Dzubiella J, Hansen J-P
University Chemical Laboratory, Lensfield Road, Cambridge CB2 1EW, United Kingdom.
J Chem Phys. 2005 Jun 15;122(23):234706. doi: 10.1063/1.1927514.
The permeation of hydrophobic, cylindrical nanopores by water molecules and ions is investigated under equilibrium and out-of-equilibrium conditions by extensive molecular-dynamics simulations. Neglecting the chemical structure of the confining pore surface, we focus on the effects of pore radius and electric field on permeation. The simulations confirm the intermittent filling of the pore by water, reported earlier under equilibrium conditions for pore radii larger than a critical radius R(c). Below this radius, water can still permeate the pore under the action of a strong electric field generated by an ion concentration imbalance at both ends of the pore embedded in a structureless membrane. The water driven into the channel undergoes considerable electrostriction characterized by a mean density up to twice the bulk density and by a dramatic drop in dielectric permittivity which can be traced back to a considerable distortion of the hydrogen-bond network inside the pore. The free-energy barrier to ion permeation is estimated by a variant of umbrella sampling for Na(+), K(+), Ca(2+), and Cl(-) ions, and correlates well with known solvation free energies in bulk water. Starting from an initial imbalance in ion concentration, equilibrium is gradually restored by successive ion passages through the water-filled pore. At each passage the electric field across the pore drops, reducing the initial electrostriction, until the pore, of radius less than R(c), closes to water and hence to ion transport, thus providing a possible mechanism for voltage-dependent gating of hydrophobic pores.
通过广泛的分子动力学模拟，研究了水分子和离子在平衡和非平衡条件下对疏水性圆柱形纳米孔的渗透情况。忽略限制孔表面的化学结构，我们重点关注孔径和电场对渗透的影响。模拟结果证实了水对孔的间歇性填充，这在早期关于平衡条件下孔径大于临界半径R(c)的报道中已有提及。低于此半径时，在嵌入无结构膜中孔两端离子浓度失衡产生的强电场作用下，水仍可渗透过孔。进入通道的水会经历显著的电致收缩，其特征是平均密度高达体密度的两倍，且介电常数急剧下降，这可追溯到孔内氢键网络的显著扭曲。通过对Na(+)、K(+)、Ca(2+)和Cl(-)离子采用伞形抽样的变体方法估计离子渗透的自由能垒，其与已知的本体水中溶剂化自由能相关性良好。从离子浓度的初始失衡开始，通过水填充的孔的连续离子通过逐渐恢复平衡。每次通过时，孔两端的电场下降，减少初始电致收缩，直到半径小于R(c)的孔对水关闭，从而对离子传输关闭，因此为疏水性孔的电压依赖性门控提供了一种可能的机制。
