The mechanism of hydrated proton transport in water
The mechanism of hydrated proton transport in water
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DOI：
10.1021/ja002506n
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发表时间：
2000-12-06
影响因子：
15
通讯作者：
Voth, GA
中科院分区：
文献类型：
--
作者：
Day, TJF;Schmitt, UW;Voth, GA
质子输运（PT）是一个对于化学、物理和生物化学领域至关重要的过程。液态水中的质子表现出比其他简单单价阳离子大得多的迁移率。这种异常高的扩散速率通常归因于 Grötthus 机制，通过该机制，水合氢物质 (H3O+) 能够将质子穿梭到其第一个溶剂化壳层中的水分子，然后水分子又可以穿梭到另一个分子。通过这种机制传输，过量的质子能够以远大于传统扩散的速率扩散到整个水的氢键网络。该过程的机理以及限速步骤是人们相当感兴趣的话题。其他地方已经表明1-4，水性体系中水合氢物质的溶剂化状态与简单水分子的溶剂化状态有很大不同。特别是，在分子动力学模拟中观察到的水合氢的配位数通常为 3.0，而散装水的配位数为 3.9。 1-4 因此，当质子转移到受体水分子时，局部氢键网络必须发生显着的重排。即，人们期望观察到受体氧位点上的氢键断裂，以及随后在供体氧上的氢键形成。据推测 5, 6 PT 机制的实际限速步骤涉及水合氢第二溶剂化壳中氢键的断裂，从而允许发生质子转移。这种转移之后立即在供体氧上形成氢键。 Agmon, 5 以及后来的 Vuilleumier 和 Borgis, 4 由于摩西分开红海的类比，将此称为“摩西机制”。确实已经发现了这种机制的一些证据4，尽管结果还不是决定性的。当前工作的目标是进一步阐明 PT 机制。水溶液中过量质子的处理需要用于 PT 过程的定量准确的势能表面 (PES)。虽然各种成熟的电子结构方法能够以合理的精度生成该表面，但这些方法对数值要求较高，因此在研究长时间尺度的凝聚相系统时并不总是具有实际用途。然而，最初由 Coulson7 和 Mulliken8 首创并由 Warshel, 9, 10 进一步发展的经验价键 (EVB) 方法已成功应用于化学反应的研究。这种方法论
Proton transport (PT) is a process of fundamental importance to the fields of chemistry, physics, and biochemistry. Protons in liquid water exhibit mobilities which are considerably greater than those of other simple monovalent cations. This anomalously high diffusion rate is most often attributed to the Grötthus mechanism, by which the hydronium species (H3O+) is able to shuttle a proton to a water molecule in its first solvation shell, which can then in turn shuttle to yet another molecule. By transporting via this mechanism, an excess proton is able to diffuse throughout the entire hydrogen bond network of water at a rate considerably greater than that of conventional diffusion. The mechanism, as well as the rate-limiting step, of this process is a topic of considerable interest.It has been shown elsewhere1-4 that the solvation state of the hydronium species in aqueous systems is considerably different from that of a simple water molecule. In particular, the coordination number of hydronium observed in molecular dynamics simulations is typically 3.0, as compared to 3.9 for bulk water. 1-4 Consequently, as the proton is transferred to a recipient water molecule, a significant rearrangement of the local hydrogen bond network must occur. Namely, one would expect to observe hydrogen bond cleavage on the recipient oxygen site, as well as subsequent hydrogen bond formation on the donor oxygen. It has been speculated5, 6 that the actual rate-limiting step of the PT mechanism involves this breaking of hydrogen bonds in the second solvation shell of the hydronium, which then allows the proton transfer to occur. This transfer is then immediately followed by hydrogen bond formation on the donor oxygen. Agmon, 5 and later Vuilleumier and Borgis, 4 referred to this as the “Moses mechanism” due to the analogy of Moses parting the Red Sea. Some evidence of this mechanism has indeed been found, 4 although the results were not conclusive. It is the goal of the current work to shed further light on the PT mechanism. The treatment of an excess proton in aqueous solution requires a quantitatively accurate potential energy surface (PES) for the PT process. While the various well-established electronic structure methods are able to generate this surface with reasonable accuracy, these methods are numerically demanding and thus not always of practical use when studying condensed-phase systems over long time scales. However, the Empirical Valence Bond (EVB) methodology, originally pioneered by Coulson7 and Mulliken8 and further developed by Warshel, 9, 10 has been successfully employed in the study of chemical reactions. This methodology