2 2 dipyridyldisulfide

Synthesis of peptide precursors was performed manually using fritted syringes on a vacuum manifold via 9‐fluorenylmethyloxycarbonyl (Fmoc) solid‐phase peptide synthesis (SPPS). All reagents were purchased from commercial sources and used without further purification. 2‐Chlorotrityl chloride resin (sub. = 1.2 mmol/g), Wang resin (sub. = 0.73 mmol/g), Oxyma Pure and all standard Fmoc amino acids were sourced from Mimotopes Pty Ltd (Australia). Fmoc‐L‐Dap(Boc)‐OH and Fmoc‐L‐Asp‐OMe were obtained from AK Scientific, Inc. (USA). Fmoc‐Cys(StBu)‐OH was sourced from CreoSalus, Inc. (USA). (1‐Cyano‐2‐ethoxy‐2‐oxoethylidenaminooxy)dimethylamino‐morpholino‐carbenium hexafluorophosphate (COMU), 1‐[bis (dimethylamino)methylene]‐1H‐1,2,3‐triazolo[4,5‐b]pyridinium‐3‐oxide hexafluorophosphate (HATU) and 2,2′‐dipyridyldisulfide (DPDS) were purchased from Combi Blocks (USA). Diisopropylcarbodiimide (DIC), 4‐dimethylaminopyridine (DMAP), diphenyldiselenide, acetoacetone, triisopropylsilane (TIPS), thioanisole, 2,2′‐(ethylenedioxy)diethanethiol (DODT), trifluoroacetic acid (TFA), piperidine, diisopropylethylamine (DIEA), N‐methylmorpholine (NMM), hydrazine hydrate, tris‐carboxyethylphosphine hydrochloride (TCEP.HCl), iodine, dimethylformamide (DMF), dichloromethane (DCM), diethyl ether, acetonitrile and trypsin were purchased from Sigma‐Merck (Australia).

Reagents and solvents were obtained at the highest available purity from Acros Organics, Alfa Aesar, Fisher Scientific, Sigma-Aldrich, ThermoFisher Scientific, or Tokyo Chemical Industry Co., and were used without further purification unless noted below. Nucleoside-5′-monophosphates, free acid, were purchased from Santa Cruz Biotechnology. 2-aminoimidazole hydrochloride and 2,2′-dipyridyldisulfide were purchased from Combi Blocks. The 5′-phosphorylated trimers, 5′-aminoimidazolium activated trimers, and the 5′-aminoimidazolium activated mononucleotides were prepared as previously described (3 (link)). RNA oligonucleotides for standard primer extension experiments were purchased from Integrated DNA Technologies. The synthesis and storage of methyl isocyanide was as previously described (9 (link)). All reactions were carried out in DNase/RNase-free distilled water.