Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma, United States.
J Phys Chem B. 2011 Dec 22;115(50):14980-90. doi: 10.1021/jp2062404. Epub 2011 Nov 22.
Calculation of surfactant pK(a)'s in micelles is a challenging task using traditional electrostatic methods due to the lack of structural data and information regarding the effective dielectric constant. Here we test the implicit- and hybrid-solvent-based continuous constant pH molecular dynamics (CpHMD) methods for predicting the pK(a) shift of a lauric acid solubilized in three micelles: dodecyl sulfate (DS), dodecyltrimethylammonium (DTA), and dodecyltriethylene glycol ether (DE3). Both types of simulations are able to reproduce the observed positive pK(a) shifts for the anionic DS and nonionic DE3 micelles. However, for the cationic DTA micelle, the implicit-solvent simulation fails to predict the direction of the pK(a) shift, while the hybrid-solvent simulation, where conformational sampling is conducted in explicit solvent, is consistent with experiment, although the specific-ion effects remain to be accurately determined. Comparison between the implicit- and hybrid-solvent data shows that the latter gives a more realistic description of the conformational environment of the titrating probe. Surprisingly, in the DTA micelle, surfactants are only slightly attracted to the laurate ion, which diminishes the magnitude of the electrostatic stabilization, resulting in a positive pK(a) shift that cannot be explained by chemical intuition or other theoretical models. Our data underscores the importance of microscopic models and ionization-coupled conformational dynamics in quantitative prediction of the pK(a) shifts in micelles.
在胶束中计算表面活性剂的 pK(a) 使用传统的静电方法是一项具有挑战性的任务，因为缺乏结构数据和关于有效介电常数的信息。在这里，我们测试了基于隐式和混合溶剂的连续恒 pH 分子动力学（CpHMD）方法，以预测溶解在三种胶束中的月桂酸的 pK(a) 位移：十二烷基硫酸盐（DS）、十二烷基三甲基铵（DTA）和十二烷基三乙二醇醚（DE3）。这两种类型的模拟都能够重现观察到的阴离子 DS 和非离子 DE3 胶束的正 pK(a) 位移。然而，对于阳离子 DTA 胶束，隐式溶剂模拟无法预测 pK(a) 位移的方向，而在混合溶剂模拟中，构象采样在显式溶剂中进行，与实验一致，尽管特定离子效应仍有待准确确定。隐式和混合溶剂数据之间的比较表明，后者更真实地描述了滴定探针的构象环境。令人惊讶的是，在 DTA 胶束中，表面活性剂对月桂酸离子的吸引力很小，这降低了静电稳定化的幅度，导致正 pK(a) 位移，这不能用化学直觉或其他理论模型来解释。我们的数据强调了微观模型和离子化耦合构象动力学在定量预测胶束中 pK(a) 位移的重要性。
