Molecular Mechanism for the Interactions of Hofmeister Cations with Macromolecules in Aqueous Solution
Molecular Mechanism for the Interactions of Hofmeister Cations with Macromolecules in Aqueous Solution
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DOI：
10.1021/jacs.0c07214
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发表时间：
2020-11-11
影响因子：
15
通讯作者：
Cremer, Paul S.
中科院分区：
文献类型：
--
作者：
Bruce, Ellen E.;Okur, Halil, I;Cremer, Paul S.
离子的一致性和浓度影响大分子的溶解度。到目前为止，人们一直致力于在分子水平上了解阴离子的特殊作用。相比之下，阳离子的作用受到的关注明显较少，阳离子与大分子相互作用的潜在机制仍然难以捉摸。为了解决这个问题，研究了侧链上带有酰胺基团的热响应性聚合物聚N-异丙基丙烯酰胺在含有9种不同阳离子氯化物的水溶液中的溶解度随盐浓度的变化。相变温度测量结果与分子动力学模拟结果相关联。结果表明，虽然所有阳离子平均从大分子/水界面上被耗尽，但更强的水合阳离子能够在酰胺氧周围局部积累。这些微弱的有利相互作用帮助部分抵消了盐析效应。此外，在溶剂共享离子对中，阳离子与氯反离子一起靠近界面。由于离子配对是浓度依赖的，随着盐浓度的增加，对主导盐析效应的缓解作用变得更大。弱水合阳离子表现出较少的离子配对倾向和较弱的酰胺氧亲和力。因此，即使在高盐浓度下，对这些离子的净盐析效应的缓解也要小得多。
Ion identity and concentration influence the solubility of macromolecules. To date, substantial effort has been focused on obtaining a molecular level understanding of specific effects for anions. By contrast, the role of cations has received significantly less attention and the underlying mechanisms by which cations interact with macromolecules remain more elusive. To address this issue, the solubility of poly(N-isopropylacrylamide), a thermoresponsive polymer with an amide moiety on its side chain, was studied in aqueous solutions with a series of nine different cation chloride salts as a function of salt concentration. Phase transition temperature measurements were correlated to molecular dynamics simulations. The results showed that although all cations were on average depleted from the macromolecule/water interface, more strongly hydrated cations were able to locally accumulate around the amide oxygen. These weakly favorable interactions helped to partially offset the salting-out effect. Moreover, the cations approached the interface together with chloride counterions in solvent-shared ion pairs. Because ion pairing was concentration-dependent, the mitigation of the dominant salting-out effect became greater as the salt concentration was increased. Weakly hydrated cations showed less propensity for ion pairing and weaker affinity for the amide oxygen. As such, there was substantially less mitigation of the net salting-out effect for these ions, even at high salt concentrations.