Water in complex environments such as living systems

With the proposal by H.E. Stanley and coworkers and the late G.W. Robinson and his coworkers that liquid water exists as two rapidly exchanging microdomains of different densities, the already complex living cell has acquired new levels of complexity. Microdomains are apparently of the order of eight molecules in diameter, large enough to dissolve small solutes. The difference in density (of the order of 30%) suggests that both physical and chemical properties of water must differ in the two microdomains. These differences cannot be measured in the pure liquid since the microdomains cannot be selectively probed. It has previously been shown, however, that water in very small pores or clefts (of the order of 1-3 nm diameter) can comprise, almost exclusively, one type of water or the other. This has enabled single types of water to be characterised. Some consequences of the coexistence of microdomains of water of different solvent properties are explored. Each solute which prefers one microdomain to the other creates a local osmotic pressure gradient which is abolished by displacement of the water equilibrium. Thus, all solutes change the properties of local water over distances of the order of nm. Surfaces have two modes of changing water properties; they may reinforce each other or interfere with each other. The combination of surfaces, solutes and two kinds of water, therefore, can result in little change in the properties of surface water, in conversion of surface water to highly reactive weakly bonded high density liquid or to inert, strongly bonded low density liquid. Since all biochemical reactions take place within 2 nm of an enzyme surface, evolution must have exploited this rich repertoire of mechanisms. (C) 2002 Published by Elsevier Science B.V.