When water molecules meet air

• Autor(a) principal: Hsie, Cho-Shuen
• Data de Publicação: 2012
• Outros Autores: Campen, R. Kramer, Verde, Ana Vila, Bolhuis, Peter, Nienhuys, Han-Kwang, Bonn, Mischa
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/1822/21120

About 70% of our planet is covered in water. Most of that water exists as water in the bulk – the neighbors of water molecules are other water molecules – and only a small fraction of molecules are at the air-water interface. Despite the small relative abundance of interfacial water, it is of the utmost importance: it governs the chemistry involving the surface of oceans and seawater aerosols, or the small water droplets forming clouds. Reactions at the air-water interface are directly relevant to world-scale phenomena such as the nutrient cycle in oceans or the evolution of the ozone layer. The air-water interface is also a good model for extended hydrophobic interfaces – the type of interface that occurs when water meets a large surface constituted by many apolar functional groups. Hydrophobic interfaces play a key role in the hydrophobic collapse of proteins (the process by which proteins acquire their 3D structure) or the aggregation of surfactants. Understanding many interfacial phenomena thus requires us to first understand the structure and dynamics of interfacial water. Studies of water at interfaces have only become possible within the past decades, with the advent of surface-sensitive experimental techniques such as vibrational sum-frequency (VSF) spectroscopy and the development of better classical models of water for molecular simulations. In this talk I will describe recent work investigating, using VSF and molecular simulation, the dynamics of two subpopulations of hydroxyl groups at the air-water interface: those OH groups that do not establish a hydrogen bond (called free OH), and those that do. The free OH population is very small in the bulk but much larger at the interface, and so is partially responsible for the interface’s properties. We found that free OH groups rotate several times faster than water in the bulk. In contrast, the net reorientation of bonded OH groups occurs at a rate similar to that of bulk water. Our results on the air-water interface have direct implications for the kinetics of protein folding or ligand binding: they suggest that the kinetics of the de-wetting transitions that necessarily accompany these processes might be tunable by protein topologies that predominantly affect the local population of free OH groups, since only free OH groups have rotational dynamics that is distinct from bulk water.