Hydration processes in biological and macromolecular systems

I discuss several themes arising from the papers presented at this Faraday Discussion that are, in my view, particularly interesting and/or important. With respect to model systems, recent progress in understanding structural aspects of the hydrophobic interaction, and of through-solvent interactions in general, is highlighted, together with the need to continue to develop more sophisticated (theoretical and computational) interpretational techniques if we are to exploit to the full the power of present-day experimental techniques. The current state of our knowledge of hydration effects on the structure and dynamics of biomolecules is discussed, and the importance of being able to see how molecular-level structural effects control behaviour at the important mesoscopic level is underlined. Issues relating to recent progress in characterising solvent effects in more complex systems and processes, including those of industrial interest, are raised, and the necessity of using a range of appropriate experimental techniques when tackling such complex problems is stressed. Progress since the 1975 Royal Society Discussion is highlighted, and interesting issues ripe for fruitful discussion at this meeting are raised. A strong case can now be made that our understanding of both the structure and dynamics of water as a function of its local environment is now sufficiently good to enable us to use water as a probe of complex system behaviour, rather than, as heretofore, an object of study in itself.