The Mechanism of the Helix-Sense Inversion of Polyaspartates ad Revealed by the Study of Model Block Copolymers

摘要

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(beta-phenethyl L-aspartate) (PA) and poly(gamma-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(gamma-benzyl L-glutamate)-b-poly(beta-phenethyl L-aspartate) (PA) and poly(gamma-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(gamma-benzyl L-glutamate)-b-poly(beta-phenethyl L-aspartate)-b-poly(gamma-benzyl L-glutamate) (BG-PA-BG) was also prepared. The screw sense of alpha-helical block copolymers was investigated in 1,1,2,2-tetrachloroethane (TCE) solution by the ~1H nuclear magnetic resonance (NMR) and circular dichroism techniques 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 deg C on heating. The thermal behaviors of triblock copolymer VBG-PA-BG was found to be very similar to those of the poly(gamma-benzyl L-glutamate) (BG) homopolymer, indicating that the reversal of the screw-sense was entirely restrained. From these observations, we have concluded that during heating, the helix-helix transition proceeds predominantly from the N-terminus in the alpha-helical polyaspartate system.