Solid State and Structural Chemistry Unit, Indian Institute of Science, Bengaluru 560012, India.
J Chem Phys. 2017 Jul 14;147(2):024901. doi: 10.1063/1.4990693.
Since the pioneering works of Pethig, Grant, and Wüthrich on a protein hydration layer, many studies have been devoted to find out if there are any "general and universal" characteristic features that can distinguish water molecules inside the protein hydration layer from bulk. Given that the surface itself varies from protein to protein, and that each surface facing the water is heterogeneous, search for universal features has been elusive. Here, we perform an atomistic molecular dynamics simulation in order to propose and demonstrate that such defining characteristics can emerge if we look not at average properties but the distribution of relaxation times. We present results of calculations of distributions of residence times and rotational relaxation times for four different protein-water systems and compare them with the same quantities in the bulk. The distributions in the hydration layer are unusually broad and log-normal in nature due to the simultaneous presence of peptide backbones that form weak hydrogen bonds, hydrophobic amino acid side chains that form no hydrogen bond, and charged polar groups that form a strong hydrogen bond with the surrounding water molecules. The broad distribution is responsible for the non-exponential dielectric response and also agrees with large specific heat of the hydration water. Our calculations reveal that while the average time constant is just about 2-3 times larger than that of bulk water, it provides a poor representation of the real behaviour. In particular, the average leads to the erroneous conclusion that water in the hydration layer is bulk-like. However, the observed and calculated lower value of static dielectric constant of hydration layer remained difficult to reconcile with the broad distribution observed in dynamical properties. We offer a plausible explanation of these unique properties.
自从 Pethig、Grant 和 Wüthrich 开创性地研究蛋白质水合层以来，许多研究致力于找出是否存在任何可以区分蛋白质水合层内水分子和本体水的“普遍和通用”特征。鉴于蛋白质表面本身因蛋白质而异，而且每个面向水的表面都是不均匀的，因此寻找普遍特征一直难以捉摸。在这里，我们进行了原子分子动力学模拟，以提出并证明，如果我们不仅关注平均性质，还关注弛豫时间分布，就可以出现这些定义特征。我们展示了四个不同蛋白质-水系统中停留时间和旋转弛豫时间分布的计算结果，并将其与本体中的相同数量进行了比较。由于同时存在形成弱氢键的肽骨架、不形成氢键的疏水性氨基酸侧链和与周围水分子形成强氢键的带电极性基团，水合层中的分布异常广泛且呈对数正态分布。这种广泛的分布是导致非指数介电响应的原因，也与水合水的高热容一致。我们的计算表明，虽然平均时间常数仅比本体水大 2-3 倍，但它对实际行为的表示很差。特别是，平均值导致错误的结论，即水合层中的水类似于本体水。然而，观察到的和计算得到的水合层静态介电常数值仍然难以与观察到的动态特性中的广泛分布相协调。我们提供了对这些独特性质的合理解释。
