Minimum Energy Pathways for Proton Transfer between Adjacent Sites Exposed to Water

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• 作者：Ran Friedman ; Stefan Fischer ; Esther Nachliel ; Steve Scheiner ; Menachem Gutman
• 刊名：Journal of Physical Chemistry B
• 出版年：2007
• 出版时间：May 31, 2007
• 年：2007
• 卷：111
• 期：21
• 页码：6059 - 6070
• 全文大小：594K
• 年卷期：v.111,no.21(May 31, 2007)
• ISSN：1520-5207

文摘

The capacity to transfer protons between surface groups is an innate property of many proteins. The transferof a proton between donor and acceptor, located as far as 6-7 Å apart, necessitates the participation of watermolecules in the process. In a previous study we investigated the mechanism of proton transfer (PT) betweenbulk exposed sites, a few ångströms apart, using as a model the proton exchange between the proton-bindingsites of the fluorescein molecule in dilute aqueous solution.¹ The present study expands the understanding ofPT reactions between adjacent sites exposed to water through the calculation the minimum energy pathways(MEPs) by the conjugate peak refinement algorithm² and a quantum-mechanical potential. The PT reactiontrajectories were calculated for the fluorescein system with an increasing number of water molecules. TheMEP calculations reveal that the transition state is highly strained and involves a supramolecular structure inwhich fluorescein and the interconnecting water molecules are covalently bonded together and the protonsare shared between neighboring oxygens. These findings are in accord with the high activation energy, asmeasured for the reaction, and indicate that PT reactions on the surface proceed by a semi- or fully concertedrather than stepwise mechanism. A similar mechanism is assumed to be operative on the surface of proteinsand renders water-mediated PT reactions as highly efficient as they are.