Patent Description:
Electrochemistry has long played an important role in preparative chemical processes, with some early applications dating back to the <NUM>th century. <NUM>-<NUM> It offers many unique advantages, including exquisite control over the fundamental parameters driving reaction outcome through facile manipulation of electrode potential, mild reaction conditions, innate scalability, and the ability to generate reactive intermediates in situ. <NUM>-<NUM>.

The ability to chemoselectively modify proteins at predefined sites is a powerful technology to understand and engineer protein function. <NUM>-<NUM> In order to achieve this goal, uniquely reactive unnatural chemical groups are incorporated into biomolecules through synthetic or biosynthetic routes, followed by their selective labeling using a bioorthogonal chemical reaction. Many such bioorthogonal reactions have been developed to date, including the conjugation of alkoxyamine/hydrazine to an aldehyde/ketone, the Staudinger ligation, the Cu(I)-catalyzed and strain promoted cycloaddition between alkynes and azides, the inverse electron demand Diels-Alder reactions between tetrazines and strained alkenes, etc. Many such functionalities can now be inserted into proteins site-specifically through the co-translational incorporation of genetically encoded non-canonical amitio acids (ncAAs). Such ncAA incorporation relies on engineered nonsense-suppressing aminoacyl-tRNA synthetase (aaRS)/tRNA pairs that selectively charge these into proteins expressed in living cells, in response to a repurposed nonsense codon. The incorporated ncAA handle can be subsequently used to precisely attach a variety of entities, from biophysical probes to therapeutic agents through the bioorthogonal conjugation reaction.

However, many of the available conjugation chemistries are limited by their slow kinetics, the need for toxic catalysts, scalability, cost of reagents, or a lack of compatibility with other chemistries to allow the simultaneous attachment of multiple distinct entities at different sites. There is continuing demand for new protein labeling methods that are efficient, use readily accessible reagents, provide excellent chemoselectivity, and are compatible with other well-established labeling strategies for introducing multiple, different modifications onto proteins in a controlled manner. <NUM>-<NUM>.

Specifically, there is a continued need for additional genetically encoded chemical functionalities that can be rapidly and chemoselectively labeled using catalyst-free conjugation reactions, and that are also compatible with other available bioorthogonal chemistries to allow concurrent labeling at multiple sites.

The invention relates to methods of producing chemoselectively modified protein conjugates and electrochemically labeling of proteins as detailed in the appended claims.

Bioorthogonal chemical groups that enable selective labeling of complex biomolecules have become a cornerstone of modern chemical biology. Despite the exciting developments over the last two decades, the demand for new bioorthogonal reactions with unique abilities, and those compatible with existing chemistries for concurrent multisite-directed labeling, remains high. Electrochemistry has recently emerged as a powerful approach in small molecule synthesis owing to numerous attractive features, including precise control over fundamental reaction parameters, mild reaction conditions, and innate scalability. Even though these advantages also make it an attractive strategy for chemoselective modification of complex biomolecules such as proteins, such applications remain poorly developed.

Many of the unique advantages of electrochemistry, such as its tunability and mild reaction conditions, make it an attractive strategy for developing such chemoselective protein labeling methods. <NUM> Indeed, recent reports have used electrochemistry to activate small-molecule labeling reagents - such as a phenothiazine radical,<NUM> triazolinedioiones,<NUM> or luminols,<NUM>-<NUM> generated through anodic oxidation - which label tyrosine residues on target proteins. Similarly, in situ generated oxoammonium reagents have also been used to label tryptophan residues. <NUM>-<NUM> Electrochemical strategies have also been employed to drive nitration<NUM> or iodination<NUM> of tyrosine residues on proteins. Although these examples corroborate the enormous untapped potential of using electrochemical strategies to develop protein labeling methods, their scope is restricted by the following limitations: Targeting canonical amino residues such as tyrosine and tryptophan intrinsically compromises the ability to precisely define the site and stoichiometry of protein labeling. Furthermore, reactive electrophilic reagents generated through such strategies may show off-target reactivity,<NUM>-<NUM> particularly in the context of proteins, where the reactivity of nucleophilic amino acid sidechains can be significantly enhanced by their microenvironment.

Described herein is a novel electrochemical strategy to label any recombinantly expressed protein at predefined sites (<FIG>). The method is referred to herein as "eCLIC" and the basic mechanism is shown in <FIG>. Unlike previous approaches that rely on electrochemically activating exogenous pro-electrophilic reagents, which are limited by suboptimal control over the site and stoichiometry of protein labeling (vide supra), eCLIC uses a two-pronged approach to establish exquisite site-control. First, the noncanonical amino acid (ncAA), i. <NUM>-hydroxytryptophan (5HTP), which has a significantly lower oxidation potential relative to canonical aromatic amino acids, is introduced into a chosen site of the target protein through nonsense suppression using an engineered tryptophanyl-tRNA synthetase (EcTrpRS)/tRNA pair that we recently developed. <NUM> Next, the electron-rich 5HTP residue is selectively oxidized at the anode to generate an electrophilic intermediate, which can be efficiently trapped using various aromatic amines to generate stable conjugates. This approach to chemoselectively convert the 5HTP residue into a reactive electrophile moiety enables its coupling with a nucleophilic group, such as aromatic amines, anilines, indoles and thiazoles, which are inherently unreactive toward proteins, making eCLIC highly chemoselective. The conditions for the eCLIC reaction were developed as described herein, and used to site-specifically modify several different proteins, including a full-length antibody, with excellent efficiency and selectivity.

It is also demonstrated herein that the eCLIC reaction is mutually compatible with the strain-promoted azide-alkyne cycloaddition (SPAAC),<NUM> as well as inverse-electron demand Diels-Alder (IEDDA) reaction between strained alkenes and tetrazines,<NUM>, <NUM> two of the most popular existing bioorthogonal conjugation reactions. Furthermore, as described herein, a protein containing both a 5HTP and an azide-ncAA residue at specific sites was expressed, thus demonstrating that eCLIC and SPAAC can be used sequentially to selectively label each side chain with a distinct cargo to generate a homogeneous dual-labeled conjugate. The eCLIC reaction is compatible with a broad range of nucleophilic groups, and in particular such as aromatic amine substrates, and these reagents, as well as 5HTP, are readily accessible and inexpensive. These advantages establish eCLIC as an attractive method for site-specific protein bioconjugation.

Specifically, the present invention involves the ability to: <NUM>) chemoselectively functionalize a ncAA, i.e. 5HTP, a stable and inexpensive non-naturally occurring amino acid that can be site-specifically incorporated into proteins, using an electrochemical oxidative coupling reaction to many different coupling partners comprising a nucleophilic group (including aromatic amines) under physiological conditions, <NUM>) systematically tune the coupling chemistry by rational substitution on the coupling partners comprising a nucleophilic group (such as aromatic amines), and <NUM>) facilitate the reaction using additives such as TEMPO and HEPES that either facilitate electron transfer between protein and electrode or suppress unwanted side-reactions. The inexpensive nature of both 5HTP and its coupling partner (e.g., any nucleophilic molecule such as an aromatic amine), the intrinsically facile and scalable nature of electrochemical processes, and the compatibility of this reaction with established bioconjugation reactions, such as azide-alkyne or tetrazine-strained-alkene reactions, will make this bioconjugation strategy a useful new tool for industrial protein modification needs.

The technology of the present invention enables covalent attachment of any entity/molecule/moiety (drug, fluorophore, other biophysical probes, polyethylene glycol, other proteins, nucleic acids, a solid matrix like a solid surface), carrying a suitable reaction partner such as an aromatic amine, onto a specific site of any recombinant protein. The target protein can be expressed in any expression system (E. coli/eukaryotic cell) site-specifically incorporating the stable and inexpensive amino acid 5HTP. Reagents with suitable reaction partners, such as aromatic amine-containing molecules, can be coupled to this protein using anodic oxidation. Proteins encompassed by the present disclosure include, for example, a polypeptide, a peptide, an antibody, antibody fragment, viral protein, chemokine, cytokine, antigen, enzyme or growth factor.

Such conjugation reactions can be used to generate <NUM>) antibody-drug conjugates, <NUM>) antibody-fluorophore conjugates, <NUM>) antibody-enzyme conjugates, <NUM>) therapeutic protein-PEG conjugate, <NUM>) protein-nucleic acid conjugates, <NUM>) immobilize proteins on surfaces site-specifically, <NUM>) generate kits for site-specific modification of proteins, etc. Synthesis of other biomolecules (e.g., DNA, RNA) with the <NUM>-hydroxyindole will also allow their precise labeling through the electrochemical coupling reaction.

Thus, the present disclosure describes compositions of chemoselective, homogeneous protein-cargo conjugates, and specifically encompasses bioconjugates with biorthogonal labeling of <NUM>-HTP residues genetically-engineered into the protein. Encompassed herein are bioconjugates comprising a biomolecule comprising one, or more unnatural amino acid residues incorporated in a site-specific manner into the biomolecule, wherein the one, or more, unnatural amino acid residues further comprise a biochemical or chemical cargo entity conjugated to the unnatural amino acid via a covalent electrochemical linkage. The bioconjugates comprise any suitable biomolecule, but specifically encompasses biomolecules selected from the group consisting of proteins, polypeptides, peptides or nucleic acids, including DNA or RNA nucleic acids.

Any suitable non-canonical amino acid (ncAA, also referred to herein as an unnatural amino acid residue) can be targeted for conjugation. In the present invention the unnatural amino acid is <NUM>-hydroxytryptophan as incorporated into one, or more site-specific positions in the biomolecule.

The bioconjugates of the present disclosure are bioorthogonally-labeled with a suitable cargo entity. More specifically encompassed by the present disclosure are cargo entities selected from the group consisting of one, or more of the following: small molecule, therapeutic drug, detectable label, detectable probe, polymer such as PEG, peptide, or nucleic acid. In another aspect, the cargo entity comprises means (e.g., suitable chemical reaction partners or chemical linkages) to facilitate the immobilization of the bioconjugate to a suitable solid support such as a surface or bead, such as a polymer support or a glass bead.

Further encompassed by the present disclosure are methods of producing a bioconjugate comprising a biomolecule comprising one, or more unnatural amino acid residues incorporated in a site-specific manner into the biomolecule, wherein the one, or more, unnatural amino acid residues further comprise a biochemical or chemical cargo entity conjugated to the unnatural amino acid via a covalent electrochemical linkage.

Specifically encompassed in the present invention are methods of producing a chemoselectively modified protein bioconjugate, comprising the steps of providing a recombinantly-expressed protein comprising one, or more non-canonical amino acid (ncAA) residues at specific sites of the protein, wherein one, or more. of the ncAA residues is a <NUM>-hydroxytryptophan (<NUM>-HTPP); selectively oxidizing the one, or more, <NUM>-HTP amino acid residues of the protein under suitable conditions and at low voltage, wherein the voltage is sufficient to generate an electrophilic <NUM>-HTP intermediate; and reacting the <NUM>-HTP intermediate with a coupling partner comprising a chemical moiety comprising a nucleophilic group such as shown in <FIG> or <FIG>, under conditions suitable for the coupling of the nucleophilic group to the one, or more, <NUM>-HTP residues in the protein, thereby producing a chemoselectively modified protein bioconjugate. As described herein typically the ncAA residues to be modified are exposed on the surface of the protein. In one embodiment of the present invention the protein is an antibody or an antibody fragment such as an Fab fragment.

The method uses an electrochemical reaction that results in a covalent linkage/bond between the targeted unnatural amino acid reside of the biomolecule and the cargo entity. More specifically, the electrochemical reaction is an electrocatalysis reaction. The electrocatalysis reaction comprises one, or more reaction partners (i.e., a nucleophilic group or a moiety comprising a nucleophilic group) selected from the group consisting of: aromatic amines, anilines, indoles, and thiazoles that link the cargo entity to the unnatural amino acid of the biomolecule.

The electron-rich <NUM>-HTP has a significantly lower oxidation potential that canonical/naturally-occurring amino acids wherein oxidation voltage is between about <NUM> volts and about <NUM> volts. In particular, the oxidation voltage is between about <NUM> volts and about <NUM> volts; and in one embodiment the oxidation voltage is between about <NUM> and <NUM> volts. Importantly, under suitable oxidation conditions (e.g., mild oxidation conditions) as described herein, only the <NUM>-HTP is oxidized, and in turn, only the <NUM>-HTP residues of the protein react with aromatic amines to conjugate the cargo of interest-none of the other amino acid residues are reactive.

In some embodiments of the present invention, the oxidation step comprises the addition of an electron transfer mediator, such as <NUM>,<NUM>,<NUM>,<NUM>-tetramethylpiperidine-<NUM>-oxyl (TEMPO). Additionally, as described herein, a suitable buffer for the oxidation reaction is HEPES.

The methods described herein couple an aromatic amine group to the <NUM>-HTP residue. The aromatic amines can be any of the following: anilines, indoles, and thiazoles, and particularly suitable aromatic amines are shown in shown in <FIG> and <FIG>.

Also as described herein, the aromatic amine group can further comprise a biochemical of chemical cargo entity. In particular, the cargo entity is selected from the group consisting of one, or more of the following: small molecule, therapeutic drug, detectable label, detectable probe, polymer such as PEG, peptide, or nucleic acid. In one embodiment, the cargo entity comprises means to immobilize the bioconjugate to a solid support.

In some embodiments of the present invention, the <NUM>-HTP further comprises a bulky protecting group, such as a tert-Butoxycarbonyl (Boc) protecting group.

In one aspect, the one, or more, non-canonical amino acid residues is an azide ncAA such as a cyclopropane-lysine amino acid (CpK) or an azido-lysine amino acid (CpK) or an azido-lysine amino acid (AzK) and the method further comprises coupling the azide ncAA residue with a coupling partner under conditions suitable for the coupling of the coupling partner to the azide ncAA, thereby producing a chemoselectively modified protein bioconjugate modified at two specific sites of the protein. In some embodiments the coupling partner further comprises a biochemical or chemical cargo entity as described herein.

The electrochemical conjugation chemistry of the present inventions is compatible with existing bioconjugation strategies, such as azide-alkyne cycloaddition and tetrazine-strained alkene reactions and can be used together to simultaneously attach multiple different entities onto distinct sites of a target protein. Technology to site-specifically incorporate 5HTP as well as another non-canonical amino acid (ncAA, also referred to herein as an unnatural amino acid, or non-naturally occurring amino acid) incorporating azide/cyclopropene into any two sites of a target protein has already been developed and shown that the two sites can be concurrently labeled with two distinct entities (e.g., two different fluorophores or therapeutic agents).

The 5HTP-incorporating aminoacyl-tRNA synthetase/tRNA can also be used to substitute all tryptophan residues with 5HTP (partially) within the proteome of any cell. Subsequent chemoselective labeling of these 5HTP-residues using suitable reagents, such as aromatic amines, will enable their facile identification (e.g., through fluorophore labeling)/isolation (e.g., through biotin labeling). This would provide a unique way to identify newly synthesized proteins in a cell.

Also, all conjunctions used are to be understood in the inost inclusive sense possible. Thus, the word "or" should be understood as having the definition of a logical "or" rather than that of a logical "exclusive or" unless the context clearly necessitates otherwise.

It will be understood that although terms such as "first" and "second" are used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. Thus, an element discussed below could be termed a second element, and similarly, a second element may be termed a first element without departing from the teachings of the present invention.

As described herein, the present invention establishes a general strategy to label recombinant proteins using 5HTP-directed electrochemical conjugation reaction. The ability to <NUM>) site-specifically incorporate 5HTP, a stable and accessible ncAA, into any protein of interest expressed in both E. coli and eukaryotic cells, <NUM>) chemoselectively functionalize this non-natural residue with readily available coupling partners such as aromatic amines, <NUM>) systematically tune the coupling chemistry by rational substitution on the aromatic amine and other coupling partners, and <NUM>) the innate scalability and other advantages of electrochemical reaction will make this bioconjugation strategy a useful new tool in chemical biology.

The genetic code expansion (GCE) technology enables site-specific incorporation of noncanonical amino acids (ticAAs) using engineered aminoacyl-tRNA synthetase/tRNA pairs. <NUM>-<NUM> This technology provides an attractive way to precisely introduce a unique chemical functionality into predefined sites of proteins, which can be subsequently activated using electrochemistry with high chemoselectivity for conjugation with suitable bioorthogonal reaction partners. coli tryptophanyl-tRNA synthetase (EcTrpRS)/tRNAEcTrp pair was engineered for the site-specific incorporation of <NUM>-hydroxytryptophan (5HTP) into proteins expressed in both E. coli and eukaryotic cells with excellent fidelity and efficiency. <NUM>, <NUM>-<NUM> The electron-rich <NUM>-hydroxyindole functionality is otherwise stable, but under mild oxidative conditions, it is known to readily undergo coupling with itself or other electron-rich aromatic compounds. <NUM>-<NUM> The ability to selectively trigger an electrochemical oxidation of the 5HTP residue on a protein provides a plausible path for chemoselective protein labeling. The behavior of 5HTP was analyzed using cyclic voltammetry (CV) relative to other redox-active aromatic amino acids such as tyrosine and tryptophan (<FIG>). 5HTP was found to oxidize at a significantly lower voltage than canonical aromatic amino acids, revealing room for its selective oxidative modification.

Using the engineered EcTrpRS/tRNAEcTrp pair, a 5HTP residue was incorporated at the surface-exposed <NUM> site of superfolder GFP (sfGFP), and the resulting protein was used to explore if the unique <NUM>-hydroxyindole functionality could be chemoselectively modified using electrochemistry. An otherwise identical wild-type sfGFP protein, with a tyrosine residue at the <NUM> site instead of 5HTP, was used as a control. A variety of reaction conditions and potential coupling partners were screened using an IKA ElectraSyn <NUM> Microvial instrument to find that sfGFP-<NUM>-5HTP forms an adduct with N,N-dimethylaniline la, as observed by whole-protein mass-spectrometry (<FIG>). Using graphite electrodes at a controlled potential (<NUM> V), stirring <NUM> sfGFP-<NUM>-5HTP and <NUM> N,N-dimethylaniline 1a in <NUM> phosphate buffer resulted in complete protein modification within <NUM> minutes (<FIG>). Treating the wild-type sfGFP protein - which harbors all twenty canonical amino acids - under the same condition resulted in no detectable protein labeling, which confirms the chemoselectivity of the eCLIC reaction.

Next, the eCLIC reaction conditions were optimized in an attempt to reduce the concentration of the aniline substrate needed for efficient conjugation. These optimizations were performed using a synthetic dialkylaniline 1b (<FIG>; see also <FIG>), which has a carboxylic acid group that improves solubility in aqueous buffers and serves as an attachment handle for other cargo. Near-complete labeling of sfGFP-<NUM>-SHTP using only <NUM> of 1b was achieved, but an elevated electrode potential of 1V was necessary. Further reducing the concentration of 1b under these conditions resulted in incomplete conversions, as well as protein overoxidation (<FIG>; <FIG>). Even at this elevated potential, the chemoselectivity of the eCLIC reaction was not compromised, as evidenced from the absence of protein modification in corresponding control reactions with wild-type sfGFP. Further optimization identified HEPES (N-<NUM>-hydroxyethylpiperazine-N'-<NUM>-ethanesulfonicacid)<NUM> buffer to be beneficial for eCLIC; the use of <NUM> HEPES (pH <NUM>) instead of phosphate buffer facilitated near-complete protein labeling using only <NUM> 1b, and also suppressed protein overoxidation, likely due to its known ability to scavenge reactive oxygen species. <NUM> Finally, the inclusion of TEMPO,<NUM> a known electron-transfer mediator, further improved the efficiency of the reaction, enabling near-complete labeling of sfGFP-<NUM>-5HTP using only <NUM> dialkylaniline 1b, at <NUM> V.

Using these reaction conditions, sfGFP-<NUM>-5HTP was functionalized with several other aromatic amines with diverse structures (<FIG>; see also <FIG>). Wild-type sfGFP was not labeled by these reagents under the same eCLIC condition (<FIG>). Anilines functionalized with additional cargo were also synthesized, such as a terminal alkyne, a biotin, and a fluorophore, attached through a short PEG linker, and demonstrated their successful conjugation to sfGFP-<NUM>-TAG using eCLIC (<FIG>; <FIG>). Most of the structurally diverse anilines tested demonstrated efficient protein labeling without further optimization of reaction conditions tailored to individual substrates. Not surprisingly, a labeling reaction with phenothiazine, which is known to facilitate the labeling of tyrosine residues, led to multiple labeling of both the wild-type and 5HTP-mutant of sfGFP (<FIG>; <FIG>). In addition to sfGFP, <NUM>-HTP was also site-specifically incorporated into myoglobin (at site <NUM>) and the 5F7 nanobody that binds the HER2 receptor (at site <NUM>). <NUM> In each case, the target protein was efficiently modified using eCLIC (<FIG>), while the corresponding wild-type proteins lacking the 5HTP residue showed no conversion under identical reaction conditions (<FIG>). These observations underscore the generality of eCLIC for site-specific protein bioconjugation. Its compatibility with a wide range of aromatic amine coupling partners - which are highly accessible and inexpensive - significantly enhances the flexibility and utility of this bioconjugation strategy.

The whole-protein mass spectrometry analyses of eCLIC labeling reactions show product masses that are consistent with the attachment of the aromatic amine to the protein substrate and a concomitant loss of <NUM> Da, which is indicative of an oxidative coupling reaction. The reaction using free 5HTP and N,N-dimethylaniline 1a was modeled. However, this was complicated by the high propensity of <NUM>-hydroxyindoles to form complex oligomeric products under mild oxidative condition through homocoupling. <NUM>-<NUM>, <NUM> Subjecting a mixture of 5HTP and 1a to eCLIC conditions led to complex product mixtures, even when the latter was used in large excess. This is in stark contrast to the clean eCLIC labeling observed with 5HTP-containing proteins, which likely stems from the site-isolated nature of the protein-bound 5HTP residue that thwarts such homocoupling reactions, and promotes heterocoupling with exogenous aromatic amine coupling partners. Interestingly, the presence of a bulky protection group, such as tert-butyloxycarbonyl (Boc), on the amine functionality of 5HTP suppresses the homocoupling reaction, perhaps mimicking the behavior of a protein-associated 5HTP residue. Using the Boc-protected 5HTP, significantly promoted the heterocoupling reaction with 1a under eCLIC conditions and isolate the resulting coupling product using HPLC (<FIG>). A thorough structural characterization using NMR, X-ray crystallography, and mass-spectrometry revealed a C-C coupled product where 5HTP and the N. N-dimethylaniline 1a are connected through the <NUM> positions of both aromatic rings (<FIG>). The coupling product formed between Boc-protected 5HTP (<NUM>) and a primary aniline substrate <NUM> was structurally analyzed. Intriguingly, the eCLIC product in this case resulted in a different connectivity, where the aromatic amine is attached to the <NUM> position of 5HTP, as confirmed by NMR analyses (<FIG>; <FIG>).

CV of both N,N-dimethylaniline 1a and the primary aniline substrate <NUM> show a significantly higher potential needed for their oxidation relative to 5HTP (<FIG>). Consequently, it is reasonable to propose a mechanism where the reaction is initiated through anodic oxidation of 5HTP side chain to create a radical cation. The aromatic amines react with this electrophilic radical either through a radical coupling or a nucleophilic attachment mechanism (FIG. A further single-electron oxidation of this intermediate is necessary to rearomatize the coupled product and complete the reaction. It is reasonably believed that this terminal electron transfer is facilitated by dissolved oxygen or oxidized TEMPO. Indeed, no protein labeling was observed when eCLIC was performed under oxygen-depleted condition and in the absence of TEMPO (<FIG>). Including TEMPO under oxygen depleted condition partially restored protein labeling. These observations suggest that, unlike established electrochemical protein modification strategies that activate a pro-electrophilic small-molecule labeling reagent, eCLIC proceeds through direct chemoselective activation of the uniquely reactive 5HTP residue on the protein.

Due to its distinctive reaction mechanism, the eCLIC labeling strategy is mutually compatible with the two other widely used bioorthogonal reactions: the strain-promoted azide-alkyne cycloaddition (SPAAC),<NUM>. and the inverse-electron demand Diels-Alder (IEDDA) reaction between tetrazines and strained alkenes. <NUM>,<NUM> To test this hypothesis, the sfGFP protein was expressed separately incorporating three different ncAAs at the surface-exposed <NUM> site: 5HTP, AzK, and CpK (<FIG>). The pyrrolysyl pair was used to incorporate AzK and CpK in response to a UAG codon, as described previously. <NUM> In each case, incorporation of the desired ncAA was confirmed by the MS analysis, and each sfGFP mutant was separately subjected to three different labeling reactions (<FIG>): SPAAC (with BCN), IEDDA (with tetrazine), and eCLIC (with 1b). Modification of each ncAA residue was observed only upon treatment with its cognate reaction partner, confirming the mutual orthogonality of the three labeling reactions (<FIG>).

Next, it was demonstrated that eCLIC and SPAAC can be used together to selectively label a protein at two different sites. To this end, we generated an sfGFP protein incorporating 5HTP and AzK at sites <NUM> and <NUM>, respectively (<FIG>). <NUM> This was achieved by co-expressing the sfGFP-<NUM>-TGA-<NUM>-TAG reporter in ATMW-BL21 E. coli<NUM> with a TGA-suppressing EcTrpRS/tRNA pair that charges 5HTP, and a TAG-suppressing pyrrolysyl pair that charges AzK. The resulting protein was purified by immobilized metal-ion chromatography using a C-terminal polyhistidine tag, and was characterized by mass-spectrometry (<FIG>). This sfGFP double mutant was first labeled at the 5HTP residue through eCLIC using 1b, followed by SPAAC modification of the AzK side chain with a cyclooctyne. MS analysis of these reactions confirmed efficient sequential labeling at intended sites to yield the dual-labeled product (<FIG>). These results confirm that eCLIC is compatible with other established bioconjugation reactions, and can be used together to facilitate precise multi-site protein labeling.

In recent decades, monoclonal antibodies have been exploited for countless applications, from enabling reagents for biological discovery to powerful new biologics that offer novel therapeutic modalities. Such applications frequently rely on the ability to functionalize the antibody with additional entities. For example, antibodies labeled with fluorescent probes are widely used for numerous assays both for biological research and disease diagnosis. Additionally, antibody-drug conjugates,<NUM>-<NUM> where a toxic drug molecule is covalently attached to a monoclonal antibody, provide a powerful strategy to deliver such payloads into target cells with high precision, reducing both the effective therapeutic dose, as well as undesirable side-effects. Traditionally, antibody modification has relied on functionalizing canonical nucleophilic side chains such as lysine or cysteine, but such strategies are limited by poor control over the site and stoichiometry of antibody modification. Using the GCE technology, it has been possible to incorporate ncAAs with bioconjugation handles into full-length antibodies, which allows subsequent attachment of cargo with full site control. <NUM>-<NUM> Such site-specific labeling strategy provides significant advantage for developing both antibody-based reagents and therapeutics. <NUM>, <NUM>, <NUM>-<NUM>.

To explore the feasibility of using eCLIC for modifying full-length antibodies, the anti-HER2 antibody Trastuzumab was recombinantly expressed site-specifically incorporating 5HTP at the <NUM> position of the heavy chain. Expi293 suspension cells were transiently transfected with two plasmids, one encoding the mutant heavy chain (HC-<NUM>-TGA) and the light chain of Trastuzumab, and the other a TGA-suppressing EcTrpRS/tRNA pair selective for 5HTP (<FIG>), which yielded Trastuzumab-HC-<NUM>-5HTP with good yield (<NUM>/L) after purification. In comparison, the same expression/purification conditions yielded <NUM>/L wild-type Trastuzumab. we were able Trastuzumab-HC-<NUM>-5HTPwas homogenously labeled with aniline probes <NUM> or 1b using eCLIC selectively on the heavy chain (<FIG>. Next, a primary aniline warhead coupled to a fluorescein (<NUM>; <FIG>) and conjugated it to Trastuzumab-HC-<NUM>-5HTP using the optimized eCLIC condition. Fluorescence imaging following SDS-PAGE (<FIG>) and mass-spectrometry analyses (<FIG>) confirmed selective and efficient labeling of the heavy chain with the fluorophore, whereas the light chain and the corresponding wild-type antibody was not modified. When this antibody-fluorophore conjugate was incubated with the SK-BR-<NUM> cell line that overexpresses the HER2 receptor, robust cell-binding was observed using fluorescence-activated cell sorting (FACS) analyses, while the same conjugate failed to stain HEK293T cells that do not overexpress this receptor (<FIG>). These observations confirm that eCLIC can be used to efficiently generate functional conjugates of full-length antibodies for various applications.

It is now demonstrated herein that electrochemistry can be used to efficiently modify proteins at predefined sites using the unique reactivity of the 5HTP residue that can be introduced into proteins through the GCE technology. Previously reported electrochemical protein bioconjugation strategies rely on activating exogenous electrophilic reagents that functionalize a canonical amino acid residue, which intrinsically lack precise control over the site and stoichiometry of protein labeling. In contrast, eCLIC proceeds through chernoselective anodic oxidation of the protein-associated 5HTP residue, followed by its efficient capture by a wide array of aromatic amines. Notably, the reagents used in eCLIC are inexpensive and readily accessible. For example, 5HTP (<$<NUM>/g) is significantly less expensive relative to other commonly used ncAAs for bioconjugation ($<NUM>-$<NUM>/g), whereas simple anilines such as <NUM>-aminophenylacetic acid <NUM> (<$<NUM>/g) are orders of magnitude cheaper than labeling agents such as cyclooctynes, and trans-cyclooctenes (>$<NUM>/g). Moreover, it has recently been shown that 5HTP can also be biosynthetically generated in cells,<NUM> obviating its exogenous supply altogether. A variety of aromatic amines can be used as coupling partners in eCLIC, which enhances the utility of this bioconjugation strategy. Even though eCLIChas demonstrated broad substrate scope with several different aromatic amines (<FIG>), it is likely that the full scope of potential coupling partners is much larger. It is reasonably expected that other electron rich aromatic amino acids (such as amino-tryptophan, amino-tyrosine, hydroxy-tyrosine, histidine-imidazole with electron donating substitutions) can also react selectively in a similar manner under electrochemical conditions. The GCE technology has been used to express ncAA-containing relevant proteins, such as full-length antibodies, in industrial scale (grams/L),<NUM>, <NUM> providing an established path to express conjugation-ready proteins for eCLIC labeling in large scale. Finally, the compatibility of eCLIC with established bioconjugation strategies such as SPAAC and IEDDA will enable its use for site-specific labeling of proteins with distinct entities with precise site control. Owing to these advantages, eCLIC will find numerous applications for generating useful conjugates of recombinant proteins.

The ability to precisely chemically label complex biomolecules (protein, DNA, RNA, etc.) is highly useful to understand and engineer their properties. To achieve this, the present invention has developed a new highly efficient "biorthogonal" chemical reaction between an ncAA and different coupling partners, such as aromatic amines, both of which are non-natural chemical groups not found in biomacromolecules.

Experiments were conducted in a home-made glass three electrode cell containing a carbon rod counter electrode (<NUM>% trace metals; Sigma Aldrich), a Ag/AgCl reference electrode (<NUM> NaCl; Basi Inc. ; West Lafayette, IN), and a glassy carbon working electrode (<NUM> OD; Pine Research; Durham, NC). Prior to each experiment, the glassy carbon electrode was polished for <NUM> minutes with a <NUM> polycrystalline diamond slurry (Ted Pella; Redding, CA). The electrode potential was controlled with a Versastat <NUM> potentiostat (AMETEK; Berwyn, PA).

The CV experiments were conducted with <NUM> of electrolyte solution containing a pH <NUM> PBS supporting electrolyte (<NUM> phosphate; <NUM> sodium chloride) and <NUM> of the analyte. The nominal turning potentials for the CV were -<NUM> V and +<NUM> V vs SHE with a scan rate of <NUM> mV/s.

The CV experiments with TEMPO and N-Boc-Orvle-5HTP were carried out in <NUM> of electrolyte solution containing a <NUM>:<NUM> mixture of pH <NUM> PBS supporting electrolyte (<NUM> phosphate; <NUM> sodium chloride) and acetonitrile. Mixed into the electrolyte was <NUM> of the analyte of interest. The nominal turning potentials for the CVs were +<NUM> V and +<NUM> V vs SHE with a scan rate of <NUM> mV/s.

To a <NUM> ElectraSyn reaction vial was added <NUM>µL of <NUM> HEPES, pH <NUM>, <NUM>µL of <NUM> TEMPO, <NUM>µL of <NUM> sfGFP-<NUM>-5HTP (<NUM> final concentration), a sufficient volume of substrate in DMSO for a <NUM> - <NUM> final concentration, and molecular biology grade water to a final volume of <NUM>µL. Reactions were run under "Constant Voltage" settings at <NUM> V for <NUM> at <NUM> substrate (time and concentration may be optimized), using SK-<NUM> microelectrodes as both the anode and cathode with no reference electrode. Due to the low current generated in the reaction, Vial Detection setting was turned off. The reaction mixture was buffer-exchanged using a Bio-Spin® P-<NUM> column (Bio-Rad <NUM>) pre-equilibrated with PBS to remove excess small-molecule substrate and HEPES buffer.

Oxygen-free deionized water was prepared by sparging with nitrogen for <NUM> hour. The sparged water, solid HEPES powder, concentrated <NUM>-5HTP-sfGFP stock (<NUM>), <NUM> TEMPO in DMSO, and <NUM> dialkylaniline carboxylate (1b) were transferred to a N<NUM> glovebox after <NUM> purge/refill cycles and left for <NUM> hours. The water was used to prepare fresh <NUM> HEPES buffer, pH <NUM>. All components were mixed as in standard reaction conditions (<NUM> <NUM>-5HTP-sfGFP, <NUM> HEPES, <NUM> 1b) in the presence or absence of <NUM> TEMPO and sealed in an Electrasyn <NUM> microvial. Reactions were carried out at <NUM> mV for <NUM> minutes and processed as described above for mass spectrometry analysis outside of the glove box.

stGFP-<NUM>-5HTP, sfGFP-<NUM>-AzK, and sfGFP-<NUM>-CpK were each diluted to <NUM> in <NUM> HEPES, pH <NUM> for all reactions. eCLIC was conducted on each protein with <NUM> dialkylaniline carboxylate 1b and <NUM> TEMPO for <NUM> hour. Tetrazine cycloadditions were carried out with a final concentration of <NUM> tetrazineaminc,<NUM>-<NUM> and allowed to react for <NUM>. Strain promoted azide-alkyne cycloadditions were carried out with a final concentration of <NUM> BCN-OH (Sigma-Aldrich), and allowed to react for <NUM>. All labeling reactions were buffer exchanged into PBS by PD-<NUM> spin columns and analyzed by ESI-MS.

A stock of sfGFP-<NUM>-5HTP-<NUM>-AzK was diluted to a <NUM> in <NUM> HEPES, pH <NUM>, and reacted with <NUM> 1b for <NUM> minutes at <NUM> V. The sample was buffer exchanged into PBS using a PD-<NUM> spin column, then reacted with BCN-OH was added to a final concentration of <NUM> and was allowed to incubate for <NUM> hour at room temperature. The protein was analyzed after both steps by ESI-MS.

Expression and protein purification of sfGFP, myoglobin, and 5F7 nanobody were performed as previously described, in the E. coli ATMW-BL21 strain. <NUM> All proteins were expressed from a pET22b plasmid, driven by a IPTG-inducible T5-lac promoter. In brief, ATMW-BL21 cells were co-transformed with the appropriate pET22b plasmid and a pEVOL-tacI EcTrpRS-h13 leuV<NUM> suppressor plasmid. For wild-type protein expression, the cells were transformed with only the appropriate pET22b plasmid. A single transformant was used to start a <NUM> overnight culture containing the appropriate antibiotics (<NUM>µg/mL ampicillin, <NUM>µg/mL chloramphenicol, <NUM>µg/mL spectinomycin, <NUM>µg/mL zeocin, and <NUM>µg/mL gentamycin), <NUM> of which was used to inoculate a <NUM> LB culture (in a <NUM> sterile Erlenmeyer flask) supplemented with antibiotics. Cultures were grown to an OD<NUM> of <NUM>-<NUM>, followed by induction with a final concentration of <NUM> IPTG and <NUM> 5HTP. The cultures were expressed at <NUM> with shaking at <NUM> rpm for <NUM>. The cultures were spun down at <NUM>,<NUM> x g for <NUM> minutes at <NUM>, and the supernatant was removed. Cell pellets were resuspended in a ratio of <NUM> lysis buffer to <NUM> expression culture volume. Lysis buffer: B-PER Bacterial Protein Extraction Reagent (Thermo Scientific), 1X Halt Protease Inhibitor Cocktail (Thermo Scientific), <NUM>% Pierce Universal Nuclease (Thermo Scientific). Cells were nutated for <NUM> minutes at room temperature, then clarified by centrifugation at <NUM>,<NUM> x g for <NUM> minutes at <NUM>. The sfGFP reporter proteins contain a C-terminal polyhistidine tag and were purified using a HisPur Ni-NTA resin (Thermo Scientific) following the manufacturer's protocol. Protein purity was confirmed by SDS-PAGE analysis and protein molecular mass was characterized using HPLC-coupled ESI-MS of intact proteins (Agilent Technologies, <NUM> Infinity ESI-TOF).

To express the sfGFP-3TGA-151TAG reporter, ATMW-BL21 cells were co-transformed with a pGTEV-3TGA-151TAG reporter plasmid<NUM> and pEvoltac EcW. h14-MbPylRS TAG suppressor plasmid encoding both EcTrp and the pyrrolysyl pairs. <NUM> The sfGFP reporter was expressed and analyzed as described above, except supplemented with <NUM> 5HTP and <NUM> AzK upon <NUM> IPTG induction, and a larger culture volume of <NUM> (grown and expressed in a <NUM> sterile Erlenmeyer flask) was used.

Expression of sfGFP-<NUM>-CpK and sfGFP-<NUM>-AzK were performed as previously described.

Trastuzumab expression in Expi293 suspension cells and mass spectrometry analysis:
Expi293 cells were grown and cultivated under standard conditions as recommended by the supplier (ThermoFisher). Cells were grown in Expi293 medium supplemented with <NUM>. 5x antibiotic-antimyotic (Thermo Fisher, <NUM>) at <NUM>, <NUM>% CO<NUM>, in sterile plastic Erlenmeyer flasks with ventilated caps, and shaken using a CO<NUM>-resistant orbital shaker at <NUM> rpm. Cultures were maintained by seeding at a <NUM> million viable cells/mL, then expanding cultures every <NUM>th day when they reached a density of <NUM>-<NUM> million viable cells/mL. Cell density and viability were measured using the trypan blue exclusion method with a Bio-Rad TC20™ automated cell counter.

An adaptation of a previously reported PEI-based transfection method was used for antibody expression. <NUM> After reaching <NUM>-<NUM> million viable cells/mL at a minimum of <NUM>% viability, cells were centrifuged at <NUM> xg for <NUM> and resuspended in fresh media to <NUM> million viable cells/mL. To the concentrated cultures was added a <NUM>:<NUM> ratio of pCDNA3. <NUM>-HC-<NUM>-TGA-Trastuzumab and pAcBac-EcWRS-H14-1x-EcWtR(TGA) at a final DNA concentration of <NUM>µg/mL. A concentrated stock solution of <NUM> kDa linear PEI (<NUM>/mL) was added to a final concentration of <NUM>µg/mL. Cells were incubated with shaking under standard expansion conditions for <NUM> hours, then diluted to <NUM>-<NUM> million viable cells/mL, and sterile (<NUM> filtered) <NUM> stocks of 5HTP and valproic acid were added to <NUM> and <NUM> final concentrations, respectively.

After <NUM>-<NUM> days of expression, Expi293 supernatant was harvested by pelleting cells at <NUM> xg for <NUM> minutes, then passing the decanted media through a sterile filter (<NUM>). The media was adjusted to pH <NUM>, <NUM> NaOAc using a <NUM> stock. After equilibrating <NUM> Pierce™ Protein G resin (Thermo Scientific, <NUM>) with <NUM> column volumes of wash buffer (pH <NUM>, <NUM> NaOAc), the filtered media was passed through by gravity flow (<NUM>-<NUM>). Protein G resin was washed with a volume of <NUM> NaOAc, pH <NUM> equal to approximately half the volume of added media (<NUM>-<NUM>). Purified antibody was eluted by <NUM>-<NUM> stepwise additions of <NUM> pH <NUM>, <NUM> glycine, collecting each elution in tubes containing <NUM>µL pH <NUM><NUM> sodium phosphate for immediate neutralization. Eltited fractions were qualitatively tested for the presence of protein by adding <NUM>µL elution fraction to <NUM>µL Bradford reagent (Thermo Scientific <NUM>). Fractions containing protein were pooled and was concentrated to ~ <NUM>-<NUM>/mL final protein concentration using Amicon® Ultra <NUM> (Millipore Sigma UFC901024) centrifugal filters. Concentrated antibody was buffer-exchanged using a PD midiTrap G-<NUM> column (GE Healthcare <NUM>) pre-equilibrated with <NUM> column volumes of PBS. Protein concentration was determined by absorption measurement using a NanoDrop <NUM> spectrophotometer (Fisher Scientific ND2000).

Protein purity and protein conjugation state was determined by HPLC-coupled ESI-MS of fully reduced antibodies (Agilent Technologies, <NUM> Infinity ESI-TOF). First, <NUM>µg antibody sample was adjusted to <NUM> phosphate pH <NUM>, <NUM> fresh TCEP, then heated to <NUM> for <NUM> minutes. After cooling to <NUM>, antibody was incubated <NUM>µL Remove-iT® PNGase (New England Biolabs, P0706S) for <NUM> minutes. To remove PNGase, digested antibody was incubated with <NUM>µL of settled chitin resin (New England Biolabs, S6651L) on an orbital shaker for <NUM> minutes, then carefully removed as supernatant after centrifuging at <NUM>,<NUM> xg for <NUM> minute. <NUM> - <NUM> ng of protein was used for HPLC-coupled MS analysis as described above.

After reaching confluence, adherent cultures of SK-BR-<NUM> and HEK293T cells were detached from the plate surfaces using <NUM>% trypsin (HyClone) for <NUM> minutes at <NUM>. Trypsin was quenched with two volumes of ice-cold DMEM + <NUM>% FBS, centrifuged at <NUM>,<NUM> xg for <NUM> minutes, then resuspended in ice-cold PBS at <NUM>-<NUM> million cells/mL. Cells were treated with <NUM> Trastuzumab-HC-<NUM>5HTP-fluorescein-aniline (<NUM>) conjugate and gently rocked for <NUM> minutes at <NUM>. Cells were washed once and resuspended in ice-cold PBS, then analyzed by flow cytometry on a BD Accuri C6 Plus flow cytometer (BD Biosciences) using the FITC settings (excitation wavelength: <NUM>, standard filter: <NUM>/<NUM>). <NUM>,<NUM> events were collected in BD Accuri C6 Plus. version <NUM>. <NUM>, and data were processed using FlowJo, version <NUM>.

Unless mentioned otherwise, all reactions were conducted with anhydrous solvents (source. Acros Organics, Fisher Scientific or Alfa Aesar) under an atmosphere of N<NUM> in oven dried glassware. All work up and purifications were conducted with reagent grade solvents under ambient conditions. Room temperature refers to <NUM>. Thin-layer chromatography was performed using Merck silica gel <NUM> F<NUM> pre-coated plates (<NUM>). NMR spectra were recorded on Varian INOVA <NUM> or <NUM> NMR spectrometers at <NUM>. NMR data were processed using Mnova (Mestrelab). Chemical shifts are reported in ppm from tetramethylsilane (δ <NUM>) with the solvent resonance as the internal standard. For <NUM>H-NMR, data are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, dd = double doublet, q = quartet, t = triplet, m = multiplet), coupling constants (Hz) and integration. Unless mentioned otherwise, reactions were monitored by liquid chromatography (Agilent Technologies <NUM> Infinity) coupled mass-spectrometry (Agilent Technologies <NUM> TOF LC/MS) using a reverse phase analytical column (Phenomenex) with mobile phase comprises of water-acetonitrile-<NUM>% formic acid. Reverse phase HPLC (RP-HPLC) purifications were performed in a Waters HPLC system (fitted with a Waters <NUM> binary pump and Waters <NUM> UV-VIS detector) using a reverse phase semi-preparative column (Phenomenex, Jupiter® <NUM>, C-<NUM>, <NUM>Å, <NUM> x <NUM>); mobile phase for the RP-HPLC purification comprised of solvent-A (water-MeCN-<NUM>% trifluoroacetic acid in <NUM>:<NUM>:<NUM> ratio) and solvent B (MeCN-water-<NUM>% trifluoroacetic acid in <NUM>:<NUM>:<NUM> ratio); flow rate <NUM>/min.

N-BOc methyl ester of <NUM>-hydroxytryptophan <NUM> was prepared from commercially available <NUM>-hydroxy-L-tryptophan (5HTP; Chem Impex International) following a reported procedure. <NUM> Electrochemical reactions were performed in an ElectraSyn <NUM> system (IKA) using SK-<NUM> graphite electrodes. PBS buffer has the following composition: <NUM> Na<NUM>HPO<NUM>, <NUM> <NUM> KH<NUM>PO<NUM>, <NUM> NaCl and <NUM> KCl; pH <NUM>. Borate buffer has the following composition: <NUM> boric acid, <NUM> sodium tetraborate and <NUM> NaCl; pH <NUM>.

N-Boc methyl ester of <NUM>-hydroxytryptophan <NUM> (<NUM>, <NUM> mmol) and N,N-dimethylaniline 1a (<NUM>, <NUM> mmol, <NUM> eqv. ) were dissolved in a <NUM> mixture of MeCN/PBS (<NUM>:<NUM>; pH <NUM>). Resulting homogeneous mixture was split into four equal (each ~<NUM>) parts. The ElectraSyn vial cap having a graphite anode and a graphite cathode were inserted into one part (<NUM>) of the mixture in a ElectraSyn vial and subjected to electrolysis under a constant voltage (<NUM> V) under ambient conditions while stirring vigorously (<NUM> rpm). After <NUM> the ElectraSyn vial cap was removed and electrodes were rinsed with MeCN (<NUM>); the washing layer was combined with the rest of the crude reaction mixture and preserved at <NUM>. Aforementioned protocol was followed for the rest three parts. Crude reaction mixtures from the four batches were pooled together and was diluted with <NUM> aqueous elution buffer (comprises of water-MeCN-<NUM>. <NUM>% formic acid in <NUM>:<NUM>:<NUM> ratio) and poured into a column pre-packed with <NUM> reverse phase C18 silica gel (particle size: <NUM>-<NUM>; Acros Organics; <NPL>). After eluting the applied solution the column was washed with ~<NUM> aqueous elution buffer to remove all the unreacted N,N-dimethylaniline (judged by silica gel TLC). Next, a <NUM>:<NUM> mixture of organic (comprises of MeCN-water-<NUM>% formic acid in <NUM>:<NUM>:<NUM> ratio) and aqueous elution buffer was passed through the column and the eluted fractions were analyzed by LC-MS. The fractions having the expected product mass were pooled together, concentrated in a rotary evaporator followed by lyophilization. Further purification with RP-HPLC using a semi-preparative C18 column provided the analytically pure product (white solid, <NUM>). Following elution conditions was used for RP-HPLC: <NUM>-<NUM> <NUM>% A, <NUM>-<NUM> <NUM>% A to <NUM>% B, <NUM>-<NUM> <NUM>% B, <NUM>-<NUM> <NUM>% B to <NUM>% A, <NUM>-<NUM> <NUM>% A); retention time of the product = <NUM>. <NUM>H-NMR (<NUM>, CD<NUM>CN): <NUM> (s; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM> (d, J = <NUM>; <NUM>), <NUM> (d, J = <NUM>; <NUM>), <NUM> (d, J = <NUM>; <NUM>), <NUM> (d, J = <NUM>; <NUM>), <NUM> (d; J = <NUM>; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM> (s; <NUM>), <NUM> (s; <NUM>), <NUM> (dd, J = <NUM>, <NUM>; <NUM>), <NUM> (dd, J = <NUM>, <NUM>; <NUM>), <NUM> (s; <NUM>), <NUM> (s; <NUM>); <NUM>H-NMR (<NUM>, D<NUM>O): <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM> (d, J = <NUM>; <NUM>), <NUM> (s; <NUM>), <NUM> (d, J = <NUM>; <NUM>), <NUM> (s; <NUM>), <NUM> (s; <NUM>), <NUM> (dd, J = <NUM>, <NUM>; <NUM>), <NUM> (dd, J = <NUM>, <NUM>; <NUM>), <NUM> (s; <NUM>), <NUM> (s; <NUM>); <NUM>C-NMR (<NUM>, CD<NUM>CN): δ <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>; <NUM>C-NMR (<NUM>, D<NUM>O): δ <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>; ESI-TOF-MS (m/z): [M-+H]+ Calculated for C<NUM>H<NUM>N<NUM>O<NUM> <NUM>, obtained <NUM>. The product structure was confirmed by X-ray diffraction analysis of a single crystal of the product grown at the room temperature (by slow evaporation of a solution in MeOH/diethylether). HPLC-MS profile of a solution of the crystal was found to be identical with that of the NMR characterized sample.

N-Boc methyl ester of <NUM>-hydroxytryptophan <NUM> (<NUM>, <NUM> mmol) andp-toludine <NUM> (<NUM>, <NUM> mmol) were dissolved in a <NUM> mixture of MeCN/borate buffer (<NUM>:<NUM>; pH <NUM>) and were split into two equal (each <NUM>) parts. The ElectraSyn vial cap having a graphite anode and a graphite cathode were inserted into one part (~<NUM>) of the mixture and subjected to electrolysis under a constant voltage (<NUM> V) under ambient conditions while stirring vigorously (<NUM> rpm). After <NUM> the ElectraSyn vial cap was removed and electrodes were rinsed with MeCN (<NUM>); the washing layer was combined with the rest of the reaction mixture and preserved at <NUM>. Aforementioned protocol was followed for the second part of the ~<NUM> mixture. Crude reaction mixtures from the two batches were pooled together, diluted with <NUM> aqueous elution buffer (comprises of water-MeCN-<NUM>% formic acid in <NUM>:<NUM>:<NUM> ratio) and poured into a column pre-packed with <NUM> reverse phase C18 silica gel (particle size: <NUM>-<NUM>; Acros Organics; <NPL>). After the elution of the applied solution the column was washed with ~<NUM> aqueous elution buffer to remove all the unreacted p-toludine (judged by silica gel TLC). Next, a <NUM>:<NUM> mixture of organic (comprises of MeCN-water-<NUM>% formic acid in <NUM>:<NUM>:<NUM> ratio) and aqueous elution buffer was passed through the column and the eluted fractions were analyzed by LC-MS. Combined fractions with expected product were concentrated in a rotary evaporator followed by lyophilization. Repurification with a RP-HPLC utilizing a semi-preparative C-<NUM> column provided the analytically pure product (white solid, <NUM>). Following elution conditions was used for RP-HPLC: <NUM>-<NUM> <NUM>% A, <NUM>-<NUM> <NUM>% A to <NUM>% B, <NUM>-<NUM> <NUM>% B, <NUM>-<NUM> <NUM>% B to <NUM>% A, <NUM>-<NUM> <NUM>% A); retention time of the product = <NUM>. <NUM>H-NMR (<NUM> MHz, CD<NUM>CN): <NUM> (s; <NUM>), <NUM> (d, J = <NUM>; <NUM>), <NUM> (d, J = <NUM>; <NUM>), <NUM> (d, J = <NUM>; <NUM>), <NUM> (d, J = <NUM>; <NUM>), <NUM> (d, J = <NUM>; <NUM>), <NUM> (s; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM> (s; <NUM>), <NUM> (dd, J = <NUM>, <NUM>; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM> (s; <NUM>), <NUM> (s; <NUM>), <NUM> (s; <NUM>); <NUM>C-NMR (<NUM>, CD<NUM>CN): δ <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. ESI-TOF-MS (m/z): [M+H]+ Calculated for C<NUM>H<NUM>N<NUM>O<NUM> <NUM>, obtained <NUM>.

To a solution of <NUM>(<NUM>)-carboxyfluorescein <NUM> (<NUM>, <NUM> mmol; mixture of <NUM>- and <NUM>- isomer; Cayman Chemical) in anhydrous DMF (<NUM>) was added <NUM>-[<NUM>-(dimethylamino)propyl]-<NUM>-ethylcarbodiimide hydrochloride (EDC. HCL, <NUM>, <NUM> mmol; Chem Impex Int. ) followed by the addition of N-hydroxysuccinimide (<NUM>, <NUM> mmol) and the resulting mixture was stirred at the room temperature for <NUM>. Solvent was evaporated off under reduced pressure and the residue was dissolved in a mixture of THF/EtOAc (<NUM>, <NUM>:<NUM>) and washed with water (4x15 mL). Combined organic layer was dried over anhydrous Na<NUM>SO<NUM> and concentrated under reduced pressure to get the N-succinimidyl ester of <NUM>(<NUM>)-carboxyfluorescein <NUM> (<NUM>, <NUM> mmol, <NUM>% yield). The purity of the product was confirmed by HPLC-MS analysis. ESI-TOF-MS (m/z): [M+H]+ Calculated for C<NUM>H<NUM>NO<NUM> <NUM>, obtained <NUM>. The intermediate was used directly in the next step without further purification.

N-Boc-protected phenethylamine (<NUM>) was prepared following a reported protocol from <NUM>-aminophenethylamine (Combi-Blocks).

To a solution of N-succinimidyl ester of <NUM>(<NUM>)-carboxyfluorescein <NUM> (<NUM>, <NUM> mmol) in THF (<NUM>) was added N-Boc-protected phenethylamine <NUM> (<NUM> ing, <NUM> mmol) and the resulting mixture was stirred at the room temperature for <NUM> to have complete conversion (LC-MS). The solvent was evaporated off under reduced pressure and the residue was partitioned between a biphasic mixture of water (<NUM>) and organic solvents (EtOAc/THF <NUM>:<NUM>, <NUM>). Organic layer was separated and aqueous layer was extracted with the EtOAc/THF mixture (3x15 mL). Combined organic layer was dried over anhydrous Na<NUM>SO<NUM> and concentrated under reduced pressure to get the expected product <NUM> as a red semi-solid (<NUM>, <NUM> mmol, <NUM>% yield). The purity of the product was confirmed by HPLC-MS analysis. ESI-TOF-MS (m/z): [M+H]+ Calculated for C<NUM>H<NUM>N<NUM>O<NUM> <NUM>, obtained <NUM>. The intermediate was used directly in the next step without further purification.

Anhydrous <NUM> (N) HCl solution in <NUM>,<NUM>-dioxane (<NUM>) was added to the intermediate <NUM> under an inert atmosphere. Resulting mixture was stirred for <NUM> at the room temperature. LC-MS analysis of the crude mixture confirmed completion of the removal of the Boc protection. Solvent was evaporated off under reduced pressure to get an oil that was repeatedly diluted with <NUM> chloroform and dried under reduced pressure to get rid of dissolved hydrochloric acid. The expected product <NUM> was obtained as an orange-yellow solid (<NUM>, quantitative yield). The purity of the product was confirmed by HPLC-MS analysis. ESI-TOF-MS (m/z): [M+H]+ Calculated for C<NUM>H<NUM>N<NUM>O<NUM> <NUM>, obtained <NUM>.

1b was prepared according to a reported procedure through a two-step sequence:<NUM>.

N-hydroxysuccinimide (<NUM>, <NUM> mmol) was added to a solution of <NUM>-(methyl(phenyl)amino)butanoic acid 1b (<NUM>, <NUM> mmol) in THF (<NUM>), followed by the addition of EDC. HCl (<NUM>, <NUM> mmol). The resulting mixture was stirred at the room temperature for <NUM> and then concentrated under reduced pressure. The residue was partitioned between a biphasic mixture of EtOAc (<NUM>) and water (<NUM>). Organic layer was separated washed sequentially with water (3x30 mL) and brine (<NUM>). The organic layer was dried over anhydrous Na<NUM>SO<NUM> and concentrated under reduced pressure to get the N-succinimidyl ester <NUM> (<NUM>, <NUM> mmol, <NUM>% yield) as a white solid. <NUM>H-NMR (<NUM>, CDCl<NUM>): <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM> (t, J = <NUM>; <NUM>), <NUM> (s; <NUM>), <NUM> (s; <NUM>), <NUM> (t, J = <NUM>; <NUM>), <NUM>-<NUM> (m; <NUM>); <NUM>C-NMR (<NUM>, CDCl<NUM>): δ <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>.

A mixture of the N-succinimidyl ester <NUM> (<NUM>, <NUM> mmol) and Boc-amino-PEG<NUM>-amine <NUM> (<NUM>, <NUM> mmol; Ambeed, Inc. ) in dichloromethane (<NUM>) was stirred at the room temperature for <NUM>. Solvent was evaporated off under reduced pressure to get an oil that was purified by silica gel (<NUM>-<NUM> mesh) flash column chromatography (gradient elution: <NUM>% EtOAc in hexane to EtOAc to <NUM>% MeOH in EtOAc) to afford the product <NUM> as a yellow oil (<NUM>, <NUM> mmol, <NUM>% yield). Rf: <NUM> (<NUM>% MeOH in EtOAc); <NUM>H-NMR (<NUM>, CDCl<NUM>): <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM> (t, J = <NUM>; <NUM>), <NUM> (s; <NUM>), <NUM> (s; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM> (t, J = <NUM>; <NUM>), <NUM> (q, J = <NUM>; <NUM>), <NUM> (t, J = <NUM>; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM> (s; <NUM>), <NUM> (t, J = <NUM>; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM> (s; <NUM>), <NUM>C-NMR (<NUM>, CDCl<NUM>): δ <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>.

Anhydrous <NUM> (N) HCl solution in <NUM>,<NUM>-dioxane (<NUM>) was added to the oil (<NUM>) under an inert atmosphere. Resulting mixture was stirred for <NUM> at the room temperature. LC-MS analysis of the crude mixture confirmed complete removal of the Boc protection. The solvent was evaporated off under reduced pressure to get an oil that was repeatedly diluted with <NUM> methanol and dried under reduced pressure to get rid of dissolved hydrochloric acid. The hydrochloride salt of the expected product <NUM> was obtained as a yellow oil (<NUM>, quantitative yield). The purity of the product was confirmed by HPLC-MS analysis. <NUM>H-NMR (<NUM>, CD<NUM>OD): <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM> (t, J = <NUM>; <NUM>), <NUM> (t, J = <NUM>; <NUM>), <NUM> (s; <NUM>), <NUM> (t, J = <NUM>; <NUM>), <NUM>-<NUM> (m; <NUM>); <NUM>C-NMR (<NUM>, CD<NUM>OD): δ <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. ESI-TOF-MS (m/z): [M+H]+ Calculated for C<NUM>H<NUM>N<NUM>O<NUM> <NUM>, obtained <NUM>.

Potassium carbonate (<NUM>, <NUM> mmol) was added to a solution of the hydrochloride intermediate <NUM> (<NUM>, <NUM> mmol) in DMF (<NUM>) and stirred for <NUM> before the addition of a solution of N-succinimidyl ester of <NUM>(<NUM>)-carboxyfluorescein <NUM> (<NUM>, <NUM> mmol) in THF (<NUM>). Resulting mixture was stirred at the room temperature for <NUM> to allow full conversion (LC-MS). Solvent was evaporated off under reduced pressure and the residue was partitioned between a biphasic mixture of water (<NUM>) and organic solvents (EtOAc/THF <NUM>:<NUM>, <NUM>). Organic layer was separated and aqueous layer was extracted with the EtOAc/THF mixture (4x15 mL). Combined organic layer was dried over anhydrous Na<NUM>SO<NUM> and concentrated under reduced pressure to get <NUM> as an oil (<NUM>, <NUM> mmol, <NUM>% yield). The purity of the product was confirmed by HPLC-MS analysis. ESI-TOF-MS (m/z): [M+H]- Calculated for C<NUM>H<NUM>N<NUM>O<NUM> <NUM>, obtained <NUM>.

A mixture of biotin-PEG<NUM>-amine <NUM> (<NUM>, <NUM> mmol; BroadPharm), N-succinimidyl ester <NUM> (<NUM>, <NUM> mmol) in anhydrous DMF (<NUM>) was heated and stirred at <NUM> for <NUM>. The solvent was evaporated off under reduced pressure and the residue was purified by silica gel (<NUM>-<NUM> mesh) column chromatography (gradient elution: EtOAc to <NUM>% MeOH in EtOAc to <NUM>% MeOH in EtOAc and then to <NUM>% MeOH in EtOAc) to afford the product. Silica gel was removed from the product by dissolving the product in warm chloroform and filtering. The product <NUM> was obtained as a yellow oil (<NUM>, <NUM> mmol; <NUM>% yield). Rf: <NUM> (<NUM>% MeOH in EtOAc); <NUM>H-NMR (<NUM>, CDCl<NUM>): <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM> (s; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM> (s; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM> (s; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>); <NUM>C-NMR (<NUM>, CDCl<NUM>): δ <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. FSI-TOF-MS (m/z): [M+H]+ Calculated for C<NUM>H<NUM>N<NUM>O<NUM>S <NUM>, obtained <NUM>.

To the intermediate (<NUM>) a solution of <NUM> (N) HCl in <NUM>,<NUM>-dioxane (<NUM>) was added dropwise at <NUM> and the resulting mixture was allowed to come to the room temperature while stirred. A solid precipitate started forming. After <NUM> stirring at the room temperature the mixture was concentrating under reduced pressure. To the resulting oil, chloroform (~<NUM>) was added and concentrated to dryness to get rid of dissolved HCl. The oil was triturated with diethylether (~<NUM>) and dried thoroughly. Resulting solid residue was dissolved in ~<NUM> water and lyophilized to get the expected product (<NUM>) as a yellow solid (<NUM>, <NUM> mmol, <NUM>% yield). <NUM>H-NMR (<NUM>, D<NUM>O): δ <NUM> (d, J = <NUM>; <NUM>), <NUM> (d, J = <NUM>; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>); <NUM>C-NMR (<NUM>, D<NUM>O): δ <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> ESI-TOF-MS (m/z): [M+H]- Calculated for C<NUM>H<NUM>N<NUM>O<NUM>S <NUM>, obtained <NUM>.

A mixture of alkyne-PEG<NUM>-amine <NUM> (<NUM>, <NUM> mmol; Lumiprobe), N-succinimidyl ester <NUM> (<NUM>, <NUM> mmol) in anhydrous THF (<NUM>) was heated and stirred at <NUM> for <NUM> to allow full conversion of the starting N-succinimidyl ester (judged by LC-MS). The solvent was evaporated off under reduced pressure and the residue was purified by silica gel (<NUM>-<NUM> mesh) column chromatography (gradient elution: <NUM>% EtOAc in hexanes to EtOAc to <NUM>% MeOH in EtOAc) to afford the product. Silica gel was removed from the product by dissolving the product in warm chloroform and filtering. Concentrating the filtrate provided the intermediate <NUM> as a yellow oil (<NUM>, <NUM> mmol; <NUM>% yield). Rf: <NUM> (<NUM>%) MeOH in EtOAc); <NUM>H-NMR (<NUM>, CDCl<NUM>): δ <NUM> (d, J = <NUM>; <NUM>), <NUM> (d, J = <NUM>; <NUM>), <NUM> (s; <NUM>), <NUM> (s; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM> (s; <NUM>), <NUM>C-NMR (<NUM>, CDCl<NUM>): δ <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>.

The intermediate <NUM> was dissolved in <NUM> CH<NUM>Cl<NUM>, and anhydrous trifluoroacetic acid (<NUM>) was added dropwise at <NUM>, and the resulting mixture was stirred at <NUM> for <NUM> to have complete removal of Boc protection (judged by LC-MS). The mixture was concentrated under reduced pressure; to fully remove TFA, additional CH<NUM>Cl<NUM> (~<NUM>) was added and the solution was evaporated four consecutive times. The expected product <NUM> was obtained as a yellow oil (<NUM>, <NUM> mmol, quantitative yield). <NUM>H-NMR (<NUM>, CD<NUM>OD): δ <NUM> (d, J = <NUM>; <NUM>), <NUM> (d, J = <NUM>; <NUM>), <NUM> (s; <NUM>), <NUM> (d, J = <NUM>; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM>-<NUM> (m; <NUM>), <NUM> (s; <NUM>), <NUM> (d, J = <NUM>; <NUM>), <NUM>C-NMR (<NUM>, CD<NUM>OD): δ <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. ESI-TOF-MS (m/z): [M+H]+ Calculated for C<NUM>H<NUM>N<NUM>O<NUM> <NUM>, obtained <NUM>.

MS analysis of eCLIC reactions associated with <FIG>. Expected molecular weights: sfGFP-<NUM>-5HTP (<NUM> Da), conjugate with 1a (<NUM> Da), conjugate with 1b (<NUM> Da), conjugate with <NUM> (<NUM> Da).

MS analysis of eCLIC reactions of sfGFP-<NUM>-5HTP with various substrates associated with <FIG>. Expected molecular weight of unmodified sfGFP-<NUM>-5HTP is <NUM> Da. Expected eCLIC adduct is highlighted with a green arrow in each case.

<FIG> shows representative examples of MS analysis of eCLIC reactions of wild-type sfGFP with various substrates shown in <FIG>. None of the substrates show detectable labeling, except for phenothiazine (<NUM>), which is known to label tyrosine residues. Expected molecular weight of unmodified wild-type sfGFP is <NUM> Da.

<FIG> shows Myoglobin and 5F7 anti-HER2 nanobody control proteins lacking a 5HTP residue do not undergo eCLIC labeling with the 1b aniline, even though the 5HTP mutants of these proteins can be efficiently labeled (FIG.

<FIG> shows HPLC-MS analysis of the eCLIC reaction between N-Boc methyl ester of <NUM>-hydroxytryptophan <NUM> (exact mass: <NUM>) and N,N-dimethylaniline (exact mass: <NUM>); A<NUM> represents absorbance at <NUM> in an arbitrary unit; N,N-dimethylaniline has little absorbance at <NUM> (elutes around <NUM>); there was a single peak having the expected product mass (at <NUM>, see below for the mass spec) in the crude reaction mixture, other peaks correspond to unreacted starting material (5HTP derivative), higher order homo- and hetero-coupling reactions between the starting materials having following m/z (protonated forms): peak (i) <NUM> (2A+B-<NUM>+H+), peak (ii) <NUM> (3A+B-<NUM>+H+), peak (iii) <NUM> (2A-<NUM>+H+) etc. [A and B refers to <NUM> and 1a, respectively]; expected product (A+B-<NUM>) having m/z <NUM> was structurally characterized by mass spectrometry, NMR and XRD analysis; dissolved crystal refers to a solution of the crystal of the product, the crystal having been retrieved from the same sample pool from which diffraction quality single crystal was picked for XRD structure analysis.

<FIG> shows HPLC-MS analysis of the eCLIC reaction modelling between N-Boc methyl ester of <NUM>-hydroxytryptophan <NUM> (exact mass: <NUM>) andp-toludine <NUM> (exact mass: <NUM>); A<NUM> represents absorbance at <NUM> in an arbitrary unit; expected product (A+C-<NUM>) having m/z <NUM> (protonated form) was structurally characterized by mass spectrometry and co-relation NMR analysis; peak (i) had m/z (protonated form) of <NUM> (A+C-<NUM>+H+) is likely generated by over-oxidation of the expected product (A+C-<NUM>); peak (ii) had m/z (protonated forms) of <NUM> (A+C-<NUM>+<NUM>+H+) is suggestive of a further oxidized product, [A and C refers to the <NUM> and <NUM>, respectively] (B) chemical shifts of <NUM>H and <NUM>C assigned through <NUM>-D and <NUM>-D NMR analysis (vide infra); carbons are numbered numerically and protons are designated alphabetically.

Fig. <NUM> shows a proposed mechanism of eCLIC.

<FIG> shows the results of experiments demonstrating the interchangeable roles of TEMPO and oxygen in the eCLIC reaction. sfGFP-<NUM>-5HTP was reacted with 1b in <NUM> HEPES, pH <NUM> at <NUM> mV for <NUM> hour under oxygen-depleted conditions in the absence of TEMPO, oxygen-depleted conditions in the absence of TEMPO (<NUM>), and optimized eCLIC conditions (aerobic + TEMPO). Expected molecular weight for sfGFP-<NUM>-TAG is <NUM> Da, and its conjugation product with 1b is <NUM> Da.

<NUM> shows the plasmid map of pCBNA3. <NUM>-C-Her2+SbfI (containing Trastuzumab light chain and heavy chain fragments).

<NUM> shows the plasmid map of pAcBac1_EcWRS1-5HTP-H14_RevU6EcW_TGA.

<FIG> shows the plasmid map of pET22b-T5-5F7.

<FIG> shows the plasmid map of pET22b-Myoglobin-K99>TAG.

Thus, the present disclosure establishes a unique electrochemical chemoselective conjugation reaction to site-specifically modify proteins expressed in both bacteria and eukaryotes. The electrocatalysis reactions are highly scalable, rapid (taking less than about <NUM> minutes to about <NUM> hours, and highly selective for the targeted unnatural amino acid.

While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

Claim 1:
A method of producing a chemoselectively modified protein bioconjugate, comprising the steps of:
a.) providing a recombinantly-expressed protein comprising one, or more non-canonical amino acid (ncAA) residues at specific sites of the protein, wherein one, or more, of the ncAA residues is a <NUM>-hydroxytryptophan (<NUM>-HTP);
b.) selectively oxidizing the one, or more, <NUM>-HTP amino acid residues of the protein under suitable conditions and at low voltage, wherein the voltage is sufficient to generate an electrophilic <NUM>-HTP intermediate; and
c.) reacting the <NUM>-HTP intermediate with a coupling partner comprising a nucleophilic group under conditions suitable for the coupling of the nucleophilic group to the one, or more, <NUM>-HTP residues in the protein, thereby producing a chemoselectively modified protein bioconjugate.