Characterization of Hydrogen-bond Correlation Involved in the Helical Structure of Polypeptides and Application to the Molecular Design of Synthetic Polymers

本文研究了聚l -天冬氨酸酯螺旋感反转特性的分子机制。已知这些人造聚合物根据侧链中酯基(X)的化学构成在右n或左旋螺旋上形成α-螺旋。聚（β-苯甲酰l -天冬氨酸）（PBLA）在常规螺旋溶液中在任何温度下都保持左旋形式，而聚（β-苯乙基l -天冬氨酸）（PPLA）则随着温度的升高从右旋形式转变为左旋形式。在这项工作中，我们选择了随机共聚物，包括两个相反螺旋感的残基，BLA和PLA，来研究α-螺旋氢键阵列在螺旋形成溶剂如氯仿或四氯乙烷（TCE）中的稳定性。在PLA> - 50%范围内，共聚物的螺旋感发生了明显的s型转变。^在向列液晶环境中对部分氘化样品的核磁共振研究表明，热诱导的螺旋感反转沿a-螺旋主链高度协同发生。这些观察结果强烈地暗示了一个从一端到另一端的拉链式传播过程。这种转变的驱动力可以追溯到侧链构象的热变化。对于给定的共聚物样品，在稀各向同性溶液中观察到的转变温度与在液晶状态下观察到的转变温度几乎相同，这表明两种介质中的转变机制大致相同。在齐姆-布拉格概念中规定的理论公式已成功地用于解释这些螺旋-螺旋过渡现象。

The molecular mechanism involved in the helix-sense reversal characteristic of poly(L-aspartic acid ester)s has been investigated. These man-made polymers are known to form an α-helix in either the right-n or left-handed screw depending on the chemical constitution of the ester group (X) in the side chain. While poly(β-benzyl L-aspartate) (PBLA) remains in the left-handed form in conventional helicoidal solution at all temperatures, poly(β-phenethyl L-aspartate) (PPLA) exhibits transformation from the right-to the left-handed form with increasing temperature. In this work, random copolymers comprising residues of the two opposite screw-sense preference, 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 tetrachloroethane (TCE). Sharp S-shaped transitions of the helix sense were observed for copolymers in the range PLA>50%. ^2H NMR studies of partially deuterated samples in the nematic liquid crystalline environment indicate that the thermally induced helix-sense inversion takes place highly cooperatively along the a-helical backbone. These observations strongly suggest a zipper-type propagation process starting from one terminal to the other. The driving force of the transformation has been traced back to the thermal variation of the side chain conformation. For given copolymer samples, the transition temperatures observed in dilute isotropic solution are nearly identical to those obtained in the liquid crystalline state, suggesting that the transition mechanism remains more or less the same in both media. A theoretical formula prescribed within the Zimm-Bragg concept has been successfully used in interpreting these helix-helix transition phenomena.