Roy S, Gruenbaum S M, Skinner J L
Theoretical Chemistry Institute and Department of Chemistry, 1101 University Ave., University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
J Chem Phys. 2014 Nov 14;141(18):18C502. doi: 10.1063/1.4895546.
Understanding the structure of water near cell membranes is crucial for characterizing water-mediated events such as molecular transport. To obtain structural information of water near a membrane, it is useful to have a surface-selective technique that can probe only interfacial water molecules. One such technique is vibrational sum-frequency generation (VSFG) spectroscopy. As model systems for studying membrane headgroup/water interactions, in this paper we consider lipid and surfactant monolayers on water. We adopt a theoretical approach combining molecular dynamics simulations and phase-sensitive VSFG to investigate water structure near these interfaces. Our simulated spectra are in qualitative agreement with experiments and reveal orientational ordering of interfacial water molecules near cationic, anionic, and zwitterionic interfaces. OH bonds of water molecules point toward an anionic interface leading to a positive VSFG peak, whereas the water hydrogen atoms point away from a cationic interface leading to a negative VSFG peak. Coexistence of these two interfacial water species is observed near interfaces between water and mixtures of cationic and anionic lipids, as indicated by the presence of both negative and positive peaks in their VSFG spectra. In the case of a zwitterionic interface, OH orientation is toward the interface on the average, resulting in a positive VSFG peak.
了解细胞膜附近水的结构对于表征诸如分子运输等水介导的事件至关重要。为了获得膜附近水的结构信息，拥有一种仅能探测界面水分子的表面选择性技术是很有用的。一种这样的技术是振动和频产生（VSFG）光谱学。作为研究膜头基/水相互作用的模型系统，在本文中我们考虑水表面的脂质和表面活性剂单分子层。我们采用一种结合分子动力学模拟和相敏VSFG的理论方法来研究这些界面附近的水结构。我们模拟的光谱与实验定性一致，并揭示了阳离子、阴离子和两性离子界面附近界面水分子的取向有序性。水分子的OH键指向阴离子界面导致VSFG峰为正，而水的氢原子远离阳离子界面导致VSFG峰为负。在水与阳离子和阴离子脂质混合物的界面附近观察到这两种界面水物种共存，这由它们的VSFG光谱中正负峰的同时出现表明。在两性离子界面的情况下，OH平均取向朝向界面，导致VSFG峰为正。
