Synthesis and conformational study of poly(Nϵ‐benzyl‐ L ‐lysine) and its benzyloxycarbonyl derivative

The solvent‐and pH‐induced conformational changes are examined in order to investigate the influence of benzyl group. Polymer was prepared via Nϵ‐benzyloxycarbonyl, Nϵ‐benzyl‐Nα‐carboxy‐L‐lysine anhydride. The resulting poly (Nϵ‐benzyloxycarbonyl, Nϵ‐benzyl‐L‐lysine) was obtained in high yield and had a high molecular weight. The protected polymer was removed into poly (Nϵ‐benzyl‐L‐lysine) by treating it with hydrogen bromide. From the results of the ORD and CD, the protected polymer has a righthanded α‐helix, showing [m′]233 = –10,300, [θ]220 = –27,600 and [θ]207 = –25,100 in dioxane. The breakdown of the helical conformation is found to occur at 8% dichloroacetic acid in chloroform‐dichloroacetic acid mixture. In the pH range 3.35–6.90, poly (Nϵ‐benzyl‐L‐lysine) is in a random coil structure. In the pH range 7.50–13.0, the polypeptide has a right‐handed α‐helix structure; [m′]233 = –12,000, [0]220 = –27,200, and [0]207 = –27,000. In comparison with poly‐L‐lysine, the coil‐to‐helix transition is observed at lower pH range in 50% n‐propanol. Above pH 8 by heating, the α ⇀ β transition of poly (Nϵ‐benzyl‐L‐lysine) is not observed in an aqueous media.