Effect of Cardiolipin on Proton Permeability of Phospholipid Liposomes: The Role of Hydration at the Lipid–Water Interface

The effect of cardiolipin on the proton permeability of dipalmitoyl-phosphatidylcholine small unilamellar vesicles was examined by utilizing the pH-dependent fluorescence emission of 5- (and 6-) carboxyfluorescein. It has been found that the proton permeability of the phospholipid bilayer was greatly enhanced in the presence of cardiolipin, an acidic phospholipid mainly found in the inner mitochondrial membranes. In the presence of bovine heart cardiolipin, the bilayer surface hydration, as assessed with the fluorescence lifetime of 1-anilinonaphthalene-8-sulfonic acid, was increased, while hydration in the acyl chain region was not altered. In addition, the bilayer fluidity was also not affected. Taken together, these results suggest that the lipid–water interface is the major energy barrier for proton permeation of the bilayer vesicles, and alteration to properties of this interface by cardiolipin headgroup appears to be responsible for the enhanced proton permeability.