Transition path sampling of water exchange rates and mechanisms around aqueous ions.
Transition path sampling of water exchange rates and mechanisms around aqueous ions.
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水交换率和水离子周围机制的过渡路径采样。
DOI：
10.1063/1.3224737
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发表时间：
2009
期刊：
影响因子：
--
通讯作者：
K. Rosso
中科院分区：
文献类型：
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作者：
S. Kerisit;K. Rosso
本文用过渡路径取样法测定了Na（+）和Fe（2+）两种水溶液离子的水交换速率和机理。特别地，计算水交换速率的压力依赖性以确定活化体积。一种常用的方法来计算水交换率，反应通量法，也被采用，并比较了两种方法。Na（+）周围的水交换速率足够快，可以通过直接分子动力学模拟来计算，从而为比较提供参考。这两种方法预测的汇率和活化量与直接模拟结果一致。考虑了四个额外的钠电位模型，以将本工作的结果与先前从分子模拟中确定的Na（+）的唯一活化体积进行比较[D. Spangberg等人，化学物理快报276，114（1997）]，并基于模型再现钠离子水溶液的已知性质的能力提供活化体积的最佳可能估计。Spangberg和Hermansson [D. Spangberg和K. 120，4829（2004）]和X-Plor/Charmm-22 [M. Patra和M. Karttunen，J. 25，678（2004）]模型表现最好，预测活化体积分别为-0.22和-0.78 cm（3）mol（-1）。对于Fe（2+）周围的水交换，过渡路径取样预测活化体积为+3.8 cm（3）mol（-1），与现有的实验数据非常一致。然而，在反应通量方法的平均力计算的潜力，未能充分采样适当的过渡途径和相反的压力依赖性的速率预测的结果。用过渡路径取样法分析得到的反应轨迹表明，Fe（2+）交换反应是通过缔合交换机制进行的，这与传统的正活化体积的机理解释相悖。总的来说，获得了相当多的见解，不仅为Na（+）和Fe（2+）的汇率和机制，但也为确定最强大的建模策略，为这些目的。
The rates and mechanisms of water exchange around two aqueous ions, namely, Na(+) and Fe(2+), have been determined using transition path sampling. In particular, the pressure dependence of the water exchange rates was computed to determine activation volumes. A common approach for calculating water exchange rates, the reactive flux method, was also employed and the two methods were compared. The water exchange rate around Na(+) is fast enough to be calculated by direct molecular dynamics simulations, thus providing a reference for comparison. Both approaches predicted exchange rates and activation volumes in agreement with the direct simulation results. Four additional sodium potential models were considered to compare the results of this work with the only activation volume for Na(+) previously determined from molecular simulation [D. Spangberg et al., Chem. Phys. Lett. 276, 114 (1997)] and provide the best possible estimate of the activation volume based on the ability of the models to reproduce known properties of the aqueous sodium ion. The Spangberg and Hermansson [D. Spangberg and K. Hermansson, J. Chem. Phys. 120, 4829 (2004)] and X-Plor/Charmm-22 [M. Patra and M. Karttunen, J. Comput. Chem. 25, 678 (2004)] models performed best and predicted activation volumes of -0.22 and -0.78 cm(3) mol(-1), respectively. For water exchange around Fe(2+), transition path sampling predicts an activation volume of +3.8 cm(3) mol(-1), in excellent agreement with the available experimental data. The potential of mean force calculation in the reactive flux approach, however, failed to sufficiently sample appropriate transition pathways and the opposite pressure dependence of the rate was predicted as a result. Analysis of the reactive trajectories obtained with the transition path sampling approach suggests that the Fe(2+) exchange reaction takes place via an associative interchange mechanism, which goes against the conventional mechanistic interpretation of a positive activation volume. Collectively, considerable insight was obtained not only for the exchange rates and mechanisms for Na(+) and Fe(2+) but also for identifying the most robust modeling strategy for these purposes.