Highly cooperative helix-sense reversal of polyaspartates. Influence of the comonomer of the opposite screw-sense preference

Poly(l-aspartic acid ester)s are known to form an α-helix in either the right(r)- or left(ℓ)-handed screw depending on the chemical constitution of the ester group in the side chain. While poly(β-benzyl l-aspartate) (PBLA) normally remains in the ℓ-form in conventional helicoidal solution at all temperatures, poly(β-phenethyl l-aspartate) (PPLA) exhibits transformation from the r- to ℓ-form with increasing temperature. In this work, random copolymers comprising residues of the two opposite screw-sense preferences, BLA and PLA, were chosen to investigate the stability of the α-helical hydrogen-bond array in a helix-forming solvent such as chloroform or 1,1,2,2-tetrachloroethane. Sharp S-shape transitions of the helix sense were observed for copolymers in the range PLA > 50%. The minor component BLA strictly obeys the majority rule in these copolymers. 2H NMR studies of partially deuterated samples in the nematic liquid-crystalline (LC) state have proved that the thermally induced r ↔ ℓ helix-sense inversion takes place highly cooperatively along the α-helical backbone: i.e., the cooperativity of the transition was not much affected by the inclusion of substantial amount of weaker points along the chain. These observations unambiguously suggest a zipper-type propagation process for PPLA starting from one terminal to the other. For given copolymer samples, the transition temperatures determined in dilute isotropic solution are nearly identical to those observed in the LC state, suggesting that the transition mechanism remains also similar. In the succeeding paper, relevant experimental data will be interpreted according to the theory consistent with these observations.