Synthesis and conformational study of poly(N ϵ ‐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 N ϵ ‐benzyloxycarbonyl, N ϵ ‐benzyl‐N α ‐carboxy‐ L ‐lysine anhydride. The resulting poly (N ϵ ‐benzyloxycarbonyl, N ϵ ‐benzyl‐ L ‐lysine) was obtained in high yield and had a high molecular weight. The protected polymer was removed into poly (N ϵ ‐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 (N ϵ ‐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 (N ϵ ‐benzyl‐ L ‐lysine) is not observed in an aqueous media.