Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, USA.
J Phys Chem B. 2013 Jun 27;117(25):7626-52. doi: 10.1021/jp302587d. Epub 2013 Jun 18.
A macromolecule in a gradient of a cosolute that is preferentially (relative to the solvent) either attracted to or excluded from the domain of the macromolecule should experience a thermodynamic force and move, respectively, up or down the gradient. A theory of chemotactic forces arising from such preferential interactions, especially short-range ligand binding and excluded volume interactions, is developed via an extension of Kirkwood-Buff theory. The ligand binding result is confirmed for both non-ionic and ionic cosolutes by standard solution thermodynamics. The effect of increasing the electrolyte concentration to diminish the electrostatic free energy of a charged macromolecule is also treated formally via an electrostatic macromolecule-electrolyte preferential interaction coefficient. For short-range interactions, the induced chemotactic velocity is attributed entirely to tangential tractions at the interface between the macromolecule and its surrounding solution. The velocity of a spherical macromolecule driven by such tractions is derived by a hydrodynamic calculation for steady-state creepy flow with a partial slip boundary condition. Qualitative comparisons of theoretical predictions with experimental observations of Zheng and Pollack pertaining to charged microspheres near the surfaces of non-ionic gels suggest that the reported exclusion zones are due to chemotaxis induced by gradients of base (NaOH) (or acid (HCl)) and salt. With a single adjustable parameter, namely, the ratio of slip length to area per surface carboxyl (or amidine) group, this theory yields nearly quantitative agreement with many observations. The estimated slip length for the microspheres is comparable to that obtained for bovine serum albumen by fitting the chemotactic theory to two reported cross-diffusion coefficients. When a solution with a gradient of NaOH is placed in contact with a smooth glass wall, chemotactic surface tractions are predicted to cause convection of the solution toward the acidic end of the gradient, as observed in preliminary experiments.
在优先（相对于溶剂）被吸引或排斥到大分子域的共溶剂的浓度梯度中，大分子应该会受到热力学力的作用，并分别向上或向下移动梯度。通过扩展 Kirkwood-Buff 理论，开发了一种源自这种优先相互作用（尤其是短程配体结合和排除体积相互作用）的趋化力理论。通过标准溶液热力学证实了非离子和离子共溶剂的配体结合结果。还通过静电大分子-电解质优先相互作用系数正式处理了增加电解质浓度以减小带电大分子的静电自由能的影响。对于短程相互作用，感应趋化速度完全归因于大分子与其周围溶液之间界面上的切向牵引力。通过对具有部分滑移边界条件的稳态蠕动流的水动力计算，推导出由这种牵引力驱动的球形大分子的速度。理论预测与 Zheng 和 Pollack 关于非离子凝胶表面附近带电微球的实验观察之间的定性比较表明，报道的排除区是由于由碱（NaOH）（或酸（HCl））和盐的梯度诱导的趋化作用引起的。通过一个单一的可调参数，即滑移长度与每个表面羧基（或脒基）基团的面积之比，该理论与许多观察结果几乎完全一致。对于微球，估计的滑移长度与通过将趋化理论拟合到两个报道的扩散系数来拟合牛血清白蛋白的滑移长度相当。当将具有 NaOH 梯度的溶液与光滑的玻璃壁接触时，预测趋化表面牵引力会导致溶液向梯度的酸性端对流，如初步实验观察到的那样。
