Physics Department T37, Technical University Munich, 85748 Garching, Germany.
J Phys Chem B. 2010 May 27;114(20):7098-103. doi: 10.1021/jp1010814.
The action of NaCl vs KCl on the static and kinetic behavior of a fully charged and unfolded polyglutamic acid (PGA) chain is investigated by extensive explicit-water computer simulations. Ion-specific shrinking of the PGA coil size with increasing salt concentration is observed and is consistent with intrinsic viscosity measurements. The PGA relaxation kinetics is found to be nearly exponential in KCl on a Rouse/Zimm time scale (approximately 1 ns), whereas NaCl induces a 10- to 100-times slower, highly nonexponential relaxation. The slow decay can be traced back to Na(+) ions bridging anionic groups with scale-free power-law residence time distributions. This "transient cross-linking" may explain cation-specific slowing down of (bio)polymer kinetics observed in a variety of experiments. A systematic test using different force-field combinations in the simulations corroborates the qualitative trends, while quantitatively, the kinetic rates in the NaCl simulations significantly depend on the particular choice of water and ion parameters.
通过广泛的显式水分子计算机模拟，研究了 NaCl 和 KCl 对完全荷电和展开的聚谷氨酸（PGA）链的静态和动态行为的影响。随着盐浓度的增加，观察到离子特异性的 PGA 线圈尺寸收缩，这与特性粘度测量结果一致。在罗瑟/齐姆时间尺度（约 1 ns）上，发现 PGA 的弛豫动力学在 KCl 中几乎呈指数形式，而 NaCl 则导致 10 到 100 倍的更慢、高度非指数弛豫。这种缓慢的衰减可以追溯到 Na(+) 离子桥接带有无标度幂律停留时间分布的阴离子基团。这种“瞬态交联”可以解释在各种实验中观察到的（生物）聚合物动力学的阳离子特异性减速。通过在模拟中使用不同力场组合的系统测试证实了定性趋势，而在定量方面，NaCl 模拟中的动力学速率很大程度上取决于水和离子参数的特定选择。
