Dry proteins are dead, or at best asleep. Substitution of D
2
O can drastically alter biological activity. Water is thus essential in maintaining the structural integrity of biologically active macromolecules, and is implicated in their functioning. Such water may occupy a range of dynamical states, from being strongly bound and localized, to more labile and ‘liquid-like’. Spatially ordering the macromolecules aids the search for the more localized water molecules. For example, diffraction experiments on single crystals can resolve ‘bound’ water molecules within a protein molecule - often at active sites, coordinated to metals or ions. Less precise information is obtained on the partially occupied external water sites, which are of importance to the folding and the dynamics of the biomolecule. Orientation of fibrous molecules increases the information obtainable from n.m.r. experiments. Combination of other experimental results on disordered aggregates (e.g. in solution) with chemical and structural data on the macromolecule and water itself yields useful, if circumstantial, information. Statistical and computer techniques may help to elucidate the complex nature of water-protein interactions, and to interpret the results of more complex experiments.
