Specificity in Cationic Interaction with Poly(N-isopropylacrylamide)

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• 作者：Hongbo Du ; Sumith Ranil Wickramasinghe ; Xianghong Qian
• 刊名：The Journal of Physical Chemistry B
• 出版年：2013
• 出版时间：May 2, 2013
• 年：2013
• 卷：117
• 期：17
• 页码：5090-5101
• 全文大小：793K
• 年卷期：v.117,no.17(May 2, 2013)
• ISSN：1520-5207

文摘

Classical molecular dynamics (MD) simulations were conducted for PNIPAM in 1 M monovalent alkali chloride salt solutions as well as in 0.5 M divalent Mg²⁺ and Ca²⁺ chloride salt solutions. It was found that the strength for the direct alkali ion鈥揳mide O binding is strongly correlated with the size of the ionic radius. The smallest Li⁺ ion binds strongest to amide O, and the largest Cs⁺ ion has the weakest interaction with the amide bond. For the divalent Mg²⁺ and Ca²⁺ ions, their interactions with the amide bond are weak and appear to be mediated by the water molecules, particularly in the case of Mg²⁺, resulting from their strong hydration. The direct binding between the cations and amide O requires partial desovlation of the ions that is energetically unfavorable for Mg²⁺ and also to a great extent for Ca²⁺. The higher cation charge makes the electrostatic interaction more favorable but the dehydration process less favorable. This competition between electrostatic interaction and the dehydration process largely dictates whether the direct binding between the cation and amide O is energetically preferred or not. For monovalent alkali ions, it is energetically preferred to bind directly with the amide O. Moreover, Li⁺ ion is also found to associate strongly with the hydrophobic residues on PNIPAM.