Sthoer Adrien, Hladílková Jana, Lund Mikael, Tyrode Eric
Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden.
Division of Theoretical Chemistry, Lund University, P.O.B. 124, SE-22100 Lund, Sweden.
Phys Chem Chem Phys. 2019 Jun 7;21(21):11329-11344. doi: 10.1039/c9cp00398c. Epub 2019 May 20.
Specific interactions between the carboxylic acid moiety and the monovalent salts CsCl, NaCl, and LiCl, have been investigated in Langmuir monolayers using vibrational sum frequency spectroscopy (VSFS) and complemented with coarse grained and all-atom molecular dynamics simulations. By exploiting VSFS's intrinsic surface specificity, an emphasis was made on targeting headgroup vibrations of both its charged and uncharged forms as well as water molecules in the interfacial layer. The degree of deprotonation of the monolayer as a function of cation concentration and pH was experimentally determined and theoretically rationalized. Starting from 100 mM, the surface charge was overestimated by the Gouy-Chapman model and varied depending on the identity of the cation, highlighting the appearance of ion specific effects. Agreement could be found using a modified Poisson-Boltzmann model that takes into account steric effects, with a fitted effective ion-size compatible with the hydrated ion diameters. The relative affinity of the cations to the carboxylic acid moiety was pH dependent: at pH 4.5 they arranged in the order Cs > Na > Li, but fully reversed (Li > Na > Cs) at pH 9. Simulations yielded microscopic insight into the origin of this behavior, with the cations showing contrasting interaction preferences for either the uncharged carboxylic acid or the charged carboxylate. Sum frequency spectra also provided evidence that all cations remained hydrated when interacting with the charged headgroup, forming solvent-separated or solvent-shared ion pairs. However, for the specific case of 1 M Li at pH 9, contact ion pairs were formed. Finally, the remarkable effect of trace metal multivalent cations in the interpretation of experiments is briefly discussed. The results provide exciting new insights into the complex interactions of alkali metal cations with the biophysically relevant carboxylic acid moiety.
利用振动和频光谱（VSFS）对羧酸部分与单价盐CsCl、NaCl和LiCl之间的特定相互作用进行了研究，并辅以粗粒度和全原子分子动力学模拟。通过利用VSFS固有的表面特异性，重点研究了其带电和不带电形式的头基振动以及界面层中的水分子。通过实验确定并从理论上合理化了单层去质子化程度与阳离子浓度和pH值的函数关系。从100 mM开始，古依-查普曼模型高估了表面电荷，且表面电荷随阳离子种类的不同而变化，突出了离子特异性效应的出现。使用考虑空间效应的修正泊松-玻尔兹曼模型可以得到一致结果，拟合得到的有效离子尺寸与水合离子直径相符。阳离子与羧酸部分的相对亲和力取决于pH值：在pH为4.5时，它们的排列顺序为Cs > Na > Li，但在pH为9时完全相反（Li > Na > Cs）。模拟对这种行为的起源提供了微观见解，阳离子对不带电的羧酸或带电的羧酸盐表现出不同的相互作用偏好。和频光谱还提供了证据，表明所有阳离子在与带电头基相互作用时仍保持水合状态，形成溶剂分隔或溶剂共享离子对。然而，对于pH为9时1 M Li的特定情况，形成了接触离子对。最后，简要讨论了痕量金属多价阳离子在实验解释中的显著影响。这些结果为碱金属阳离子与生物物理相关的羧酸部分之间的复杂相互作用提供了令人兴奋的新见解。
