Side-Chain and Main-Chain Conformational Transitions of Poly(L-1- and 2-naphthylalanine)

Abstract

Solvent and temperature-induced conformational transitions of poly(L-1- and 2-naphthylalanine) were investigated by circular dichroism in a 1,2-dichloroethane (DCE)/trifluoroacetic acid (TFA) mixed solvent. The two polypeptides were in helical conformation in DCE in which the side chain chromophores are also arranged helically. Poly(L-1-naphthylalanine) showed a two-stage conformational transition upon the addition of TFA to the polymer solution in DCE. An intermediate conformation, which appeared at a TFA content between 0.5 and 3.0 (v/v) %, was tentatively assigned to another helical conformation in which the orientation of the side-chain chromophores is different from that of the helical conformation in DCE. The final state, which appeared at higher TFA content than 3%, was assigned to an extended β-structure. Poly(L-2-naphthylalanine) showed a single conformational transition at a TFA content of 2—3%. The final state was assigned to a randomly-coiled conformation. A temperature-induced conformational transition was observed for poly(L-2-naphthylalanine) in DCE/TFA mixed solvent with a TFA content of 1—5%. In the mixed solvent, the polymer was in a randomly-coiled state at low temperatures and in a helical conformation at high temperatures.