Olmsted M C, Anderson C F, Record M T
Department of Chemistry, University of Wisconsin, Madison 53706.
Proc Natl Acad Sci U S A. 1989 Oct;86(20):7766-70. doi: 10.1073/pnas.86.20.7766.
Applications of the grand canonical Monte Carlo method demonstrate the importance of end effects on fundamental molecular and thermodynamic properties of oligoelectrolyte solutions. Simulations are carried out for a series of solutions containing double-helical DNA oligomers of varying numbers of phosphate charges N (8 less than or equal to N less than or equal to 100) and univalent electrolyte at fixed activity (a +/- = 1.76 mmol/dm3). These results are used to evaluate as follows: C+N(a), the local concentration of cations at various axial positions along the oligomer surface; C+N(a), the axial average of these concentrations; TN, the preferential interaction coefficient expressed per oligomer charge, which is directly related to the fractional thermodynamic extent of association of counterions. A sufficiently long oligomer (N greater than or equal to 48 under the conditions simulated) is characterized by an interior region over which C+N(a) is uniform and equal to C+ infinity (a), the polyion limit. This interior region is flanked by two symmetric terminal regions, in which C+N(a) varies linearly with axial position from the end of the oligomer to a distance approximately 18 monomer units (approximately 3.1 nm) from that end. For long oligomers, the characteristics of the terminal regions [length and axial profile of C+N(a)] do not vary with N and, by inference, also pertain to the polyion under the same conditions. Both C+N(a) and TN approach their polyelectrolyte limits as linear functions of 1/N. These linear dependences can be attributed to the increasing predominance of the contribution due to the polyion-like interior of the oligomer as N increases.
巨正则蒙特卡罗方法的应用表明了端效应对于寡电解质溶液基本分子和热力学性质的重要性。针对一系列含有不同磷酸电荷数N（8≤N≤100）的双螺旋DNA寡聚物以及固定活度（a± = 1.76 mmol/dm³）的单价电解质的溶液进行了模拟。这些结果用于如下评估：C⁺N(a)，沿寡聚物表面不同轴向位置处阳离子的局部浓度；C⁺N(a)，这些浓度的轴向平均值；TN，以每个寡聚物电荷表示的优先相互作用系数，它与抗衡离子缔合的分数热力学程度直接相关。在模拟条件下，足够长的寡聚物（N≥48）的特征在于其内部区域，在该区域内C⁺N(a)是均匀的且等于C∞(a)，即聚离子极限。这个内部区域两侧是两个对称的末端区域，其中C⁺N(a)从寡聚物末端到距该末端约18个单体单元（约3.1 nm）的距离内随轴向位置线性变化。对于长寡聚物，末端区域的特征[C⁺N(a)的长度和轴向分布]不随N变化，据此推断，在相同条件下也适用于聚离子。C⁺N(a)和TN都作为1/N的线性函数趋近于它们的聚电解质极限。这些线性依赖关系可归因于随着N增加，由于寡聚物类似聚离子的内部所产生的贡献的主导作用日益增强。
