Poly(S-ethylsulfonyl-l-homocysteine): An α-Helical Polypeptide for Chemoselective Disulfide Formation

Homocysteine and cysteine are the only natural occurring amino acids that are capable of disulfide bond formations in peptides and proteins. The chemoselective formation of asymmetric disulfide bonds, however, is chemically challenging and requires an activating group combining stability against hard nucleophiles, e.g., amines, with reactivity toward thiols and soft nucleophiles. In light of these considerations, we introduced the S-alkylsulfonyl cysteines in our previous work. Here, we present the synthesis and ring-opening polymerization of S-ethylsulfonyl-l-homocysteine N-carboxyanhydrides. We demonstrate that the polymerization leads to narrowly distributed polypeptides (Đ = 1.1–1.3) with no detectable side reactions in a chain length regime from 11 to 165. In contrast to the already reported cysteine derivatives, poly(S-ethylsulfonyl-l-homocysteine)s do not form β-sheets, which reduce solubility and limit the degree of polymerization of poly(S-ethylsulfonyl-l-cysteine)s to 50. Instead, these polymers form α-helices as confirmed by circular dicroism (CD) experiments and infrared spectroscopy (FT-IR). In comparison to the cysteine derivatives, the α-helix formation leads to slightly faster polymerization kinetics (rate constants from 1.44 × 10–5 to 5.29 × 10–5 s–1). In addition, the ability for the chemoselective formation of asymmetric disulfides is preserved as monitored via 1H NMR experiments. Consequently, this new polypeptide overcomes the chain length limitations of poly(S-ethylsulfonyl-l-cysteine)s and thus provides convenient access to reactive poly(S-ethylsulfonyl-l-homocysteine)s for chemoselective disulfide formation.