The mechanism of the helix-sense inversion of polyaspartates as revealed by the study of model block copolymers

Abstract

For the purpose of elucidating the mechanism of the helix-sense inversion phenomena characteristic of poly(aspartic ester)s, we synthesized two diblock copolymers comprising poly(β-phenethyl L-aspartate) (PA) and poly(γ-benzyl L-glutamate) (BG) blocks designated as BG-PA and PA-BG in the order from the C-terminus to the N-terminus. A triblock copolymer sample such as poly(γ-benzyl L-glutamate)-b-poly(β-phenethyl L-aspartate)-b-poly(γ-benzyl L-glutamate) (BG-PA-BG) was also prepared. The screw sense of α-helical block copolymers was investigated in 1, 1, 2, 2-tetrachloroethane (TCE) solution by the 1H nuclear magnetic resonance (NMR) and circular dichroism techniques as a function of temperature. For the BG-PA diblock copolymer, the helix-sense of the PA block was completely reversed from right-to left-handed at 100°C on heating. The thermal behaviors of triblock copolymer BG-PA-BG was found to be very similar to those of the poly(γ-benzyl L-glutamate) (BG) homopolymer, indicating that the reversal of the screwsense was entirely restrained. From these observations, we have concluded that during heating, the helix-helix transition proceeds predominantly from the N-terminus in the α-helical polyaspartate system.