Galamba N
Grupo de Física-Matemática da Universidade de Lisboa , Av. Prof. Gama Pinto 2, 1649-003 Lisboa, Portugal.
J Phys Chem B. 2014 Apr 17;118(15):4169-76. doi: 10.1021/jp500067a. Epub 2014 Apr 4.
Despite being intensively studied, the magnitude of specific structural and dynamic perturbations of water next to hydrophobic surfaces remains a matter of debate. Here we show, from molecular dynamics, that the structure of a subset of water molecules in the first hydration layer, those preserving four nearest water neighbors, resembles that of water at ∼10 °C, and that the origin of the orientational slowdown is mainly a decrease of the hydrogen-bond (HB) acceptor switch frequency, while water structuring plays a minor role, slightly accelerating HB acceptor switches. By portraying the mean HB dynamics of water as a doubly periodic event, we demonstrate that the orientational retardation factor is effectively defined by the ratio of the HB acceptor switch period in the hydration layer and bulk. Excluded volume delays HB acceptor switches, accelerating the orientational relaxation of ∼1/3 of the water molecules on the hydration layer in this time scale, but this is largely exceeded by the decrease of the HB switch frequency, consistent with 2D IR spectroscopy experiments, and at the origin of longer HB lifetimes. The orientational mobility of water populations with long HB lifetimes is also probed, and although a relaxation plateau is observed at ∼10 ps consistent with fs IR spectroscopy experiments, no water molecule is rotationally frozen at any time scale. The proposed molecular picture is consistent with fs IR, 2D IR, and NMR experimental results on the orientational retardation of water and reveals the magnitude of "hidden" enhanced ordered water pentamers formed near hydrophobic solutes.
尽管已经进行了深入研究,但疏水表面附近水的特定结构和动态扰动的程度仍存在争议。在此,我们通过分子动力学表明,第一水化层中一部分水分子(即那些保留四个最近邻水分子的水分子)的结构类似于约10°C时水的结构,并且取向减慢的起源主要是氢键(HB)受体开关频率的降低,而水的结构化作用较小,只是略微加速了HB受体开关。通过将水的平均HB动力学描绘为一个双周期事件,我们证明取向延迟因子实际上由水化层和本体中HB受体开关周期的比率定义。排除体积会延迟HB受体开关,在这个时间尺度上加速水化层中约1/3水分子的取向弛豫,但这在很大程度上被HB开关频率的降低所超过,这与二维红外光谱实验一致,并且是更长HB寿命的起源。我们还探测了具有长HB寿命的水群体的取向迁移率,尽管在约10皮秒处观察到一个弛豫平台,这与飞秒红外光谱实验一致,但在任何时间尺度上都没有水分子发生旋转冻结。所提出的分子图景与关于水取向延迟的飞秒红外、二维红外和核磁共振实验结果一致,并揭示了在疏水溶质附近形成的“隐藏”增强有序水五聚体的程度。