Beckstein Oliver, Tai Kaihsu, Sansom Mark S P
Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, UK.
J Am Chem Soc. 2004 Nov 17;126(45):14694-5. doi: 10.1021/ja045271e.
A hydrophobic pore of subnanometer dimensions can appear impermeable to an ion even though its radius is still much wider than that of the ion. Pores of molecular dimensions can be found, for instance, in carbon nanotubes, zeolites, or ion channel proteins. We quantify this barrier to ion permeation by calculating the potential of mean force from umbrella-sampled molecular dynamics simulations and compare them to continuum-electrostatic Poisson-Boltzmann calculations. The latter fail to describe the ion barrier because they do not account for the properties of water in the pore. The barrier originates from the energetic cost to desolvate the ion. Even in wide pores, which could accommodate an ion and its hydration shell, a barrier of several kT remains because the liquid water phase is not stable in the hydrophobic pore. Thus, the properties of the solvent play a crucial role in determining permeation properties of ions in confinement at the molecular scale.
即使亚纳米尺寸的疏水孔半径仍比离子半径宽得多，它对离子来说可能仍是不可渗透的。例如，在碳纳米管、沸石或离子通道蛋白中可以发现分子尺寸的孔。我们通过从伞形采样分子动力学模拟计算平均力势来量化这种离子渗透屏障，并将其与连续静电泊松 - 玻尔兹曼计算进行比较。后者无法描述离子屏障，因为它们没有考虑孔中水分子的性质。这种屏障源于使离子去溶剂化的能量成本。即使在能够容纳离子及其水合壳的宽孔中，由于疏水孔中的液态水相不稳定，仍存在几kT的屏障。因此，溶剂的性质在决定分子尺度受限环境中离子的渗透性质方面起着关键作用。
