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

研究了聚L-天冬氨酸酯螺旋反转特性的分子机理。已知这些人造聚合物在右旋或左旋螺旋中形成α-螺旋，这取决于侧链中酯基（X）的化学组成。聚L-天冬氨酸-β-苄酯（PBLA）在常规的螺旋溶液中，在所有温度下均保持左旋构型，而聚L-天冬氨酸-β-苯乙酯（PPLA）则随着温度的升高，从右旋构型转变为左旋构型。本工作中，我们选择了含有两种相反螺旋结构的无规共聚物BLA和PLA，研究了它们在氯仿或四氯乙烷（TCE）等溶剂中的α-螺旋氢键阵列的稳定性。对于PLA> 50%范围内的共聚物，观察到螺旋方向的尖锐S形转变。对部分氘代样品在无规液晶环境中的^2 H NMR研究表明，热诱导的螺旋-有义反转沿α-螺旋主链高度协同地沿着发生。这些观测结果强烈表明，从一个终端到另一个终端的拉链式传播过程。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几乎相同的液晶状态中获得的那些，这表明在两种介质中的转变机制保持或多或少相同。在齐姆布拉格概念内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.