Net proton-hydroxyl permeability of large unilamellar liposomes measured by an acid-base titration technique.

Abstract

The net proton-hydroxyl permeability of large unilamellar liposomes was measured by an acid-base pulse titration technique and was determined to be several orders of magnitude greater than that measured for other monovalent ions. This permeability is relatively insensitive to variations in lipid composition. Proton permeability and hydroxyl permeability vary with pH 6 to 8; this variation can occur in the absence of alterations in surface charge density resulting from titrations of acidic and basic groups on the lipids. In order to account for the exceptionally high proton-hydroxyl permeability with respect to other monovalent ions, it is proposed that protons or hydroxyls or both interact with clusters of H-bonded water molecules in the lipid bilayer, such that they are transferred across the bilayer by rearrangement of H-bonds in a manner similar to their transport in water and ice.