## 2 Materials and Methods  
### General Information  
Chemicals were purchased from commercial suppliers (Roth, Sigma Aldrich, Merck, TCI). Organic solutions were concentrated by rotary evaporation at 40 \({}^{\circ}\)C. Pyridine was stored over KOH, distilled prior to use and stored over a molecular sieve. Buffers for the purification of trinucleotides were prepared with micro-pore water (Nanopore, Thermo Scientific). Products were visualized by thin layer chromatography (TLC) on aluminum sheets covered with silica gel 60 F254 (Merck) at 254 nm. Unless otherwise described, the standard TLC solvent system dichloromethane/methanol = 95:5 was used. Column chromatography was carried out on silica gel from Merck (Geduran Si 60, 0.063-0.200 mm). Solvents for column chromatography were purified by rotary evaporation. All buffers for HPLC analyses were filtered through a 0.2 \(\upmu\)m acetonitrile resistant membrane and degassed in the ultrasonic bath for 15 min. MALDI-TOF analyses were done with a Bruker Daltonics Microflex in either positive or negative mode. For this, 1 \(\upmu\)L sample solution was mixed with 1 \(\upmu\)L hydroxypicolinic acid solution (in H\({}_{2}\)O/acetonitrile = 1:1) on a plate (MSP 96 target ground steel) and dried to crystallization [11]. Determination of support loading densities was carried out by detritylation of the support-linked nucleoside and UV/VIS absorption measurements. Therefore, 3% dichloroacetic acid was added to the support. The absorption of the DMT-cation was measured at 498 nm, and the corresponding DMT concentration was determined by comparison to a standard curve. For the detailed procedure see [PERSON] et al. [10] and Supplementary Materials S1.  
Figure 1: Reaction scheme for preparation of fully protected trinucleotide phosphoramidites.  
List of solid supports used in this work: **A** = Amino CPG (controlled pored glass), 500 Angstrom (105 mol/g) from Chem Genes, (USA); **B** = Polystyrene A SH, 200-400 mesh (850 mol/g) from RAPP Polymere (Germany); **C** = Polystyrene AM NH\({}_{2}\), 200-400 mesh (980 mol/g, high loaded) from RAPP Polymere (Germany); **D** = Custom Primer Support Amino, high cross-linked polystyrene/divinylbenzene (200 mol/g), from GE Healthcare (Sweden).  
Synthesis of 5\({}^{\prime}\)-O-Dimethoxyttriyl-3\({}^{\prime}\)-O-Methylthomethyl-2\({}^{\prime}\)-Deoxynucleosides  
The synthesis of 3\({}^{\prime}\)-O-thoomethyl-modified nucleosides (in Scheme 1(3a-d)) is described in detail in [PERSON] et al. [10]. See Supplementary Materials S2 for \({}^{1}\)H and \({}^{13}\)C NMR spectra.  
**Scheme 1.** Reaction scheme of nucleoside functionalization and immobilization. Nucleotides are linked to the support via a disulfide bridge, with (**a**) short linker (sec. carbon), (**b**) long linker (prim. carbon), (**c**) direct connection; (i) dC/dG: 5 eq. dimethylformamide-dimethylacetal, 1 h, 50 \({}^{\circ}\)C, in dimethylformamide; dA: 1.4 eq. dibutylformamide-dimethylacetal, rt, overnight, 1 h, 80 \({}^{\circ}\)C, in dimethylformamide; (ii) 1.3 eq. 4,4\({}^{\prime}\)-dimethoxyttriyl chloride, 3 h up to overnight in pyridine; (iii) 70 eq. dimethyl sulfoxide, 53 eq. acetic acid, 53 eq. acetic anhydride, rt, 20 h; (iv) 3 eq. triethylamine, 1 eq. sulfuryl chloride, 1.5 eq. potassium thiotsylate in dichloromethane. (**a**) 2 eq. 2-mercaptopropionic acid, rt, 2 h; (**b**) 2 eq. 12-mercaptoethanolodecanoic acid, rt, 2 h; (**c**) solid support B, rt, 1 h; (v) 2 eq. TSTU, 5 eq. triethylamine, solid support (A/C/D), rt, overnight in dimethylformamide/dichloromethane/water.