The lipid-water interface: revelations by osmotic stress.

Lipids at the bilayer-water interface are highly disordered and mobile, and large areas of the bilayer undergo thermal undulations. Water penetrates significantly down to the hydrocarbon chain level. This water, and water out to about 10 A from the surface, is perturbed by the lipid surface in a way that produces a strong hydration repulsion and precludes molecular contact between bilayers. Its removal costs work, but most of this water is neither a permeable barrier nor unavailable to solvate other solutes. All hydrophilic surfaces show this "hydration force." Most lipids have an excess higher free energy when packed within a bilayer membrane since in isolation they pack into high curvature assemblies with polar groups on the concave side. Osmotic stress measurements of those curved assemblies yield a measure of monolayer elastic parameters and the excess higher free energy, which likely controls embedded proteins. Osmotic stress experiments can determine whether water is energetically significant, or not, in almost any system. The osmotic effect of solutes, independent of specific binding, is to compete with lipids and proteins for water. Solute affinity for water can modify lipid packing and protein conformation, coupling lipid and protein structure and function to osmolality at the molecular level.