## 2 Materials and Methods
### Loading of Support  
#### 2.3.1 Thymidine as Starting Nucleoside  
Loading of Polystyrene A SH (B)  
5\({}^{\prime}\)-O-Dimethoxyrtyl-3\({}^{\prime}\)-O-methylthiomethyl-thymidine (**3d**) (0.85 mmol) was coevaporated three times with dry dichloromethane and dissolved under dry conditions in dry dichloromethane (53.55 mmol, 3.42 mL). Triethylamine (2.55 mmol, 0.35 mL) was added and the solution stirred in an ice bath. Freshly, up to colorlessness, distilled sulfuryl chloride (0.85 mmol, 0.07 mL) was mixed with dichloromethane (10.2 mmol, 0.65 mL), and this solution was added slowly and dropwise to the reaction mixture, which turned orange. After 10 min in the ice bath, the solution was stirred for 5 min at room temperature. Potassium thiotosylate (1.28 mmol, 0.243 g) was mixed with dimethylformamide (10.2 mmol, 0.78 mL), and this solution was added to the reaction mixture dropwise, changing the color from orange to yellow. Formation of the intermediate product **5d** was observed by TLC. After stirring for 10 min, 820 mg solid support **B** (polystyrene A SH) was added, and the reaction mixture was stirred for 20 h at room temperature. Upon removal of the liquid, the support was washed with dichloromethane and methanol and dried in vacuo [12; 13]. The loading density of the support **8d** was 238.43 mol/g. Loading of Amino-Functionalized Supports (A, C, D)  
* Preparation of 2\({}^{\prime}\)-O-linker modified thymidine derivatives **6d** and **7d** 5\({}^{\prime}\)-O-Dimethoxyrtyl-3\({}^{\prime}\)-O-methylthiomethyl-thymidine (**3d**) (2.32 mmol) was dissolved in dry dichloromethane (9.33 mL). Triethylamine (6.96 mmol, 0.96 mL) was added, and the reaction mixture was cooled in an ice bath. Freshly distilled sulfuryl chloride (2.32 mmol, 0.19 mL) in dichloromethane (1.77 mL) was added dropwise to the reaction mixture. The solution was stirred for 10 min in the ice bath. After warming up to room temperature, a solution of potassium thiotosylate (3.4 mmol, 0.65 g) in dry dimethylformamide (2.17 mL) was added dropwise to the reaction mixture. Formation of the intermediate product **5d** was observed by TLC. After stirring for 10 min 2-mercaptoethanolroppanoic acid (4.64 mmol, 0.38 mL) was added. The color of the reaction mixture turned to light red due to the loss of traces of the DMT protecting group. After 20 min the color of the solution changed to yellow. After stirring for 1.5 h the reaction mixture was washed with saturated aqueous sodium bicarbonate solution and brine. After drying over Na\({}_{2}\)SO\({}_{4}\) and removing the solvent under vacuum, the residue (**6d**) was used for in situ coupling to solid support (**A**, **C**, and **D**) [12; 13]. For synthesis of compound **7d**, mercaptododecanoic acid was used instead of 2-mercaptoethanolroppanoic acid. Synthesis was conducted in the same way as described above for 2-mercaptoethanolroppanoic acid, although at smaller scale (0.45 mmol of **3d** and 0.9 mmol of mercaptododecanoic acid). * Loading of solid support: Amino CPG (**A**)  
One eighth of the reaction mixture from above containing 5\({}^{\prime}\)-O-dimethoxyytrityl-3\({}^{\prime}\)-O-(propanoic acid-2-dithiomethyl)-thymidine (**6d**) was dissolved in dimethylformamide (0.4 mL), dichloromethane (0.8 mL) and H\({}_{2}\)O (0.1 mL). Triethylamine (2.3 mmol, 0.32 mL) and 2-succinimide-1,1,3,3-tetramethyluronium tetrafluoroborate (TSTU) (2.5 mmol, 0.72 g) were added. Afterward solid support **A** (60 mg, Amino CPG) was added, and the reaction mixture was stirred for 48 h at room temperature. The loaded solid support (**9d**) was washed with ethanol, dichloromethane, and water and dried in vacuo [14]. The loading was 10.5 mol/g. The coupling of 5\({}^{\prime}\)-O-dimethoxyytrityl-3\({}^{\prime}\)-O-(dodecanic acid-12-dithiomethyl)-thymidine (**7d**) to Amino CPG was done under the same conditions. The loading of the resulting material, **10d**, was 7.9 mol/g.