAA TCT           GAT TCG (SEQ ID NO: 28)               PCSK9A3-DNA   CGA ATC AGA TTG GAC CAG TCT CTA CAA CCT CTT TCA TGT CTT           TAG CAC TAC TTA TCC TGA TCA GAG TCC AGA TGT AGG TAG (SEQ           ID NO: 29)               PCSK9A4-   /52-Bio//iSp18/CTA CCT ACA TCT GGA CTC GAA TAG GTA CCG CTA       BtTempl   AAG ACG TGA TAG AAA CGA AAA GCA TTG GGG CTG GTC CAA TCT           GAT TCG (SEQ ID NO: 30)               PCSK9A4-DNA   CGA ATC AGA TTG GAC CAG CCC CAA TGC TTT TCG TTT CTA TCA           CGT CTT TAG CGG TAC CTA TTC GAG TCC AGA TGT AGG TAG           (SEQ ID NO: 31)               PCSK9A5-   /52-Bio//iSp18/CTA CCT ACA TCT GGA CTC CAG AAG GTG CCG GGG       BtTempl   GCA AAA ACG GTA GAA GAA TCA GTA ACG TGG CTG GTC CAA TCT           GAT TCG (SEQ ID NO: 32)               PCSK9A5-DNA   CGA ATC AGA TTG GAC CAG CCA CGT TAC TGA TTC TTC TAC CGT           TTT TGC CCC CGG CAC CTT CTG GAG TCC AGA TGT AGG TAG           (SEQ ID NO: 33)               PCSK9A6-   /52-Bio//iSp18/CTA CCT ACA TCT GGA CTC GGA ACA ACG CAG AAG       BtTempl   GAG CCG TGG AGA AGG CAG AAG AGG TTG AAA CTG GTC CAA TCT           GAT TCG (SEQ ID NO: 34)               PCSK9A6-DNA   CGA ATC AGA TTG GAC CAG TTT CAA CCT CTT CTG CCT TCT CCA           CGG CTC CTT CTG CGT TGT TCC GAG TCC AGA TGT AGG TAG           (SEQ ID NO: 35)               PCSK9A7-   /52-Bio//iSp18/CTA CCT ACA TCT GGA CTC CAG AAG GTG AAA GTA       BtTempl   AGA GAA CAA ACG GTA GAG GAA TAG GGA AGA CTG GTC CAA TCT           GAT TCG (SEQ ID NO: 36)               PCSK9A7-DNA   CGA ATC AGA TTG GAC CAG TCT TCC CTA TTC CTC TAC CGT TTG           TTC TCT TAC TTT CAC CTT CTG GAG TCC AGA TGT AGG TAG (SEQ           ID NO: 37)                    
Synthesis of Biotinylated PCSK9-A5 and Variants for Surface Plasmon Resonance Assay
 
                                 Name   Sequence                  pp1A   /5Phos/GAG TCC AGA TGT AGG TAG           (SEQ ID NO: 16)               BtBt-pp2Z   /52-Bio//iSp18/CGA ATC AGA TTG GAC            CAG (SEQ ID NO: 38)               PCSK9A5-Templ   CTA CCT ACA TCT GGA CTC CAG AAG GTG           CCG GGG GCA AAA ACG GTA GAA GAA TCA           GTA ACG TGG CTG GTC CAA TCT GAT TCG           (SEQ ID NO: 39)                    
Synthesis of Biotinylated PCSK9-Evo5 and Variants for Surface Plasmon Resonance Assay
 
                                 Name   Sequence                  pp1A-3primeStBt   /5Phos/GAG TCC AGA TGT AGG TAG/           iSp18/iBiodT/3Bio/ (SEQ ID NO: 40)               pp1A-3primeStBt-   /5Phos/GAG TCC AGA TGT AG/i5p18/       14nt   iBiodT/3Bio/ (SEQ ID NO: 41)               Evo5-pp2-dU   CCA CGT TAC TGA TTC UGC (SEQ ID           NO: 42)               PCSK9Evo5-   CTA CCT ACA TCT GGA CTC CAG AAG       Templ   GTG GCA GGG TAA ACA ACG GTA GCA           GAA TCA GTA ACG TGG (SEQ ID NO:           43)                    
Synthesis of PCSK9-Evo5-Fluor and Negative Control for EMSA Assay
 
                                 Name   Sequence                  pp1A-Alexa647   /5phos/ GAG TCC AGA TGT AGG TAG /           iSp18//3AlexF647N/ (SEQ ID NO:           44)               Evo_5 DNA-LeftHalf     TGCTACCGTTGTTTACCCTGCCACCTTCTG             (SEQ ID NO: 45)               BtBt_Evo5-Template   /52-Bio//iSp18/ CTA CCT ACA TCT               GGA CTC CAG AAG GTG GCA GGG TAA               ACA ACG GTA GCA GAA TCA GTA ACG               TGG CTG  (SEQ ID NO: 46)                    
Negative Control for PCSK9-Evo5-Syn SPR Assay
 
                                 Name   Sequence                  Evo5DNA-InvdT   TGC TAC CGT TGT TTA CCC TGC CAC CTT           CTG GAG TCC AGA TGT AGG TAG/           3InvdT/ (SEQ ID NO: 47)                    
IL-6 Binder Selection
 
                                 Name   Sequence                  Naïve library CW15   CTC GGA TGA ACC TGG ACT YNN           YNN YNN YNN YNN YNN YNN YNN           YNN YNN YNN YNN YNN YNN YNN           GGA CTG AGT CCA GAG TAA           (SEQ ID NO: 48)               BtBt-ExtC   /52-Bio//i5p18/TTA CTC TGG ACT           CAG TCC (SEQ ID NO: 49)               ExtC   TTA CTC TGG ACT CAG TCC (SEQ ID           NO: 50)               pp1C   /5Phos/GGA CTG AGT CCA GAG TAA           (SEQ ID NO: 51)               pp2W   CTC GGA TGA ACC TGG ACT (SEQ           ID NO: 52)               MiSeqC   ACA CTC TTT CCC TAC ACG ACG CTC           TTC CGA TCT NNNNNN TTA CTC TGG           ACT CAG TCC (SEQ ID NO: 53)               MiSeqW   TGG AGT TCA GAC GTG TGC TCT TCC           GAT CT NNNN CTC GGA TGA ACC TGG           ACT (SEQ ID NO: 54)               IlluminaAdapterFwd   AATGATACGGCGACCACCGAGATCTACAC           [8-base barcode]ACACTCTTTCCCT           ACACGAC (SEQ ID NO: 55)               IlluminaAdapterRev   CAAGCAGAAGACGGCATACGAGAT[8-           base barcode]GTGACTGGAGTTCA           GACGTGTGCT (SEQ ID NO: 56)                    
Synthesis of Putative IL-6-Binding HFNAPs for Bead Retention Assay
 
                                 Name   Sequence                  pp1C   /5phos/GGA CTG AGT CCA GAG TAA           (SEQ ID NO: 51)               pp2W   CTC GGA TGA ACC TGG ACT (SEQ ID NO:           52)               IL6-A1-BtTempl   /52-Bio//iSp18/TTA CTC TGG ACT CAG           TCC TGG TGG CCA CTG GCA GCA CCG TCA           CGG AGG CTG AGA CTG CCG CAA AGT           CCA GGT TCA TCC GAG (SEQ ID NO: 57)               IL6-A2-BtTempl   /52-Bio//iSp18/TTA CTC TGG ACT CAG           TCC ACA GGA GGG ATA CCG GAA GGG CAA           AGG CCG TAG CCA GAA CAA AAA AGT CCA           GGT TCA TCC GAG (SEQ ID NO: 58)               1L6-A3-BtTempl   /52-Bio//iSp18/TTA CTC TGG ACT CAG           TCC ACA ACG CAG CCG GAA GCA CCA CAG           GAG AGG CGG TAA CCA GCA ACA AGT           CCA GGT TCA TCC GAG (SEQ ID NO: 59)               1L6-A4-BtTempl   /52-Bio//iSp18/TTA CTC TGG ACT CAG           TCC TCG GAA GCG TCA CGA CGG TAA CCG           GCA CTG AGA GCA ACA CCA CAA AGT CCA           GGT TCA TCC GAG (SEQ ID NO: 60)               1L6-A5-BtTempl   /52-Bio//iSp18/TTA CTC TGG ACT CAG           TCC TGG CTA AGG CGA CCA CGG GCA CTG           CAA CCA CAG CCA AAG GGG TGA AGT CCA           GGT TCA TCC GAG (SEQ ID NO: 61)               1L6-A6-BtTempl   /52-Bio//iSp18/TTA CTC TGG ACT CAG           TCC CAA TGA GGG AGA GAG GGG GAA GGG           CGA CAA AGG CCG TAA CCA GGA AGT CCA           GGT TCA TCC GAG (SEQ ID NO: 62)               1L6-A7-BtTempl   /52-Bio//iSp18/TTA CTC TGG ACT CAG           TCC CCG GAG GAA TGA GTA CGA GGA AGG           GCA ACG AAA TAA ACA GCA GCA AGT CCA           GGT TCA TCC GAG (SEQ ID NO: 63)                    
Synthesis of Biotinylated IL6-A7 and Variants for Surface Plasmon Resonance Assay
 
                                 Name   Sequence                  pp1C-3ddC   /5Phos/GGACTGAGTCCAGAGTAA/3ddC/            (SEQ ID NO: 64)               BtBt-pp2W   /52-Bio//iSp18/CTCGGATGAACCTGGACT            (SEQ ID NO: 65)               IL6-A7-Templ   TTA CTC TGG ACT CAG TCC CCG GAG GAA            TGA GTA CGA GGA AGG GCA ACG AAA TAA           ACA GCA GCA AGT CCA GGT TCA TCC GAG            (SEQ ID NO: 66)                    
Synthesis and Characterization of Phosphoramidite Intermediates
 
     Compounds were prepared and characterized by Wuxi AppTec Co. under the direction of Xun Hong. The protocols and characterization furnished along with these compounds are printed here. NMR spectra were recorded on a Bruker Avance 400 MHz for  1 H NMR. Chemical shifts are reported in ppm (S). Chromatographic purifications were by flash chromatography using 100˜200 mesh silica gel. Anhydrous solvents were pre-treated with 3 Å MS column before use. All commercially available reagents were used as received unless otherwise stated. 
     General Synthetic Routes: 
                                                             
Synthesis of Phosphoramidites 5a-d
 
     
       
         
         
             
             
         
       
     
     To a solution of 1 (40.0 g, 113.2 mmol, 1 equiv) and DMAP (0.113 g, 1.13 mmol, 0.01 equiv) in pyridine (400 mL) was added dropwise DMTrCl (40.2 g, 119 mmol, 1.05 equiv) and at 0° C. The mixture was stirred at 25° C. for 16 h. TLC (DCM/MeOH=20/1) indicated that 1 was consumed completely. The reaction mixture was concentrated with MeOH (50 mL) under reduced pressure to remove pyridine. The residue was purified by column chromatography (SiO 2 , DCM/MeOH=50/1 to 20/1) to give the 2 (62 g, yield 84%) as a white foam.  1 H NMR (400 MHz, DMSO-d 6 ) δ 8.65 (d, J=4.02 Hz, 1H), 7.99 (s, 1H), 7.53 (dd, J=7.28, 5.77 Hz, 1H), 7.36-7.42 (m, 2H), 7.20-7.35 (m, 6H), 6.90 (d, J=9.03 Hz, 4H), 6.09 (t, J=6.78 Hz, 1H), 4.15-4.24 (m, 1H), 3.91 (d, J=3.51 Hz, 1H), 3.74 (s, 6H), 3.18 (d, J=3.01 Hz, 2H), 2.22 (ddd, J=13.30, 5.77, 3.01 Hz, 1H), 2.06-2.16 (m, 1H). 
     
       
         
         
             
             
         
       
     
     To a solution of 2 (6 g, 9.15 mmol, 1 equiv), Cs 2 CO 3  (8.95 g, 27.5 mmol, 3 equiv), (E)-4,4,5,5-tetramethyl-2-(5-methylhex-1-en-1-yl)-1,3,2-dioxaborolane (2.46 g, 11 mmol, 1.2 equiv) and PPh 3  (1.2 g, 4.58 mmol, 0.5 equiv) in dioxane (700 mL) and water (30 mL) was added Pd(OAc) 2  (2.35 g, 10.5 mmol, 0.1 equiv) at 25° C. under N 2  current. The mixture was heated to 90° C. and stirred for 16 h. TLC (ethyl acetate/MeOH=20/1) showed 2 was consumed completely. The reaction mixture was diluted with water 50 mL and extracted with ethyl acetate (100 mL×2). The combined organic layers were washed with sat. aqueous NaCl (50 mL), dried over MgSO 4 , filtered and concentrated under reduced pressure to give a residue. The residue was purified by column chromatography (SiO 2 , DCM/MeOH=50/1 to 20:1) to give compound 3a (5.1 g, 8.15 mmol, 89% yield) was obtained as a light-yellow solid.  1 H NMR (400 MHz, CDCl 3 ) δ 7.86 (s, 1H), 7.42 (d, J=7.53 Hz, 2H), 7.18-7.35 (m, 7H), 6.81 (d, J=7.53 Hz, 4H), 6.46 (t, J=6.53 Hz, 1H), 5.53-5.70 (m, 2H), 4.47-4.57 (m, 1H), 4.13 (d, J=3.01 Hz, 1H), 3.79 (s, 5H), 3.47 (dd, J=10.54, 3.01 Hz, 1H), 3.28 (dd, J=10.54, 3.01 Hz, 1H), 2.70 (ddd, J=13.55, 5.52, 3.01 Hz, 1H), 2.24 (dt, J=13.55, 6.78 Hz, 1H), 1.56-1.83 (m, 4H), 1.13-1.40 (m, 3H), 0.91 (dtd, J=9.47, 6.43, 6.43, 3.26 Hz, 2H), 0.75 (dd, J=6.78, 2.76 Hz, 6H). 
     
       
         
         
             
             
         
       
     
     To a solution of 3a (4.23 g, 6.76 mmol, 1.00 equiv) in DMF (40.00 mL) was added Et 3 N (1.03 g, 10.14 mmol, 1.41 mL, 1.50 equiv) and benzoic anhydride (1.84 g, 8.11 mmol, 1.53 mL, 1.20 equiv). The mixture was stirred at 0 to 25° C. for 16 h. TLC (petroleum ether/ethyl acetate=1/1) indicated 3a was consumed completely and the reaction was clean. The reaction mixture was quenched by addition water 20 mL at 0-5° C., and then extracted with ethyl acetate (50 mL). The combined organic layers were washed with sat. aqueous NaCl (20 mL), dried over MgSO 4 , filtered and concentrated under reduced pressure to give a residue. The residue was purified by column chromatography (Basic SiO 2 , petroleum ether/ethyl acetate=5/1 to 2/1) to a yellow solid.  1 H NMR (400 MHz, CDCl 3 ) δ 13.52 (br. s., 1H), 8.30 (d, J=7.03 Hz, 2H), 7.96 (s, 1H), 7.50-7.57 (m, 1H), 7.40-7.50 (m, 4H), 7.21-7.37 (m, 7H), 6.84 (dd, J=8.53, 1.51 Hz, 4H), 6.40 (t, J=6.78 Hz, 1H), 6.21 (d, J=16.06 Hz, 1H), 5.92-6.03 (m, 1H), 4.55 (d, J=3.01 Hz, 1H), 4.10 (d, J=3.01 Hz, 1H), 3.79 (s, 6H), 3.56 (dd, J=10.54, 3.01 Hz, 1H), 3.31 (dd, J=10.54, 3.01 Hz, 1H), 2.52 (ddd, J=13.55, 5.77, 2.76 Hz, 1H), 2.35 (dt, J=13.68, 6.96 Hz, 1H), 2.09 (d, J=3.51 Hz, 1H), 1.72-1.91 (m, 2H), 1.41 (dt, J=13.43, 6.59 Hz, 1H), 0.83-1.00 (m, 2H), 0.77 (dd, J=6.53, 1.51 Hz, 6H). 
     
       
         
         
             
             
         
       
     
     To a solution of 4a (2.87 g, 3.93 mmol, 1 equiv) and 4,5-dicyanoimidazole (0.696 g, 5.90 mmol, 1.5 equiv) in DCM (30 mL) was added drop wise of 3-bis(diisopropylamino) phosphanyloxypropanenitrile (1.42 g, 4.72 mmol, 1.2 equiv) at 0° C. under N 2  current. Then the mixture was stirred at 0-25° C. for 2 h under N 2  current. A clear yellow solution was obtained. TLC (petroleum ether/ethyl acetate=2/1) showed 4a was consumed completely. The reaction mixture was concentrated under reduced pressure to give a residue. The resulting residue was purified by column chromatography (basic SiO 2 , petroleum ether/ethyl acetate=10/1 to 5/1) to give phosphoramidite 5a (1.75 g, 1.88 mmol, 48% yield) as a light-yellow foam.  1 H NMR (400 MHz, CDCl 3 ) δ 13.51 (br. s., 1H) 8.30 (d, J=7.53 Hz, 2H) 7.99 (d, J=18.57 Hz, 1H) 7.50-7.58 (m, 1H) 7.41-7.50 (m, 4H) 7.21-7.37 (m, 8H) 6.84 (ddd, J=6.65, 4.64, 2.26 Hz, 4H) 6.36-6.48 (m, 1H) 6.18 (d, J=16.06 Hz, 1H) 5.88-6.02 (m, 1H) 4.62 (td, J=6.65, 3.26 Hz, 1H) 4.15-4.26 (m, 1H) 3.79 (d, J=2.51 Hz, 7H) 3.50-3.66 (m, 4H) 3.25 (dt, J=10.79, 3.39 Hz, 1H) 2.54-2.70 (m, 2H) 2.27-2.44 (m, 2H) 1.66-1.86 (m, 2H) 1.38 (dd, J=13.30, 6.78 Hz, 1H) 1.12-1.22 (m, 9H) 1.04 (d, J=7.03 Hz, 3H) 0.79-0.93 (m, 3H) 0.75 (t, J=5.77 Hz, 6H).  31 P NMR (162 MHz, CDCl 3 ) δ 148.40-149.28 (m, 1 P). TLC petroleum ether/ethyl acetate=2/1 (R f =0.43). 
     
       
         
         
             
             
         
       
     
     To a solution of 2 (8.00 g, 12.2 mmol, 1 equiv), Cs 2 CO 3  (11.9 g, 36.6 mmol, 3 equiv), (E)-2-(2-cyclopropylvinyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2.84 g, 14.7 mmol, 1.2 equiv) and PPh 3  (1.60 g, 6.10 mmol, 0.5 equiv) in dioxane (60 mL) and water (30 mL) was added Pd(OAc) 2  (0.274 g, 1.22 mmol, 0.1 equiv) at 25° C. under N 2  current. The mixture was heated to 90° C. and stirred for 16 h. TLC (ethyl acetate/MeOH=20/1) showed compound 2 was consumed completely. The reaction mixture was diluted with water 50 mL and extracted with ethyl acetate (100 mL×2). The combined organic layers were washed with sat. aqueous NaCl (50 mL), dried over MgSO 4 , filtered and concentrated under reduced pressure to give a residue. The residue was purified by column chromatography (SiO 2 , DCM/MeOH=50/1 to 20:1) to give 3b (7.00 g, 11.8 mmol, 96% yield), obtained as a light-yellow solid. 
     
       
         
         
             
             
         
       
     
     To a solution of 3b (3.4 g, 5.71 mmol, 1.00 equiv) in DMF (30.00 mL) was added Et 3 N (0.693 g, 6.85 mmol, 1.50 equiv) and benzoic anhydride (1.42 g, 6.28 mmol, 1.20 equiv). The mixture was stirred at 0-25° C. for 16 h. TLC (DCM/MeOH=20/1) indicated 3b was consumed completely and the reaction was clean. The reaction mixture was quenched by addition sat. aqueous NaCl 20 mL at 25° C., and then extracted with ethyl acetate (50 mL). The combined organic layers were washed with sat. aqueous NaCl (20 mL), dried over MgSO 4 , filtered and concentrated under reduced pressure to give a residue. The residue was purified by column chromatography (Basic SiO 2 , petroleum ether/ethyl acetate=3/1 to 2:1) to give 4b (2.6 g, yield 66%) as a yellow solid.  1 H NMR (400 MHz, CDCl 3 ) δ 13.53 (br. s., 1H), 8.29 (d, J=7.53 Hz, 2H), 7.92 (s, 1H) 7.50-7.57 (m, 1H), 7.41-7.49 (m, 4H), 7.21-7.37 (m, 8H), 6.85 (d, J=7.53 Hz, 4H), 6.39 (t, J=6.53 Hz, 1H), 6.30 (d, J=16.06 Hz, 1H), 5.60 (dd, J=15.56, 9.03 Hz, 1H), 4.48-4.56 (m, 1H), 4.08 (d, J=3.01 Hz, 1H), 3.80 (s, 6H), 3.56 (dd, J=10.54, 3.01 Hz, 1H), 3.28 (dd, J=10.79, 3.26 Hz, 1H), 2.51 (ddd, J=13.55, 6.02, 3.01 Hz, 1H), 2.32 (dt, J=13.93, 6.84 Hz, 1H), 2.19 (d, J=4.02 Hz, 1H), 1.28 (td, J=8.41, 4.77 Hz, 2H), 0.43-0.61 (m, 2H), −0.22-0.01 (m, 2H). 
     
       
         
         
             
             
         
       
     
     To a solution of 4b (2.25 g, 3.22 mmol, 1 equiv) and 4,5-dicyanoimidazole (0.570 g, 4.83 mmol, 1.5 equiv) in DCM (20 mL) was added dropwise of 3-bis(diisopropylamino) phosphanyloxypropanenitrile (1.16 g, 3.86 mmol, 1.2 equiv) at 0° C. under N 2  current. Then the mixture was stirred at 0-25° C. for 2 h under N 2  current. A clear yellow solution was obtained. TLC (DCM/MeOH=20/1) showed 4b was consumed completely. The reaction mixture was concentrated under reduced pressure to give a residue. The resulting residue was purified by column chromatography (basic SiO 2 , petroleum ether/ethyl acetate=6/1 to 5/1) to give phosphoramidite 5b (2.20 g, 2.44 mmol, 75.9% yield) as a light-yellow foam.  1 H NMR (400 MHz, CDCl 3 ) δ 13.53 (br. s., 1H) 8.29 (d, J=7.53 Hz, 2H) 7.95 (d, J=18.57 Hz, 1H) 7.50-7.57 (m, 1H) 7.41-7.49 (m, 4H) 7.21-7.38 (m, 7H) 6.85 (dd, J=7.53, 4.52 Hz, 4H) 6.36-6.46 (m, 1H) 6.28 (dd, J=15.81, 3.76 Hz, 1H) 5.57 (dd, J=15.81, 9.29 Hz, 1H) 4.59 (td, J=6.53, 3.01 Hz, 1H) 4.19 (dd, J=15.56, 2.01 Hz, 1H) 3.80 (d, J=2.51 Hz, 6H) 3.48-3.66 (m, 4H) 3.22 (dt, J=10.54, 3.01 Hz, 1H) 2.52-2.70 (m, 2H) 2.27-2.43 (m, 2H) 1.21-1.31 (m, 2H) 1.17 (dd, J=6.78, 2.76 Hz, 10H) 1.04 (d, J=6.53 Hz, 3H) 0.78-0.92 (m, 1H) 0.39-0.56 (m, 2H) −0.29-−0.08 (m, 2H).  31 P NMR (162 MHz, CDCl 3 ) δ 148.36-149.25 (m, 1 P). TLC 20:1 DCM:methanol (R f =0.85). 
     
       
         
         
             
             
         
       
     
     To a solution of 2 (6.00 g, 9.15 mmol, 1 equiv), Cs 2 CO 3  (8.95 g, 27.5 mmol, 3 equiv), (E)-2-(2-cyclopropylvinyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2.59 g, 11.0 mmol, 1.2 equiv) and PPh 3  (1.20 g, 4.58 mmol, 0.5 equiv) in dioxane (40 mL) and water (20 mL) was added Pd(OAc) 2  (0.274 g, 1.22 mmol, 0.1 equiv) at 25° C. under N 2  current. The mixture was heated to 90° C. and stirred for 16 h. TLC (DCM/MeOH=20/1) showed 2 was consumed completely. The reaction mixture was diluted with water (50 mL) and extracted with ethyl acetate (100 mL×2). The combined organic layers were washed with sat. aqueous NaCl (50 mL), dried over MgSO 4 , filtered and concentrated under reduced pressure to give a residue. The residue was purified by column chromatography (SiO 2 , DCM/MeOH=50/1 to 20:1) to give the 3c (4.90 g, 7.68 mmol, 83% yield) was obtained as a light-yellow solid.  1 H NMR (400 MHz, CDCl 3 ) δ 7.82 (s, 1H), 7.43 (d, J=7.53 Hz, 2H), 7.19-7.36 (m, 8H), 6.82 (d, J=8.53 Hz, 4H), 6.52 (t, J=6.53 Hz, 1H), 5.55-5.69 (m, 2H), 4.46-4.55 (m, 1H), 4.16 (d, J=2.51 Hz, 2H), 3.72-3.84 (m, 6H), 3.45 (dd, J=10.04, 3.01 Hz, 1H), 3.29 (dd, J=10.29, 3.26 Hz, 1H), 3.09 (q, J=7.19 Hz, 1H), 2.74 (dt, J=10.79, 2.89 Hz, 1H), 2.21 (dt, J=13.80, 6.65 Hz, 1H), 1.72-1.86 (m, 2H), 1.35-1.59 (m, 9H), 0.92 (br. s., 2H). 
     
       
         
         
             
             
         
       
     
     To a solution of compound 3c (4.8 g, 7.53 mmol, 1.00 equiv) in DMF (50.0 mL) was added Et 3 N (1.14 g, 11.3 mmol, 1.50 equiv) and benzoic anhydride (2.04 g, 9.04 mmol, 1.20 equiv). The mixture was stirred at 0-25° C. for 16 h. TLC (DCM/MeOH=20/1) indicated 3c was consumed completely and the reaction was clean. The reaction mixture was quenched by addition water 20 mL at 25° C., and then extracted with ethyl acetate (50 mL). The combined organic layers were washed with sat. aqueous NaCl (20 mL), dried over MgSO 4 , filtered and concentrated under reduced pressure to give a residue. The residue was purified by column chromatography (basic SiO 2 , petroleum ether/ethyl acetate=5/1 to 2:1) to give compound 4c (3.60 g, yield 64%) as a yellow solid.  1 H NMR (400 MHz, CDCl 3 ) δ 13.52 (br. s., 1H), 8.30 (d, J=7.53 Hz, 2H), 7.90 (s, 1H), 7.50-7.57 (m, 1H), 7.40-7.48 (m, 4H), 7.20-7.37 (m, 8H), 6.84 (d, J=8.03 Hz, 4H), 6.39 (t, J=6.53 Hz, 1H), 6.13 (s, 2H), 4.51-4.58 (m, 1H), 4.10 (d, J=3.01 Hz, 1H), 3.79 (s, 6H), 3.53 (dd, J=10.54, 3.51 Hz, 1H), 3.33 (dd, J=10.54, 3.51 Hz, 1H), 2.52 (ddd, J=13.68, 5.90, 3.01 Hz, 1H), 2.33 (dt, J=13.55, 6.78 Hz, 1H), 2.18 (d, J=3.51 Hz, 1H), 1.79-1.94 (m, 2H), 1.37-1.61 (m, 7H), 1.00 (br. s., 2H). TLC DCM/MeOH=20/1 (R f =0.43). 
     
       
         
         
             
             
         
       
     
     To a solution of 4c (2.40 g, 3.24 mmol, 1 equiv) and 4,5-dicyanoimidazole (0.574 g, 4.86 mmol, 1.5 equiv) in DCM (20 mL) was added dropwise of 3-bis(diisopropylamino) phosphanyloxypropanenitrile (1.17 g, 3.89 mmol, 1.2 equiv) at 0° C. under N 2  current. Then the mixture was stirred at 0-25° C. for 2 h under N 2  current. A clear yellow solution was obtained. TLC (petroleum ether/ethyl acetate=2/1) showed 4c was consumed completely. The reaction mixture was concentrated under reduced pressure to give a residue, which was purified by column chromatography (basic SiO 2 , petroleum ether/ethyl acetate=10/1 to 6/1) to give phosphoramidite 5c (1.70 g, 2.44 mmol, 56% yield) as a white foam.  1 H NMR (400 MHz, CDCl 3 ) δ 13.51 (br. s., 1H), 8.30 (d, J=7.03 Hz, 2H), 7.93 (d, J=18.57 Hz, 1H), 7.50-7.57 (m, 1H), 7.41-7.49 (m, 4H), 7.22-7.37 (m, 8H), 6.84 (dd, J=7.53, 5.02 Hz, 4H), 6.36-6.46 (m, 1H), 6.03-6.18 (m, 2H), 4.62 (td, J=6.78, 3.01 Hz, 1H), 4.16-4.26 (m, 1H), 3.79 (d, J=2.51 Hz, 6H), 3.49-3.66 (m, 4H), 3.27 (dt, J=10.54, 3.76 Hz, 1H), 2.53-2.70 (m, 2H), 2.27-2.44 (m, 2H), 1.72-1.91 (m, 2H), 1.35-1.56 (m, 6H), 1.14-1.23 (m, 9H), 1.05 (d, J=6.53 Hz, 2H), 0.97 (d, J=3.01 Hz, 3H).  31 P NMR (162 MHz, CDCl 3 ) δ 148.49-149.26 (m, 1 P). 
     
       
         
         
             
             
         
       
     
     To a solution of 2 (6.00 g, 12.211.0 mmol, 1 equiv), Cs 2 CO 3  (8.95 g, 27.5 mmol, 3 equiv), (E)-2-(4-fluorostyryl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2.72 g, 11.0 mmol, 1.2 equiv) and PPh 3  (1.20 g, 4.58 mmol, 0.5 equiv) in dioxane (60 mL) and water (30 mL) was added Pd(OAc) 2  (0.206 g, 0.915 mmol, 0.1 equiv) at 25° C. under N 2  current. The mixture was heated to 90° C. and stirred for 16 h. TLC (DCM/MeOH=20/1) showed 2 was consumed completely. The reaction mixture was diluted with water (50 mL) and extracted with ethyl acetate (100 mL×2). The combined organic layers were washed with sat. aqueous NaCl (50 mL), dried over MgSO 4 , filtered and concentrated under reduced pressure to give a residue. The residue was purified by column chromatography (SiO 2 , DCM/MeOH=50/1 to 20:1) to give 3d (3.10 g, 4.77 mmol, 52% yield) as a light-yellow solid. TLC DCM/MeOH=20/1 (R f =0.20). 
     
       
         
         
             
             
         
       
     
     To a solution of 3d (2.7 g, 4.16 mmol, 1.00 equiv) in DMF (30.00 mL) was added Et 3 N (0.631 g, 6.24 mmol, 1.50 equiv) and benzoic anhydride (1.13 g, 4.99 mmol, 1.20 equiv). The mixture was stirred at 0-25° C. for 16 h. TLC (petroleum ether/ethyl acetate=1/1) indicated 3d was consumed completely and the reaction was clean according to TLC. The reaction mixture was quenched by addition water 100 mL at 0-5° C., and then extracted with ethyl acetate (50 mL). The combined organic layers were washed with sat. aqueous NaCl (20 mL), dried over MgSO 4 , filtered and concentrated under reduced pressure to give a residue. The residue was purified by column chromatography (basic SiO 2 , petroleum ether/ethyl acetate=2/1 to 1:1) to give 4d (2.00 g, yield 64%) as a yellow solid.  1 H NMR (400 MHz, CDCl 3 ) δ 13.59 (br. s., 1H), 8.32 (d, J=7.53 Hz, 2H), 8.22 (s, 1H), 7.52-7.59 (m, 1H), 7.42-7.51 (m, 4H), 7.33 (dd, J=8.78, 1.76 Hz, 4H), 7.22-7.29 (m, 3H), 7.14-7.21 (m, 1H), 7.05 (d, J=16.56 Hz, 1H), 6.71-6.89 (m, 9H), 6.43 (t, J=6.53 Hz, 1H), 4.58 (br. s., 1H), 4.15 (d, J=2.51 Hz, 1H), 3.59-3.75 (m, 7H), 3.29 (dd, J=11.04, 3.01 Hz, 1H), 2.59 (ddd, J=13.55, 6.02, 3.01 Hz, 1H), 2.41 (dt, J=13.55, 6.78 Hz, 1H), 2.14 (br. s., 1H);  19 F NMR (376 MHz, CDCl 3 ) δ −114.32 (s, 1 F) 
     
       
         
         
             
             
         
       
     
     To a solution of 4d (2.30 g, 3.05 mmol, 1 equiv) and 4,5-dicyanoimidazole (0.570 g, 4.27 mmol, 1.5 equiv) in DCM (20 mL) was added drop wise of 3-bis(diisopropylamino) phosphanyloxypropanenitrile (1.10 g, 3.66 mmol, 1.2 equiv) at 0° C. under N 2  current. Then the mixture was stirred at 0-25° C. for 3 h under N 2  current. A clear yellow solution was obtained. TLC (petroleum ether/ethyl acetate=2/1) showed 4d was consumed completely. The reaction mixture was concentrated under reduced pressure to give a residue, which was purified by column chromatography (basic SiO 2 , petroleum ether/ethyl acetate=6/1 to 5/1) to give phosphoramidite 5d (2.10 g, 2.20 mmol, 72% yield) as a light-yellow foam.  1 H NMR (400 MHz, CDCl 3 ) δ 13.59 (s, 1H), 8.33 (d, J=7.53 Hz, 2H), 8.25 (d, J=19.07 Hz, 1H), 7.52-7.59 (m, 1H), 7.41-7.51 (m, 4H), 7.22-7.38 (m, 7H), 7.18 (dd, J=7.03, 4.02 Hz, 1H), 7.03 (d, J=16.06 Hz, 1H), 6.66-6.85 (m, 9H), 6.45 (q, J=6.53 Hz, 1H), 4.65 (td, J=6.53, 3.01 Hz, 1H), 4.19-4.30 (m, 1H), 3.69 (s, 6H), 3.49-3.65 (m, 3H), 3.19-3.28 (m, 1H), 2.58-2.76 (m, 2H), 2.36-2.46 (m, 2H), 1.51 (d, J=6.53 Hz, 1H), 1.17 (d, J=7.03 Hz, 8H), 1.04 (d, J=6.53 Hz, 3H).  31 P NMR (162 MHz, CDCl 3 ) δ 148.45-149.26 (m, 1 P).  19 F NMR (376 MHz, CDCl 3 ) δ −114.45 (s, 1 F). TLC 2:1 pentane ether:ethyl acetate (R f =0.43). 
     Synthesis of Phosphoramidite 11 
     
       
         
         
             
             
         
       
     
     To a solution of 6 (50.0 g, 141.2 mmol, 1 equiv) and DMAP (0.172 g, 1.41 mmol, 0.01 equiv) in pyridine (500 mL) was added dropwise DMTrCl (50.2 g, 148.2 mmol, 1.05 equiv) and at 0° C. The mixture was stirred at 25° C. for 16 h. TLC (Petroleum ether/Ethyl acetate=1/1) indicated compound 6 was consumed completely. The reaction mixture was concentrated with MeOH (5 mL) under reduced pressure to remove pyridine. The residue was purified by column chromatography (SiO 2 , petroleum ether/ethyl acetate=2/1 to 1/2) to give 7 (85.0 g, yield 92%) as a white foam.  1 H NMR (400 MHz, CDCl 3 ) δ 8.16 (s, 1H), 7.44 (d, J=7.53 Hz, 2H), 7.22-7.38 (m, 7H), 6.87 (d, J=8.53 Hz, 4H), 6.35 (dd, J=7.78, 5.77 Hz, 1H), 4.53-4.62 (m, 1H), 4.11-4.18 (m, 2H), 3.81 (s, 6H), 3.35-3.48 (m, 2H), 2.53 (ddd, J=13.55, 5.52, 2.51 Hz, 1H), 2.26-2.37 (m, 1H). TLC petroleum ether/ethyl acetate=1/1 (R f =0.15). 
     
       
         
         
             
             
         
       
     
     To a solution of 7 (68.7 g, 105 mmol, 1 equiv), methyl acrylate (54 g, 627 mmol, 2 equiv), PPh 3  (5.5 g, 20.9 mmol, 0.2 equiv), and trimethylamine (21.2 g, 209 mmol, 2 equiv) in dioxane (700 mL) was added Pd(OAc) 2  (2.35 g, 10.5 mmol, 0.1 equiv) at 25° C. under N 2  current. The mixture was heated to 90° C. and stirred for 16 h. TLC (petroleum ether/ethyl acetate=1/1) showed 7 was consumed completely. The reaction mixture was filtered under reduced pressure to give a residue. The residue was purified by column chromatography (SiO 2 , petroleum ether/ethyl acetate=1/1 to 1.5:1) to give the compound 8 (42 g, 68.3 mmol, 65.3% yield) as a light-yellow solid. 
     
       
         
         
             
             
         
       
     
     To solution of 8 (42 g, 68.3 mmol, 1 equiv) in THF (500 mL) was added NaOH aqueous (1N, 102.5 mL, 1.5 equiv) at 25° C., and then the resulting mixture was stirred at 25° C. for 16 h. TLC (DCM/MeOH=10/1) indicated 8 was consumed completely. The reaction mixture was partitioned between ethyl acetate (200 mL) and water (100 mL), the water phase was separated, acidified with sat. aqueous citric acid to pH7, the white suspension was filtered and dried under reduced pressure to give the compound 9 (30.5 g, 50.8 mmol, 74% yield) as a white solid.  1 H NMR (400 MHz, CDCl 3 ) δ 7.67 (s, 1H), 7.40 (d, J=7.53 Hz, 2H), 7.24-7.33 (m, 8H), 7.15-7.22 (m, 1H), 6.91 (d, J=15.56 Hz, 1H), 6.82 (d, J=9.04 Hz, 4H), 6.29 (t, J=6.27 Hz, 1H), 4.47 (d, J=6.02 Hz, 1H), 3.99 (d, J=5.02 Hz, 1H), 3.75 (s, 5H), 3.45 (dd, J=10.29, 5.27 Hz, 1H), 3.34 (dd, J=10.04, 4.52 Hz, 1H), 2.41-2.51 (m, 1H), 2.26 (dt, J=13.80, 6.65 Hz, 1H). TLC DCM/MeOH=10/1 (R f =0.15). 
     
       
         
         
             
             
         
       
     
     A solution of 9 (5 g, 8.32 mmol, 1 equiv), Et 3 N (4.21 g, 41.6 mmol, 5 equiv) and HATU (4.75 g, 12.5 mmol, 1.5 equiv) in DMF (60 mL) was stirred for 30 min at 25° C. Then to this mixture was added 2-aminoethyl acetate hydrochloride (1.39 g, 9.99 mmol, 1.2 equiv) at 25° C. The mixture was stirred at 25° C. for 16 h. TLC (DCM/MeOH=20/1) showed the acid was consumed completely. The reaction mixture was quenched by addition of sat. aqueous NaHCO 3  (50 mL) at 25° C., and then extracted with ethyl acetate (100 mL×2). The combined organic layers were concentrated under reduced pressure to give a residue. The residue was purified by column chromatography (basic SiO 2 , DCM/MeOH=50/1 to 30/1) to give compound 10 (2.7 g, yield 47%) as a white foam. TLC DCM/MeOH=20/1 (R f =0.30). 
     
       
         
         
             
             
         
       
     
     To a solution of 10 (2.10 g, 3.06 mmol, 1 equiv) and 4,5-dicyanoimidazole (0.543 g, 4.59 mmol, 1.5 equiv) in DCM (30 mL) was added drop wise of 3-bis(diisopropylamino)phosphanyloxypropanenitrile (1.11 g, 3.67 mmol, 1.2 equiv) at 0° C. under N 2  current. Then the mixture was stirred at 0-25° C. for 2 h under N 2  current. A clear yellow solution was obtained. TLC (DCM/MeOH=20/1) showed 10 was consumed completely. The reaction mixture was concentrated under reduced pressure to give a residue, which was purified by column chromatography (basic SiO 2 , DCM/Acetone=15/1 to 8/1) to give phosphoramidite 11 (1.7 g, 1.44 mmol, 75% yield) as a white foam.  1 H NMR (400 MHz, CDCl 3 ) δ 7.79-7.90 (m, 1H) 7.43 (d, J=7.53 Hz, 2H), 7.20-7.36 (m, 7H), 7.04 (d, J=15.56 Hz, 1H), 6.82-6.92 (m, 4H), 6.71-6.80 (m, 1H), 6.28 (t, J=6.53 Hz, 1H), 5.41-5.55 (m, 1H), 4.57 (dt, J=6.53, 3.26 Hz, 1H), 4.18-4.29 (m, 1H), 4.07 (q, J=5.02 Hz, 2H), 3.79 (s, 6H), 3.54-3.70 (m, 3H), 3.39-3.51 (m, 3H), 3.28-3.37 (m, 1H), 2.55-2.80 (m, 2H), 2.45 (t, J=6.27 Hz, 1H), 2.28 (dt, J=13.68, 6.96 Hz, 1H), 2.06 (s, 3H), 1.25-1.31 (m, 1H), 1.18 (t, J=6.27 Hz, 9H), 1.09 (d, J=7.03 Hz, 2H). 
     Synthesis of Phosphoramidite 15 
     
       
         
         
             
             
         
       
     
     Pivaloyl chloride (7.25 g, 61.1 mmol, 1 equiv) was added drop wise to a solution of 4-(2-aminoethyl) phenol (7.5 g, 54.5 mmol, 1 equiv) in DCM (50 mL) and TFA (50 mL) at 25° C., then the resulting brown mixture was stirred for 12 h. LCMS showed reactant was consumed completely and one main peak with desired MS was detected. The reaction mixture was concentrated under reduced pressure to give a residue, which was purified by column chromatography (SiO 2 , DCM:MeOH=20/1 to 1:1) to give compound 13 (9.5 g, 42.9 mmol, 79% yield) as a brown solid.  1 H NMR (400 MHz, CDCl 3 ) δ 7.21 (d, J=8.53 Hz, 2H), 6.95 (d, J=8.53 Hz, 2H), 3.18 (t, J=7.28 Hz, 2H), 2.90-2.99 (m, 2H), 1.34 (s, 9H).  19 F NMR (376 MHz, CDCl 3 ) δ −75.82 (s, 1 F). 
     
       
         
         
             
             
         
       
     
     A solution of 9 (2.8 g, 4.66 mmol, 1 equiv), Et 3 N (0.94 g, 9.32 mmol, 2 equiv) and HATU (3.54 g, 9.32 mmol) in DMF (50 mL) was stirred for 30 min at 25° C. Then to this mixture was added 13 (1.13 g, 5.13 mmol, 1.1 equiv) at 25° C. The mixture was stirred at 25° C. for 16 h. TLC (DCM/MeOH=20/1) showed the acid was consumed completely. The reaction mixture was concentrated under reduced pressure to remove solvents. The residue was purified by column chromatography (basic SiO 2 , DCM/MeOH=20/1 to 10/1) to give compound 14 (2 g, yield 86%) as a white foam. TLC DCM/MeOH=20/1 (R f =0.20). 
     
       
         
         
             
             
         
       
     
     To a solution of 14 (2.90 g, 3.61 mmol, 1 equiv) and 4,5-dicyanoimidazole (0.639 g, 5.41 mmol, 1.5 equiv) in DCM (30 mL) was added drop wise of 3-bis(diisopropylamino)phosphanyloxypropanenitrile (1.30 g, 4.33 mmol, 1.2 equiv) at 0° C. under N 2  current. Then the mixture was stirred at 0-25° C. for 2 h under N 2  current. A clear yellow solution was obtained. TLC (DCM/MeOH=20/1) showed 14 was consumed completely. The reaction mixture was concentrated under reduced pressure to give a residue, which was purified by column chromatography (basic SiO 2 , DCM/acetone=15/1 to 10/1) to give phosphoramidite 15 (1.95 g, 1.94 mmol, 54% yield) as a light-brown gum.  1 H NMR (400 MHz, CDCl 3 ) δ 7.76-7.91 (m, 2H), 7.45 (d, J=6.02 Hz, 2H), 7.27-7.38 (m, 7H), 7.13-7.26 (m, 3H), 6.97-7.09 (m, 3H), 6.87 (dd, J=8.78, 3.76 Hz, 3H), 6.61 (dd, J=15.56, 11.04 Hz, 1H), 6.28-6.35 (m, 1H), 6.18 (s, 1H), 5.01-5.12 (m, 1H), 4.59 (br. s., 1H), 4.10-4.28 (m, 4H), 3.80 (s, 5H), 3.28-3.62 (m, 9H), 2.78 (td, J=6.15, 1.76 Hz, 3H), 2.60-2.72 (m, 10H), 2.45 (t, J=6.27 Hz, 1H), 2.28 (dt, J=12.92, 6.34 Hz, 1H), 1.37 (s, 7H), 1.30 (t, J=6.27 Hz, 15H), 1.16-1.22 (m, 7H), 1.09 (d, J=6.53 Hz, 3H).  31 P NMR (162 MHz, CDCl 3 ) δ 148.76-149.28 (m, 1 P) 14.16 (s, 2 P). 
     Synthesis and Characterization of Functionalized Oligonucleotides. 
     Standard phosphoramidite reagents and 1000-A controlled-pore glass (CPG) supports for dA, Ac-dC, dmf-dG, and dT were purchased from Glen Research, as were chemical phosphorylation reagent II (10-1901) and the phosphoramidite for NHS-carboxy-dT (10-1535). The phosphoramidite reagent for the incorporation of the aminoallyl side-chain-functionalized nucleotide (BA 0311) was purchased from Berry and Associates, as was the perfluoroalkyl-DMT dT phosphoramidite (FL 1300). The phosphoramidite reagents for the incorporation of isopentyl (5a), cyclopropyl (5b), cyclopentyl (5c), fluorophenyl (5d), ethanolamine (11), and tyramine (15) side-chain-functionalized nucleotides were custom synthesized by WuXi AppTec as detailed in the previous section. 
     Solid-phase synthesis of side-chain-functionalized DNA was performed on a PerSeptive Biosystems Expedite 8909 DNA synthesizer. All side-chain-functionalized phosphoramidites were incubated with molecular sieves overnight before use. Syntheses were performed on 1-μmol columns using standard coupling cycles, except for 5′-phosphorylation, which required 7 minutes of coupling with chemical phosphorylation reagent II. When the histamine side-chain-functionalized nucleotide was called for, NHS-carboxy-dT was incorporated in its place, and after the full-length synthesis was completed, a solution of histamine (free base; 5 mg) and diisopropylethylamine (1 μl) in 200 μl of 10% DMSO in acetonitrile was manually injected into the column and allowed to react overnight, and then the column was washed with acetonitrile and dried. Similarly, when a methylamine-functionalized nucleotide was required (for probing side chain SAR; see  FIG.  5   ), NHS-carboxy-dT was incorporated in its place, and methylamine (30 equiv. with respect to solid-phase synthesis scale, from a 2M stock in THF) and diisopropylethylamine (1 μl) in 200 μl of acetonitrile was added to the column and allowed to react overnight before washing and drying. The functionalized DNA was then deprotected and cleaved from solid support by incubation in 30% ammonium hydroxide overnight at room temperature. The phosphorylated trinucleotide building blocks were synthesized DMT-off and, after deprotection, cleavage, and evaporation of ammonium hydroxide, were purified by reverse-phase HPLC using a gradient of 0-20% acetonitrile in 0.1 M TEAA, pH 7, over 24 minutes, followed by 20-40% acetonitrile in 0.1 M TEAA, pH 7, over 10 minutes. The full length HFNAP Evo5-syn was synthesized DMT-on and the 5′-most nucleotide was incorporated with a perfluoroalkyl-DMT phosphoramidite (Berry and Associates, FL 1300). After deprotection/cleavage, the polymer was purified and deprotected on column with a fluorous phase purification cartridge (Fluoro-Pak II from Berry and Associates) according to manufacturer&#39;s instructions. (See  FIG.  10    for synthetic scheme.) The Evo5-syn used for mass spectrometry characterization and for SPR experiments was further purified by denaturing PAGE on a 10% TBE-urea gel. 
     Mass Spectrometry Characterization of Chemically Synthesized Functionalized Oligonucleotides 
     Oligonucleotide samples were analyzed in negative ion mode using a Bruker Impact II q-TOF mass spectrometer equipped with an Agilent 1290 uHPLC using flow injection analysis. The purified samples were introduced at a constant flow rate of 0.200 mL/minute using 50% acetonitrile and 0.1% formic acid. Each individual data file was calibrated for the m/z scale using a plug of sodium formate clusters introduced through a secondary isocratic pump and introduced using a 6-port valve. Using this internal calibration method, less than 2 ppm relative error was obtained on all samples. Bruker Data Analysis software was used to simulate the isotope pattern for each target ion. 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                   
                 Expected 
                   
                 Expected 
                   
               
               
                   
                 m/z 
                 Observed 
                 m/z 
                 Observed 
               
               
                 Name 
                 ([M-2H] 2− ) 
                 m/z 
                 ([M-H] − ) 
                 m/z 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 phos-CAA-Isopentyl 
                 513.6257 
                 513.6253 
                 1028.2588 
                 1028.2580 
               
               
                 phos-CAC-Isopentyl 
                 501.6201 
                 501.6198 
                 1004.2475 
                 1004.2467 
               
               
                 phos-CTC-Isopentyl 
                 497.1143 
                 497.1142 
                 995.2360 
                 995.2355 
               
               
                 phos-CGG-Isopentyl 
                 529.6207 
                 529.6204 
                 1060.2486 
                 1060.2478 
               
               
                 phos-CAT- 
                 521.0918 
                 251.0914 
                 1043.1908 
                 1043.1896 
               
               
                 Fluorophenyl 
               
               
                 phos-CAG- 
                 533.5950 
                 533.5946 
                 1068.1973 
                 1086.1961 
               
               
                 Fluorophenyl 
               
               
                 phos-CGA- 
                 533.5950 
                 533.5946 
                 1068.1973 
                 1068.1960 
               
               
                 Fluorophenyl 
               
               
                 phos-CGC- 
                 521.5894 
                 521.5890 
                 1044.1861 
                 1044.1849 
               
               
                 Fluorophenyl 
               
               
                 phos-CTA- 
                 494.0965 
                 494.0961 
                 989.2003 
                 989.1990 
               
               
                 Cyclopropyl 
               
               
                 phos-CCA- 
                 486.5967 
                 486.5961 
                 974.2006 
                 974.1993 
               
               
                 Cyclopropyl 
               
               
                 phos-CCT- 
                 482.0909 
                 482.0907 
                 965.1890 
                 965.1885 
               
               
                 Cyclopropyl 
               
               
                 phos-CCC- 
                 474.5910 
                 474.5907 
                 950.1894 
                 950.1885 
               
               
                 Cyclopropyl 
               
               
                 phos-CTT- 
                 510.6142 
                 510.6137 
                 1022.2356 
                 1022.2343 
               
               
                 Cyclopentyl 
               
               
                 phos-CTG- 
                 523.1174 
                 523.1170 
                 1074.2421 
                 1047.2406 
               
               
                 Cyclopentyl 
               
               
                 phos-CGT- 
                 523.1174 
                 523.1169 
                 1047.2421 
                 1047.2406 
               
               
                 Cyclopentyl 
               
               
                 phos-CCG- 
                 515.6176 
                 515.6170 
                 1032.2425 
                 1032.2408 
               
               
                 Cyclopentyl 
               
               
                 phos-TTT-Phenol 
                 551.5987 
                 551.5983 
                 — 
                 — 
               
               
                 phos-TTG-Phenol 
                 564.1020 
                 564.1019 
                 1129.2112 
                 1129.2099 
               
               
                 phos-TGT-Phenol 
                 564.1020 
                 564.1015 
                 1129.2112 
                 1129.2094 
               
               
                 phos-TCG-Phenol 
                 556.6021 
                 556.6017 
                 1114.2115 
                 1114.2103 
               
               
                 phos-TAA-Imidazole 
                 547.6081 
                 547.6079 
                 1096.2234 
                 1096.2227 
               
               
                 phos-TAC-Imidazole 
                 535.6025 
                 535.6023 
                 1072.2122 
                 1072.2117 
               
               
                 phos-TCA-Imidazole 
                 535.6025 
                 535.6021 
                 1072.2122 
                 1072.2113 
               
               
                 phos-TCC-Imidazole 
                 523.5969 
                 523.5964 
                 1048.2010 
                 1048.1999 
               
               
                 phos-TAT-Primary 
                 518.0889 
                 518.0883 
                 1037.1850 
                 1037.1837 
               
               
                 alcohol 
               
               
                 phos-TAG-Primary 
                 530.5921 
                 530.5917 
                 1062.1915 
                 1062.1904 
               
               
                 alcohol 
               
               
                 phos-TGA-Primary 
                 530.5921 
                 530.5917 
                 1062.1915 
                 1062.1903 
               
               
                 alcohol 
               
               
                 phos-TGC-Primary 
                 518.5865 
                 518.5862 
                 1038.1802 
                 1039.1793 
               
               
                 alcohol 
               
               
                 phos-TTA-Allylamine 
                 489.0861 
                 489.0857 
                 979.1795 
                 979.1786 
               
               
                 phos-TTC-Allylamine 
                 — 
                 — 
                 955.1683 
                 955.1672 
               
               
                 phos-TCT-Allylamine 
                 — 
                 — 
                 955.1683 
                 955.1673 
               
               
                 phos-TGG-Allylamine 
                 509.5868 
                 509.5864 
                 1020.1809 
                 1020.1799 
               
               
                   
               
            
           
         
       
     
     Mass spectrometry data for PCSK9-Evo5-syn are given in  FIG.  11   . 
     Additional Experimental Procedures 
     Isolation of Single-Stranded HFNAP by Templated Translation Via DNA Following Ligase-Mediated Polymerization 
     To synthesize the double-stranded HFNAP-template hybrid, template (10 pmol), polymerization initiation and termination primers (15 pmol each), functionalized trinucleotide building blocks (100 pmol for each occurrence of the corresponding codon) and 10× T4 RNA ligase reaction buffer (New England Biolabs, B0216L; 1 μL) were mixed in a total volume of 8 L in a PCR tube. The mixture was subjected to the following temperature program on a thermocycler: 95° C. for 10 sec; 65° C. for 4 min; a ramp from 65° C. to 4° C. at 0.1° C. per 10 s. To the PCR tube were added 1 μL of 10 mM ATP and 1 μL of T3 DNA Ligase (New England Biolabs, M0317L; 3000000 units/ml) while the reaction mixture was kept at 4° C. The reaction was incubated at 4° C. for 12 h and then at 16° C. for 2 h. For the evaluation of translation yield on template libraries ( FIG.  1 C ), the reaction mixture was run on nondenaturing 10% TBE polyacrylamide gel electrophoresis and stained with SYBR gold for characterization. 
     To synthesize an unbiotinylated HFNAP, unbiotinylated primers and a doubly biotinylated ssDNA template (200 pmol), polymerization initiation and termination primers (300 pmol each), functionalized trinucleotide building blocks (2 nmol for each occurrence of the corresponding codon) and 10× T4 RNA ligase reaction buffer (20 μL) were mixed in a total volume of 180 μL. The mixture was split in 10 equal volumes into PCR tubes and subjected to the following temperature program on a thermocycler: 95° C. for 10 sec; 65° C. for 4 min; a ramp from 65° C. to 4° C. at 0.1° C. per 10 s. To each PCR tube were added 1 μL of 10 mM ATP and 1 L of T3 DNA Ligase while the reaction mixture was kept at 4° C. The reaction was incubated at 4° C. for 12 h and then at 16° C. for 2 h. The portions were was used in a solution phase polymerization reaction. After the reaction incubation period, the reaction mixture was combined and 50 μL of a 1% suspension of with MyOne Streptavidin C1 magnetic beads (ThermoFisher Scientific, 65002; 1 μL of the stock 1% suspension per 4 pmol of biotinylated template), and then an equal volume of 2× bind-and-wash buffer (2M NaCl, 2 mM EDTA, 20 mM Tris-HCl, pH 7.5) was added. After 30 minutes of incubation on a rotor, the supernatant was removed by magnetic separation, and the beads were suspended 18 μL of 20 mM NaOH. The supernatant was combined with 12 μL of formamide denaturing mix (95% formamide, 1 mM EDTA) and run on a 10% TBE-urea PAGE gel. Desired product was visualized by UV shadowing at 265 nm against a TLC plate (with F254 indicator), excised from the gel, eluted in 200 μL of TE buffer (10 mM Tris-HCl, 1 mM EDTA, pH 8.0) overnight, filtered, mixed with 2 mL of ssDNA column loading mix (40:60:0.5 v/v/v of saturated aqueous guanidinium chloride/isopropanol/3M sodium acetate, pH 5.2) and cleaned up with a Qiagen QiaQuick column. Typical isolated yield of HFNAP from 200 pmol of template was between 5 and 15 pmol as determined by Nanodrop or quantitative PCR. For the validation of sequence specificity of translation and amplification ( FIG.  1 D ), a small sample (˜1 fmol) was amplified by PCR using Q5 DNA polymerase and the primers T7-out-PCR2 and pp2-library. The amplicon was purified by PAGE on a nondenaturing 10% TBE gel and subjected to Sanger sequencing. 
     Synthesis and isolation of a biotinylated HFNAP followed the same procedure as above, except that an unbiotinylated ssDNA template was used, and one of polymerization primers was doubly biotinylated. After polymerization reaction, streptavidin bead capture, and alkaline denaturation, the bead-bound biotinylated molecules were immobilized on beads as described above. The supernatant was removed, and the beads were washed three times with 20 μL of 20 mM NaOH. The beads were then suspended in 20 μL of formamide denaturing mix (95% formamide, 1 mM EDTA) and heated to 95° C. or 30 min. After cooling to room temperature and magnetic separation, desired product was isolated from the supernatant was directly loaded ontoby PAGE on a 10% denaturing TBE-urea PAGE gel and separated by electrophoresis. Desired product was excised from the gel and eluted. Typical isolated yield of HFNAP from 200 pmol of template was between 5 and 15 pmol as determined by Nanodrop or quantitative PCR. 
     Synthesis and isolation of a biotinylated, truncated HFNAP (such as PCSK9-Evo5) followed the same procedure, except that the primer contained a 2′-deoxy-U nucleotide, and the ligation reaction mixture was treated with USER enzyme (New England Biolabs) at 37° C. for 2 h before proceeding to streptavidin bead capture. 
     During the selections, polymerization reactions were performed with templates immobilized on streptavidin beads and processed as detailed in the main text Methods section. 
     Selection of HFNAPs that Bind Protein Targets 
     Selection of PCSK9-Binding HFNAPs from a Naïve Library 
     Recombinant human PCSK9 protein (ACROBiosystems, PC9-H5223) was immobilized onto AminoLink Plus aldehyde-functionalized agarose resin via reductive amination with a MicroLink Protein Coupling Kit (ThermoFisher Scientific, 20475) at a loading of 1 mg protein per mL resin according to the resin&#39;s manufacturer&#39;s instructions. 
     To initiate the selection, primer extension was performed with 10 pmol BtBt-ExtA on 5 pmol of sense strand randomized DNA library (“naïve library AZ15”) with Klenow (exo-) polymerase (New England Biolabs, M0212S) at 37° C. overnight. The reaction mixture was combined with an equal volume of 2× bind-and-wash buffer (2 M NaCl, 2 mM EDTA, 20 mM Tris-HCl, pH7.5) and immobilized onto 10 μL of a 1% suspension of MyOne Streptavidin C1 magnetic beads. After removal of supernatant, the beads were washed three times with 20 μL of 20 mM NaOH (leaving a biotinylated ssDNA template library on the beads) and then twice with 20 μL of 1× T4 RNA ligase reaction buffer. 
     To the bead-immobilized template library were added pp1A and pp2Z (7.5 pmol each), a mixture of all 32 functionalized trinucleotide building blocks (100 pmol each), 10× T4 RNA ligase reaction buffer (1 μL), and water to a total of 8 μL. The suspension was transferred to a PCR tube and subjected to the following temperature program on a thermocycler: 95° C. for 10 sec; 65° C. for 4 min; a ramp from 65° C. to 4° C. at 0.1° C. per 10 s. To the PCR tube were added 1 μL of 10 mM ATP and 1 μL of T3 DNA Ligase. The reaction was incubated at 4° C. for 12 h and then at 16° C. for 2 h. An additional 10 μL of a 1% suspension of MyOne Streptavidin C1 magnetic beads and 20 μL of 2× bind-and-wash buffer was added, and the mixture was incubated at room temperature for 30 min before magnetic separation. The supernatant was discarded, and then the unbiotinylated HFNAP strand was eluted from the beads by treatment with 2×30 μL of 20 mM NaOH. To the combined HFNAP fractions was added 600 uL of ssDNA column loading mix (40:60:0.5 v/v/v of saturated aqueous guanidinium chloride/isopropanol/3M sodium acetate, pH 5.2) and the mixture was cleaned up with a Qiagen MinElute column, eluting into 15 μL of water. 
     The HFNAP was added to 35 μL of DPBS (with calcium and magnesium; Lonza 17-513Q) containing 0.1 mg/ml BSA and 0.01% Tween-20, and then incubated with PCSK9 resin in a micro-spin filtration column (Pierce 89879) at room temperature for 1 h on a rotor. (The amounts of resin-bound protein used in each round are indicated in  FIG.  2 B .) The flow-through was collected by centrifugation at 1000 g into an Eppendorf tube. The beads in the column were washed three times with 50 μL each of DPBS, each wash being collected by centrifugation as well. The column was cut open and the beads were collected by centrifugation into an Eppendorf tube. To the beads was added 50 μL of a lithium dodecyl sulfate (LDS)-containing buffer (Life Technologies B0007, diluted 4-fold), and the tube was incubated at 95° C. for 15 min. After cooling, 600 uL of ssDNA column loading mix was added, and the mixture was cleaned up with a QiaQuick column, eluting the HFNAP into 50 μL of water. 
     Samples of 1 μL each from the flow-through, the three washes, and the elution were quantified by qPCR (20 μL reaction volume) using the iTaq Supermix (Biorad, 172-5125) with pp2Z and ExtA (500 nM each) as primers under the following temperature program: 95° C. for 3 min; 35 cycles of 95° C. for 15 s, 59.5° C. for 30 s, 72° C. for 15 s. The number of cycles for the qPCR curve on the elution sample to reach the end of exponential growth was used as the number of cycles for the preparative PCR (400 μL reaction volume split into 8×50 μL) using Q5 Hot Start High-Fidelity 2× Master Mix (New England Biolabs, M0494), with the selection elution pool (20 μL) as template and pp2Z and BtBt-ExtA (500 nM each) as primers, under the same annealing and extension temperatures as in the qPCR. The finished reaction was mixed with 2 mL of ssDNA column loading mix and cleaned up with a Qiagen MinElute column, eluting into 15 μL of water. The amplified dsDNA was purified by PAGE on a non-denaturing 10% TBE gel. A portion (indicated in  FIG.  2 B ) of the gel-purified product was immobilized on L of a 1% suspension of MyOne Streptavidin C1 magnetic beads, strand-separated with 20 mM NaOH, and the immobilized template strand was used for HFNAP translation to initiate the next round of selection. 
     Evolution of PCSK9-A5 for Higher Affinity 
     The evolution of PCSK9-A5 was performed in a similar fashion with the following differences. Rediv library AZ15 (custom synthesized by TriLink BioTechnologies) was used to initiate the selection. The primer pp1A-3ddC was used instead of pp1A for ligase-based polymerization in order to facilitate the removal of cheaters ( FIG.  7   ). After two rounds of selection using the same PCSK9 bead loading as before, beads with reduced loading (150 μg protein per mL resin for rounds 3-5; 40 μg protein per mL resin for round 6) were used for affinity enrichment. 
     Selection of IL-6-Binding HFNAPs from a Naïve Library 
     The selection of IL-6-binding HFNAPs was performed in a similar fashion with the following differences. Recombinant human IL-6 protein (PeproTech, 200-06) immobilized on AminoLink Plus aldehyde at 0.25 mg protein per ml resin was used as the immobilized target. Throughout the selection, 240 pmol of immobilized IL-6 protein was used in each round. A primer extension on naïve library CW15 with BtBt-ExtC was used to initiate the selection. The primers pp1C and pp2W were used for ligase-based polymerization (translation) reactions. The primers ExtC and pp2W were used for qPCR reactions. The primers BtBt-ExtC and pp2W were used in PCR reactions that amplify affinity-enriched HFNAP into dsDNA for initiating the next round of selection. 
     High-Throughput DNA Sequencing and Data Analysis 
     Small samples from the elution pool of selection rounds were amplified by PCR using Q5 Hot Start High-Fidelity 2× Master Mix with MiSeqA and MiSeqZ as primers to sub-saturation number of cycles (determined during the selection by qPCR) with primers that install flanking sequences. The amplicons were PAGE-purified and amplified by PCR with Illumina adapter primers. The amplicons were again PAGE-purified and subjected to high-throughput sequencing on an Illumina MiSeq. 
     For the IL-6 selection, samples were similarly prepared by PCR amplification with MiSeqC and MiSeqW. The amplicons were PAGE-purified and PCR amplified with Illumina adapter primers. The amplicons were again PAGE-purified and subjected to high-throughput sequencing on an Illumina MiSeq. 
     Processing and Analysis of High-Throughput Sequencing Data 
     The FASTQ files from high-throughput sequencing were first processed with CutAdapt for the following operations: a quality-based trim (with a threshold Phred score of 30), removal of constant regions (with a one-base error tolerance in each region; sequences were discarded if either constant region was not found), and filtering for the correct length (45) in the remaining sequence. Sequences that could not be completely parsed into trimer codons (whose first nucleobase should always be C or T) were discarded. 
     For the initial PCSK9 selection and the IL-6 selection, the copy numbers of all unique sequences were tallied and the unique sequences above 5 reads per million were clustered using FASTAptamer. Sequence logos for PCSK9 selection-enriched sequences were then generated for individual clusters with WebLogo 3. For the PCSK9-A5 evolution, sequence logos were generated directly from sequencing data with WebLogo 3. 
     Affinity Characterization by Bead Retention Assay 
     Candidate PCSK9-binding HFNAPs (from 0.5 pmol template via a ligase-catalyzed polymerization reaction) or sequence-matched unfunctionalized DNA (0.5 pmol) in 50 μL of DPBS (with calcium and magnesium) containing 0.1 mg/ml BSA and 0.01% Tween-20 was incubated with 1 μL of either PCSK9 beads or thrombin beads (AminoLink Plus aldehyde-functionalized agarose resin with protein loaded at 1 mg protein per mL resin via reductive amination) in a micro-spin filtration column (Pierce 89879) at room temperature for 1 h on a rotor. Following the same procedure described for the selection, flow-through was collected, the beads were washed three times and the retained HFNAP or DNA was eluted by heating, and the amount of amplifiable HFNAP or DNA in the flow-through, wash, and elution samples were quantified by qPCR. 
     Candidate IL-6-binding HFNAPs and sequence-matched DNA were similarly assayed on PCSK9 beads (prepared as above, but serving as negative control) or on IL-6 beads (AminoLink Plus aldehyde-functionalized agarose resin with protein loaded at 0.25 mg protein per mL resin via reductive amination). 
     Detailed Procedures for Surface Plasmon Resonance (SPR) Assays 
     All SPR assays were performed at 25° C. on a Biacore X100 or Biacore T200 (GE Healthcare Life Sciences). Binding kinetics between enzymatically synthesized biotinylated HFNAPs and unlabeled PCSK9 (ACROBiosystems, PC9-H5223) were measured using single-cycle kinetics with the Biotin CAPture kit (GE Life Sciences, 28920233 or 28920234). HBS-EP buffer (GE Life Sciences, BR100188), diluted by MilliQ water to 0.9×, was used as the bulk running buffer. Each experiment consisted of three start-up cycles followed by multiple data collection and blank cycles. In each data collection cycle, the CAP reagent was injected onto both active and control flow cells of the CAP chip to generate streptavidin-coated surfaces, and then a doubly biotinylated HFNAP was injected onto the active flow cell as the immobilized ligand. Afterwards, four ascending concentrations of PCSK9 protein [10, 30, 100, 300 nM protein supplemented with 1 mg/ml salmon sperm DNA (Invitrogen, 15632-011) as nonspecific binding reducer for PCSK9-A5 and its variants; 2, 6, 20, 60 nM protein without salmon sperm DNA for PCSK9-Evo5 and its variants] in 0.9×HBS-EP were injected onto both flow cells in series at 30 μL/min for 150 seconds each, followed by 240 seconds of dissociation. Both flow cells were then regenerated with a 3:1 mixture of 8 M guanidinium chloride and 1 M NaOH following manufacturer&#39;s instructions. Blank cycles were run similarly except that 0.9×HBS-EP (containing 1 mg/ml salmon sperm DNA when PCSK9-A5 and its variants were assayed) without PCSK9 protein was injected. As signals from blank cycles were similar regardless of the immobilized HFNAP, one blank cycle was run for every two data collection cycles. Kinetic parameters were fitted to double-blank-subtracted sensograms using BIAEvaluation software under a 1:1 binding model, unless stated otherwise. Binding between biotinylated Evo5 and truncated PCSK9 protein missing the prodomain (“human mature PCSK9”, ACROBiosystems, PC9-H5226) was also assayed using this protocol. 
     Binding kinetics between enzymatically synthesized biotinylated HFNAPs and unlabeled IL-6 protein, expressed in either  E. coli  (PeproTech, 200-06) or human HEK293 cells (ACROBiosystems, IL6-H4218), were assayed similarly with the following differences. Four ascending concentrations (10, 30, 100, 300 nM) of IL-6 without additional nonspecific binding reducer were injected in the single-cycle kinetic runs. As the binding kinetics did not fit a classical 1:1 binding model, a heterogeneous ligand model was used to fit the double-blank-subtracted sensograms. 
     Binding kinetics between chemically synthesized Evo5-syn and biotinylated Avi-tagged PCSK9 (ACROBiosystems, PC9-H82E7) were measured using single-cycle kinetics with on a Series S SA chip (GE Life Sciences, BR100531) using 0.9×HBS as the bulk running buffer. Both active and control flow cells were conditioned with three consecutive one-minute injections of 1 M NaCl in 50 mM NaOH. Biotinylated PCSK9 in 0.9×HBS buffer was immobilized onto the active flow cell to ˜1000 RU. Either five portions of buffer or five ascending concentrations of Evo5-syn (1.8, 6, 18, 60, 180 nM) were injected onto both flow cells in series at 30 μL/min for 150 seconds each, followed by 600 seconds of dissociation. Kinetic parameters were fitted to double-blank-subtracted sensograms under a 1:1 binding model using BIAEvaluation software. 
     Binding of PCSK9 on surface-immobilized LDLR in the presence of various competing agents was measured on a Series S SA chip. The bulk running buffer was 10 mM HEPES, 150 mM NaCl, 0.1 mM CaCl 2 ), 0.005% Tween-20, pH 7.5. Both active and control flow cells were conditioned with three consecutive one-minute injections of 1 M NaCl in 50 mM NaOH, and then biotinylated Avi-tagged LDLR (BPS Bioscience, 71206) was immobilized onto the active flow cell to ˜2000 RU. In each data collection cycle, a solution consisting of PCSK9 (20 nM final), a carboxymethyl dextran-based non-specific binding reducer (GE Healthcare, BR-1006-91, 1 mg/ml final), and varying concentrations (0, 2, 6, 20, 60, or 200 nM final) of PCSK9-Evo5-syn, Evo5DNA-InvdT, unlabeled LDLR (AcroBiosystems, LDR-H5224), or a known PCSK9-neutralizing antibody (BPS Bioscience, 71207) in bulk running buffer was injected at 10 L/min for 420 s, followed by 15 s of dissociation. The surface was regenerated using two consecutive one-minute injections of 50 mM HCl. Blank cycles were run similarly except that running buffer containing 1 mg/ml non-specific binding reducer without protein was injected. Response levels at the end of the injection periods from double-blank-subtracted sensograms were recorded. 
     Electrophoretic Mobility Shift Assay (EMSA) 
     A 7.5% Tris-Glycine polyacrylamide gel (Bio-rad, 5671024) was pre-run at 150 V for 1 hour at 4° C. in a cold room. Mixtures (12 μl each) of PCSK9-Evo5-Fluor or a sequence-matched DNA (1 nM final), PCSK9 protein (ACROBiosystems, PC9-H5223, between 0.3 and 300 nM final), and salmon sperm DNA (Invitrogen, 15632-011, 30 μg/ml final) in 0.5×HBS-EP buffer (diluted from HBS-EP buffer, GE Life Sciences, BR100188) containing 3% v/v glycerol was incubated at 25° C. for 30 minutes, and then at 4° C. for 5 minutes. The mixtures were loaded onto the pre-run gel and run at 150 V for 15 minutes at 4° C. The gel was imaged with a Typhoon imager using the Cy5 channel. DNA secondary structure prediction was performed on the mfold Web server. 
     Supplementary Text 
     The ligase-catalyzed polymerization can produce “cheater” byproducts by incorporating a polymerization primer into the reading frame, resulting in shorter products that more rapidly amplify during PCR ( FIG.  7   ). While our first PCSK9-binding selection campaign was not substantially affected by cheaters, our initial attempt at evolving PCSK9-A5 for higher affinity was unsuccessful because the cheaters eventually took over the pool. To suppress cheaters, in our subsequent attempt at evolving PCSK9-A5 for higher affinity, we used a non-extendable 2′,3′-dideoxyribose-terminated 3′-primer in polymerization reactions, so that cheater sequences would be shorter than desired full-length translation products and could be removed by the PAGE purification step at the end of each selection cycle ( FIG.  2 A  and  FIG.  7   ). 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     
       
         
         
             
             
         
       
     
     
       
         
         
             
             
         
       
     
     SUPPLEMENTARY REFERENCES 
     
         
         1. Martin, M. Cutadapt removes adapter sequences from high-throughput sequencing reads.  EMBnet. journal  17, 10-12 (2011). 
         2. Alam, K. K., Chang, J. L. &amp; Burke, D. H. FASTAptamer: A Bioinformatic Toolkit for High-throughput Sequence Analysis of Combinatorial Selections.  Mol. Ther. —Nucleic Acids  4, e230 (2015). 
         3. Crooks, G. E., Hon, G., Chandonia, J.-M. &amp; Brenner, S. E. WebLogo: a sequence logo generator.  Genome Res.  14, 1188-1190 (2004). 
         4. Zuker, M. Mfold web server for nucleic acid folding and hybridization prediction.  Nucleic Acids Res.  31, 3406-3415 (2003).