Iizuka E
Institute of High Polymer Research, Faculty of Textile Science and Technology, Shinshu University, Nagano, Japan.
Adv Biophys. 1988;24:1-56. doi: 10.1016/0065-227x(88)90003-2.
Nematic liquid-crystalline solutions of polypeptides such as PBLG and PELG orient in the direction of a very low static electric field as if the polypeptide molecules were a collection of rod-like molecular clusters having a huge, permanent electric dipole moment. This dipole moment originates from fluctuation of the distribution of polymer helices in both directions of the rod-like molecular cluster with roughly equal likelihood. The molecular cluster of PBLG is estimated to be composed of some 5 x 10(5) molecules and has dimensions of a few to several micrometers x 0.1 microns phi in methylene bromide when the degree of polymerization (mean) is 650. The nematic solutions also orient in the direction of a static magnetic field. The peptide groups of the polypeptide form planes which are parallel to the polymer helix and the magnetic-field orientation is due to the demagnetizing field produced in the peptide groups perpendicular to their planes by the ring current of the pi electrons. The formation of rod-like molecular clusters is absolutely necessary to invite the magnetic-field orientation as in the electric-field orientation. The anisotropy of the magnetic susceptibility has been estimated to be on the order of 10(-8) emu/g, and the twist elastic constant of the cholesteric structure on the order of 10(-7) dyn/cm. Derivatives of PBLG with long alkyl chains as pendant groups assume thermotropic liquid crystals, in which melted alkyl chains behave as a solvent in a similar manner as in lyotropic liquid crystals of the parent PBLG. An analogy of thermotropic liquid crystals of the modified polypeptides with those of discotic liquid crystals in the columnar phase has been suggested. Concentrated solutions of a double-stranded helix of poly(A).poly(U) and a triple-stranded helix of poly(A).2poly(U) form liquid crystals both of the nematic and the cholesteric types. The nematic solutions of these polyribonucleotide complexes orient perpendicular to the direction of a static magnetic field owing to the diamagnetic dipole moments induced in the purine or pyrimidine bases of the polyribonucleotides perpendicular to their planes, which are roughly perpendicular to the polymer helices. Any rod-like polymer has a good chance to form liquid crystals in which the occurrence of forces acting among the molecules is not necessary. The PLGA-Na in a neutral aqueous solution undergoes a "coil-to-alpha helix transition" when the polymer concentration is increased.(ABSTRACT TRUNCATED AT 400 WORDS)
诸如聚γ-苄基-L-谷氨酸酯（PBLG）和聚γ-乙基-L-谷氨酸酯（PELG）等多肽的向列型液晶溶液会在非常低的静电场方向上取向，就好像多肽分子是具有巨大永久电偶极矩的棒状分子簇的集合。这个偶极矩源于聚合物螺旋在棒状分子簇的两个方向上分布的涨落，其可能性大致相等。当聚合度（平均值）为650时，PBLG的分子簇估计由约5×10⁵个分子组成，在二溴甲烷中的尺寸为几到几微米×0.1微米（直径）。向列型溶液也会在静磁场方向上取向。多肽的肽基形成与聚合物螺旋平行的平面，磁场取向是由于π电子的环电流在垂直于肽基平面的肽基中产生的退磁场。如同在电场取向中一样，形成棒状分子簇对于引发磁场取向是绝对必要的。磁化率的各向异性估计在10⁻⁸emu/g量级，胆甾型结构的扭曲弹性常数在10⁻⁷dyn/cm量级。带有长烷基链作为侧基的PBLG衍生物呈现热致液晶，其中熔化的烷基链起到溶剂的作用，其方式与母体PBLG的溶致液晶类似。有人提出将改性多肽的热致液晶与柱状相中的盘状液晶进行类比。聚（A）·聚（U）的双链螺旋和聚（A）·2聚（U）的三链螺旋的浓溶液形成向列型和胆甾型液晶。这些聚核糖核苷酸复合物的向列型溶液由于在聚核糖核苷酸的嘌呤或嘧啶碱基中垂直于其平面（大致垂直于聚合物螺旋）感应出的抗磁偶极矩而垂直于静磁场方向取向。任何棒状聚合物都很有可能形成液晶，其中分子间作用力的出现并非必要条件。当聚合物浓度增加时，中性水溶液中的聚乳酸-羟基乙酸共聚物钠（PLGA-Na）会发生“线团到α螺旋转变”。（摘要截于400字）
