Olmsted M C, Bond J P, Anderson C F, Record M T
Department of Chemistry, University of Wisconsin-Madison 53706.
Biophys J. 1995 Feb;68(2):634-47. doi: 10.1016/S0006-3495(95)80224-9.
Grand canonical Monte Carlo (GCMC) simulations are reported for aqueous solutions containing excess univalent salt (activities a +/- = 1.76-12.3 mM) and one of the following species: an octacationic rod-like ligand, L8+; a B-DNA oligomer with N phosphate charges (8 < or = N < or = 100); or a complex resulting from the binding of L8+ at the center of an N-mer (24 < or = N < or = 250). Simplified models of these multiply charged species are used in the GCMC simulations to predict the fundamental coulombic contributions to the following experimentally relevant properties: 1) the axial distance over which ligand binding affects local counterion concentrations at the surface of the N-mer; 2) the dependence on N of GCMC preferential interaction coefficients, gamma 32MC identical to delta C3/delta C2l a +/-, T, where C3 and C2 are, respectively, the molar concentrations of salt and the multiply charged species (ligand, N-mer or complex); and 3) the dependence on N of SaKobs identical to d in Kobs/d in a +/- = delta (magnitude of ZJ + 2 gamma 32J), where Kobs is the equilibrium concentration quotient for the binding of L8+ to the center of an N-mer and delta denotes the stoichiometric combination of terms, each of which pertains to a reactant or product J having magnitude of ZJ charges. The participation of electrolyte ions in the ligand binding interaction is quantified by the magnitude of SaKobs, which reflects the net (stoichiometrically weighted) difference in the extent of thermodynamic binding of salt ions to the products and reactants. Results obtained here from GCMC simulations yield a picture of the salient molecular consequences of binding a cationic ligand, as well as thermodynamic predictions whose applicability can be tested experimentally. Formation of the central complex is predicted to cause a dramatic reduction in the surface counterion (e.g., Na+) concentration over a region including but extending well beyond the location of the ligand binding site. For binding a cationic ligand, SaKobs is predicted to be negative, indicating net electrolyte ion release in the binding process. At small enough N, -SaKobs is predicted to decrease strongly toward zero with decreasing N. At intermediate N, -SaKobs appears to exceed its limiting value as N-->infinity.
报道了关于含有过量单价盐（活度(a_{\pm}=1.76 - 12.3) mM）以及以下物种之一的水溶液的巨正则蒙特卡罗（GCMC）模拟：一种八价阳离子棒状配体(L^{8 +})；一种带有(N)个磷酸电荷的B - DNA寡聚物（(8\leq N\leq100)）；或者由(L^{8 +})在(N)聚体中心结合形成的复合物（(24\leq N\leq250)）。在GCMC模拟中使用这些多价带电物种的简化模型来预测对以下实验相关性质的基本库仑贡献：1）配体结合影响(N)聚体表面局部反离子浓度的轴向距离；2）GCMC优先相互作用系数(\gamma_{32}^{MC}\equiv\frac{\Delta C_3}{\Delta C_2}\vert_{a_{\pm},T})对(N)的依赖性，其中(C_3)和(C_2)分别是盐和多价带电物种（配体、(N)聚体或复合物）的摩尔浓度；3）(S_{a}\frac{dK_{obs}}{da_{\pm}}\equiv\Delta(\vert Z_J\vert + 2\gamma_{32}^J))对(N)的依赖性，其中(K_{obs})是(L^{8 +})与(N)聚体中心结合的平衡浓度商，(\Delta)表示各项的化学计量组合，每个项对应于具有(Z_J)电荷大小的反应物或产物(J)。电解质离子在配体结合相互作用中的参与通过(S_{a}\frac{dK_{obs}}{da_{\pm}})的大小来量化，它反映了盐离子与产物和反应物的热力学结合程度的净（化学计量加权）差异。这里从GCMC模拟获得的结果描绘了结合阳离子配体的显著分子后果，以及其适用性可通过实验测试的热力学预测。预测中心复合物的形成会导致在包括但远超出配体结合位点位置的区域内表面反离子（例如(Na^+)）浓度急剧降低。对于结合阳离子配体，预测(S_{a}\frac{dK_{obs}}{da_{\pm}})为负，表明在结合过程中有净电解质离子释放。在足够小的(N)时，预测(-S_{a}\frac{dK_{obs}}{da_{\pm}})随着(N)的减小强烈趋向于零。在中等(N)时，(-S_{a}\frac{dK_{obs}}{da_{\pm}})似乎超过其(N\rightarrow\infty)时的极限值。
