Abstract:
The interaction of biological molecules with water is an important determinant of structural
properties both in molecular assemblies, and in conformation of individual macromolecules. By
observing the effects of manipulating the activity of water (which can be accomplished by
limiting its concentration or by adding additional solutes, "osmotic stress"), one can learn
something about intrinsic physical properties of biological molecules as well as measure an
energetic contribution of closely associated water molecules to overall equilibria in biological
reactions. Here two such studies are reported. The first of these examines several species of
lysolipid which, while present in relatively low concentrations in biomembranes, have been
shown to affect many cellular processes involving membrane-protein or membrane-membrane
interactions. Monolayer elastic constants were determined by combining X-ray diffraction and
the osmotic stress technique. Spontaneous radii of curvature of lysophosphatidylcholines were
determined to be positive and in the range +30A to +70A, while lysophosphatidylethanolamines
proved to be essentially flat. Neither lysolipid significantly affected the bending modulus of the
monolayer in which it was incorporated. The second study examines the role of water in theprocess of polymerization of actin into filaments. Water activity was manipulated by adding
osmolytes and the effect on the equilibrium dissociation constant (measured as the criticalmonomer concentration) was determined. As water activity was decreased, the critical
concentration was reduced for Ca-actin but not for Mg-actin, suggesting that 10-12 fewer water
molecules are associated with Ca-actin in the polymerized state. Thisunexpectedly small amount
of water is discussed in the context of the common structural motif of a nucleotide binding cleft.
