Anomalous Acid-Base Equilibria in Biologically Relevant Water Nanopools

摘要

Water plays a crucial role in many principal biological phenomena such as enzymatic catalysis and proton pumping through a membrane protein channel. Moreover, in biological systems, water is usually contained in a small pocket of a membrane, and such confined water, which is generally called a water nanopool, shows peculiar properties differing considerably from the properties of bulk water. The confinement effect and the enclosing interfacial surfaces of waterpools are the main factors to determine the properties, such as polarity, viscosity, and H-bonding ability, of water nanopools. For example, the dielectric constant of a water nanopool has been reported to be much lower than that of bulk water (ε = 78.5) but similar to that of an alcohol (ε = 30-40). On the other hand, biological processes often take place based on proton relay along a hydrogen (H)-bonded chain, and the dynamics of biological proton relay is determined by the size, the structure, and the motion of a water cluster which is the prime agent in most of biological systems. Thus, for better understanding of cellular dynamics, it is necessary to investigate the properties of a biologically relevant water nanopool as a biomimetic system of water confined in a cell membrane. In this regard, water nanopools confined in reverse micelles, which are formed by surfactant molecules having polar headgroups pointing inward and dispersed in hydrocarbon solvents, can be good model systems of biological water.