Decoding the molecular water structure at complex interfaces through surface-specific spectroscopy of the water bending mode.

Takakazu Seki,Ellen H G Backus,Shumei Sun,Xiaoqing Yu,Tatsuhiko Ohto,Konrad Meister,Mischa Bonn,Chun-Chieh Yu,Yuki Nagata

doi:10.1039/d0cp01269f

Abstract

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-CaF2 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.

Highlights

The structure of interfacial water determines atmospheric chemistry, wetting properties of materials, and protein folding
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
For the former, we show that this method can single out water O–H groups from surface hydroxyls, and that with increasing pH, the hydrogen-bonded network of interfacial water strengthens

Summary

Introduction

Total reflection-infrared (ATR-IR), and sum-frequency generation (SFG) spectroscopy have been used for investigating the molecular structure and dynamics of water at interfaces.[1]. The H–O–H bending mode frequency is 1590–1650 cmÀ1, which is significantly distinct from the bending mode frequency of hydroxyl groups attached on solid surfaces (300–900 cmÀ1).[21] Since the first publication of the sum-frequency generation spectrum in the frequency region of the bending mode from the water–air interface,[22] there have been debates about the surface- versus bulk-contribution to the signal.[23,24] We have recently demonstrated that the surface contribution to the overall spectrum can be readily extracted, paving the way for using the bending mode as a reporter for determining the interfacial water structure.[24] The water bending mode may be well-suited to study other complex interfaces such as protein interfaces. To establish a protocol to probe the interfacial water contribution through the bending mode SFG, one should first obtain the information on whether the bending mode SFG response is affected by vibrational coupling, by performing experiments with isotopic dilution. The successful determination of the interfacial hydrogen bond network through the SFG water bending mode manifests that probing the water bending mode in the SFG spectra is a promising tool for understanding the interfacial water structure

Results

Discussion

Computational procedure