Membrane transport and the activity of water near the membrane surface.

In this paper we have mentioned several properties of membrane boundary water in the context of the transport processes discussed at this meeting. Any surface covered with water soluble groups will necessarily lower the free energy density of water solvent in its immediate vicinity. The clearest manifestation of this effect is the work required to push together two such surfaces squeezing out the intervening fluid. This "hydration" repulsion grows exponentially and depends negligibly on surface charge and ionic strength of the medium. Below 20 A separation it dominates all other interactions (unless there be present another material that competes successfully with water for the membrane surface). Boundary water appears to exclude some solutes such as small sugars but is hospitable to ions. Ionic transport is thus surprisingly well-modeled by several conflicting theories all of which treat water as an ideal continuum material. Transport by vesicular contact and fusion is probably arrested by membrane hydration. Factors which weaken or overcome hydration forces are known to facilitate fusion while fusion is suppressed by factors which increase membrane hydration.