Specific ion effects on the solution conformation of poly‐L‐proline
Specific ion effects on the solution conformation of poly‐L‐proline
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特定离子对聚-L-脯氨酸溶液构象的影响
DOI：
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发表时间：
1969
期刊：
影响因子：
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通讯作者：
P. V. von Hippel
中科院分区：
文献类型：
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作者：
T. Schleich;P. V. von Hippel
利用旋光技术研究了不同电解质对聚L -脯氨酸II在水溶液和非水溶液中构象的影响。结果表明，随着盐浓度的增加，这些试剂诱导聚L -脯氨酸的校正比左旋率呈线性下降，其摩尔效应因盐的不同而不同。盐诱导的旋光度变化可分为阴离子和阳离子组分，结果表明，阴离子是主要的特异性影响因子，其降低校正特异性旋光度的有效性依次为Cl−< NO3−< Br−< I−< ClO4−< SCN−。无机一价阳离子（Li+, Na+, K+）对降低比左旋都有同样的效果。每摩尔Ca++的作用比无机一价阳离子略大，而有机阳离子的作用则明显小一些。四烷基铵阳离子比无机一价阳离子更能有效地降低左旋比，其摩尔效率随甲基加亚甲基总含量的增加而线性增加。虽然每摩尔CH2或CH3基团的影响比四烷基铵阳离子的影响要小得多，但脂肪醇也表现出类似的随甲基加亚甲基含量增加而线性增加的现象。溶解在本质上是无氢的盐对于酰亚胺也是相当有效的。选定的粘度实验也表明，观察到的对特定左旋的影响具有结构和光学基础。这些结果是根据一个模型来解释的，这个模型涉及到阴离子在亚胺氮上的结合，以及阳离子在羰基氧上的结合。有人提出，这种结合诱导了双键特性的增加；肽键（从而缩短键）大致与结合阴离子的极化率成正比，并且这种增加被降低总介电常数的阳离子增强（例如，四烷基铵系列），并被呈现竞争性局部阴离子结合位点的阳离子减少（例如，胍离子）。我们进一步提出，这种缩氨酸键的缩短伴随着相邻键的延长，从而减少了围绕该键旋转的空间约束（增加了角ψ的可达范围），足以诱导聚L -脯氨酸II结构的渐进非合作坍塌。提出了几条证据来支持这种解释。各种中性盐也显示出诱导聚L -脯氨酸从溶液中析出或“盐析”的时间依赖性。根据盐出剂的摩尔效率的递减顺序，各种离子的排列顺序为：SO4−> Ac−> Cl−> Br−> SCN−> I−> ClO4−；K+ + > Li+ > Ca++。这些排名遵循通常的Hofmeister或溶性系列，与适用于聚L -脯氨酸溶液构象影响的那些有很大不同。
The effects of various electrolytes on the conformation of poly‐L‐proline II in aqueous and nonaqueous solution have been investigated by optical rotatory techniques. It is shown that these agents induce a linear decrease in the corrected specific levorotaton of poly‐L‐proline with increasing salt concentration, with a molar effectiveness which varies from one salt to another. The salt‐induced rotatory changes may be divided into anion and cation components, and it is shown that the major specific affectors are the anions, which increase in effectiveness in reducing the corrected specific levorotation in the following sequence: Cl− < NO3− < Br− < I− < ClO4− < SCN−. The inorganic monovalent cations tested (Li+, Na+, K+) are all equally effective in decreasing the specific levorotation. Ca++ has a marginally greater effect per mole than the inorganic monovalent cations, while the effectiveness of the ganidinium cation is appreciably less. The tetraalkylammonium cations decrease the specific levorotation more effectively than the inorganic monovalent cations, with the molar effectiveness increasing linearly with total content of methyl plus methylene groups. A similar linear increase with increasing methyl plus methylene content is shown by the aliphatic alcohols, though the effect per mole of CH2 or CH3 group is appreciably smaller than that shown by the tetraaklylammonium cations. Salts dissolved in essentially anhydron for mamide are also appreciably effective. Selected viscosity experiments have also been carried out to show that the observed effects on specific levorotation have a structural as well as an optical basis. These results are interpreted in terms of a model which involves binding of anions at the imide nitrogen, and cations at the carbonyl oxygen. It is proposed that this binding induces an increase in the double‐bond character; of the peptide bond (and thus a shortening of the bond) which is roughly proportional in the polarizability of the bound anion and that this increase is potentiated by cations which decrease the total dielectric constant (e.g., the tetraalkylammonium series), and reduced by cations presenting competitive local anion binding sites (e.g., the guanidinium ion). We propose further that this shortening of the peptide bond is accompanied by a lengthening of the adjacent bond, thus reducing the steric restraints to rotation about this bond (increasing the accessible range of the angle ψ) sufficiently to induce a progressive non‐cooperative collapse of the poly‐L‐proline II structure. Several lines of evidence are presented to support this interpretation. The various neutral salts are also shown to induce a time‐dependent precipitation or “salting‐out” of poly‐L‐proline from solution. In order of decreasing molar effectiveness as salting‐out agents in this system, the various ions may be ranked: SO4− > Ac− > Cl− > Br− > SCN− > I− > ClO4−; and K+ ≃ Na+ > Li+ > Ca++. These rankings follow the usual Hofmeister or lyotropic series, and are quite different from hose which apply to the effects on solution conformation of poly‐L‐proline.