Englert A, Mandel M, Van Nypelseer A
Macromolecules. 1975 Sep-Oct;8(5):655-60. doi: 10.1021/ma60047a016.
The Zimm and Bragg parameter sigma is calculated numerically for poly(L-alanine), polyglycine, and the copolymers of L-alanine and glycine using the molecular theory of s and sigma as developed by Go, Go, and Scheraga in a modified formulation. In this formulation, sigma is obtained from the partition function of the whole chain in the helix-coil transition region and represents therefore the contributions from the ends of helical and coil sequences and from the interactions between atoms in a coil sequence with those in the neighboring helical sequence. When the parameter sigma is calculated numerically from a hard-sphere potential, it appears that steric intractions between atoms in the coil sequence with atoms in the neighboring helical sequence, which have been neglected in previous calculations, contribute significantly to the value of sigma. Owing to these interactions the entropy of the coil sequence as well as sigma decrease, but the decrease of sigma is larger in poly(L-alanine) than in polyglycine, because of the higher flexibility of the monomer in polyglycine. The numerical value of sigma for polyglycine compared with that of poly(L-alanine) might be overestimated however by the model presented here due to approximations inherent in the hard-sphere treatment and because only regular helical sequences are considered.
使用Go、Go和Scheraga提出的分子理论的改进公式，对聚（L-丙氨酸）、聚甘氨酸以及L-丙氨酸和甘氨酸的共聚物，以数值方式计算齐姆和布拉格参数σ。在该公式中，σ由螺旋-线团转变区域中整个链的配分函数得出，因此代表螺旋和线团序列末端的贡献以及线团序列中原子与相邻螺旋序列中原子之间相互作用的贡献。当从硬球势以数值方式计算参数σ时，似乎在先前计算中被忽略的线团序列中原子与相邻螺旋序列中原子之间的空间相互作用，对σ的值有显著贡献。由于这些相互作用，线团序列的熵以及σ都降低，但聚（L-丙氨酸）中σ的降低比聚甘氨酸中更大，这是因为聚甘氨酸中单体的柔韧性更高。然而，由于硬球处理中固有的近似性以及仅考虑了规则螺旋序列，这里给出的模型可能高估了聚甘氨酸与聚（L-丙氨酸）相比的σ数值。
