High and low density intracellular water.

The proposal that liquid water consists of microdomains of rapidly-exchanging polymorphs of high and low density is examined for its impact upon roles of water in biology. It is assumed that the two polymorphs persist in solution and adjacent to surfaces and that solutes partition asymmetrically between them. It transpires that chaotropes are solutes which partition preferentially into low density water and displace the water equilibrium toward the high density polymorph. Kosmotropes. both ionic and non-polar, partition into high density water and induce low density water. Displacement of the water equilibrium at constant temperature and pressure has a thermodynamic cost which can be high. This appears to be a dominant factor in folding of proteins and DNA, aggregation of biopolymers and insolubility of non-polar kosmotropes. Cells control both the concentration of proteins and the selection of small solutes to produce an intracellular environment most conducive to co-ordinated enzyme function. Intracellular water has similar microdomains to bulk water, but surfaces and solutes redistribute them. Average properties, as measured by NMR are similar, but local properties on a nm scale may differ widely. Enzymes apparently use these local differences to activate cations for transport, induce movement and for synthesis.