Department of Chemical and Biomolecular Engineering and The Institute of NanoBioTechnology, Johns Hopkins University, Baltimore, Maryland 21218, USA.
J Chem Phys. 2011 Mar 28;134(12):124514. doi: 10.1063/1.3572058.
We assess the contribution of each coordination state to the hydration free energy of a distinguished water molecule, the solute water. We define a coordination sphere, the inner-shell, and separate the hydration free energy into packing, outer-shell, and local, solute-specific (chemical) contributions. The coordination state is defined by the number of solvent water molecules within the coordination sphere. The packing term accounts for the free energy of creating a solute-free coordination sphere in the liquid. The outer-shell contribution accounts for the interaction of the solute with the fluid outside the coordination sphere and it is accurately described by a Gaussian model of hydration for coordination radii greater than the minimum of the oxygen-oxygen pair-correlation function: theory helps identify the length scale to parse chemical contributions from bulk, nonspecific contributions. The chemical contribution is recast as a sum over coordination states. The nth term in this sum is given by the probability p(n) of observing n water molecules inside the coordination sphere in the absence of the solute water times a factor accounting for the free energy, W(n), of forming an n-water cluster around the solute. The p(n) factors thus reflect the intrinsic properties of the solvent while W(n) accounts for the interaction between the solute and inner-shell solvent ligands. We monitor the chemical contribution to the hydration free energy by progressively adding solvent ligands to the inner-shell and find that four-water molecules are needed to fully account for the chemical term. For a chemically meaningful coordination radius, we find that W(4) ≈ W(1) and thus the interaction contribution is principally accounted for by the free energy for forming a one-water cluster, and intrinsic occupancy factors alone account for over half of the chemical contribution. Our study emphasizes the need to acknowledge the intrinsic solvent properties in interpreting the hydration structure of any solute, with particular care in cases where the solute-solvent interaction strength is similar to that between the solvent molecules.
我们评估每个配位状态对特定水分子（即溶质水分子）水合自由能的贡献。我们定义了一个配位球，即内壳层，并将水合自由能分为包装、外壳层和局部、溶质特定（化学）贡献。配位状态由配位球内溶剂水分子的数量定义。包装项用于描述在液体中创建无溶质配位球的自由能。外壳层贡献用于描述溶质与配位球外流体的相互作用，对于配位半径大于氧-氧对相关函数最小值的情况，它可以通过水化的高斯模型准确描述：理论有助于确定将化学贡献与总体、非特异性贡献区分开来的长度尺度。化学贡献被重新表述为配位状态的和。这个和中的第 n 项由在没有溶质水分子的情况下观察到 n 个水分子在配位球内的概率 p(n)乘以一个考虑自由能 W(n)的因子给出，W(n)用于描述在溶质周围形成 n 个水分子簇的自由能。因此，p(n)因子反映了溶剂的固有性质，而 W(n)则反映了溶质与内壳层溶剂配体之间的相互作用。我们通过逐步向内壳层添加溶剂配体来监测水合自由能的化学贡献，并发现需要四个水分子才能完全解释化学项。对于具有化学意义的配位半径，我们发现 W(4) ≈ W(1)，因此相互作用贡献主要由形成一水簇的自由能和固有占据因子来解释，它们单独解释了超过一半的化学贡献。我们的研究强调了在解释任何溶质的水合结构时需要承认固有溶剂性质的重要性，特别是在溶质-溶剂相互作用强度与溶剂分子相似的情况下。
