10 - The Water Molecule in (Bio)Macromolecules

Abstract

This chapter discusses water in large, poorly ordered systems, most of them macromolecules. These two species, ice and liquid water, illustrate how the exceptional structure of the H2O molecule, with its two H-bond donor sites and two H-bond acceptor sites, is at the origin of the formation of a dense H-bond network. The properties of this H-bond network are, however, different in ice and in liquid water: its rigidity in ice and its great flexibility in liquid water lead to completely different chemical properties, already pointing to the important role this H-bond network plays. Studies on the hydration of macromolecules consist of observing the H-bond network that develops inside a macromolecule when its passes from a dried state to a completely soaked state. This can be achieved by, for instance, increasing the water vapor tension of the surrounding atmosphere, starting from a waterless atmosphere within which the dried sample is in equilibrium and ending with an atmosphere with 100% humidity, which is an atmosphere saturated with water vapor in equilibrium with liquid water in a restrained volume. This way of studying hydration has the advantage of defining the variable state of hydration with precise values, the hygrometry also called “relative humidity.” The chapter also describes experiments on bioprotection that stress the impact this H-bond network has on the structure of the macromolecule itself.