Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.
Phys Chem Chem Phys. 2020 May 21;22(19):10934-10940. doi: 10.1039/d0cp01269f. Epub 2020 May 6.
The structure of interfacial water determines atmospheric chemistry, wetting properties of materials, and protein folding. The challenge of investigating the properties of specific interfacial water molecules has frequently been confronted using surface-specific sum-frequency generation (SFG) vibrational spectroscopy using the O-H stretch mode. While perfectly suited for the water-air interface, for complex interfaces, a potential complication arises from the contribution of hydroxyl or amine groups of non-water species present at the surface, such as surface hydroxyls on minerals, or O-H and N-H groups contained in proteins. Here, we present a protocol to extract the hydrogen bond strength selectively of interfacial water, through the water bending mode. The bending mode vibrational frequency distribution provides a new avenue for unveiling the hydrogen bonding structure of interfacial water at complex aqueous interfaces. We demonstrate this method for the water-CaF and water-protein interfaces. For the former, we show that this method can indeed single out water O-H groups from surface hydroxyls, and that with increasing pH, the hydrogen-bonded network of interfacial water strengthens. Furthermore, we unveil enhanced hydrogen bonding of water, compared to bulk water, at the interface with human serum albumin proteins, a prototypical bio-interface.
界面水的结构决定了大气化学、材料的润湿性和蛋白质折叠。使用表面特定的和频发生（SFG）振动光谱技术，通过 O-H 伸缩模式，经常会遇到研究特定界面水分子性质的挑战。虽然这种方法非常适合水-空气界面，但对于复杂的界面，可能会出现一个潜在的问题，即表面上存在的非水分子（如矿物表面的羟基或胺基，或蛋白质中的 O-H 和 N-H 基团）的羟基或胺基基团会产生干扰。在这里，我们提出了一种通过水弯曲模式选择性提取界面水中氢键强度的方案。弯曲模式振动频率分布为揭示复杂水相界面上界面水的氢键结构提供了新的途径。我们通过水-CaF 和水-蛋白质界面证明了这种方法。对于前者，我们表明这种方法确实可以从表面羟基中单独挑选出水的 O-H 基团，并且随着 pH 值的增加，界面上水的氢键网络会增强。此外，我们揭示了与人体血清白蛋白蛋白质（一种典型的生物界面）相比，界面上水的氢键增强。
