Water in biological membranes: Adsorption isotherms and circular dichroism as a function of hydration.

Adsorption isotherms of water on human red cell membranes have been measured and the results used to determine the Brunauer-Emmett-Teller monolayer coverage, saturation hydration, and relevant heats and entropies of hydration. The tightly bound monolayer corresponds to a complete covering of only one membrane surface and the saturation hydration is greater than values for globular proteins and phospholipids. A hysteresis loop is observed in the adsorption-desorption cycle.Circular dichroism spectra of membrane films have been measured as a function of hydration in the ultraviolet region of the spectrum. The spectra resemble those of membrane suspensions with reduced scattering. The circular dichroism did not change upon dehydration of the membrane films from 92 to 0% relative humidities, implying a stable membrane protein structure independent of both water content and lipid phase.