Interactions in the hydrogen belts of membranes: Cholesterol leaving phosphatidylcholine bilayers

Cholesterol transfer from sonicated liposomes of phosphatidylcholine containing 10 or 30 mole percent cholesterol was measured with erythrocytes as acceptor. The activation energies of the (rate-limiting) bilayer-cholesterol dissociation were determined. In parallel experiments, phosphatidylcholine was replaced by an analog lacking the carbonyl oxygens, dietherphosphatidylcholine. The activation energies for dissociation of cholesterol from this phospholipid were three Cal/mole smaller than those for cholesterol-phosphatidylcholine dissociation, at both concentrations of cholesterol. These results demonstrate the involvement of the carbonyl oxygen in cholesterol-phospholipid bonding and support the hypothesis of lipid-lipid hydrogen bonding in the hydrogen belts of membranes.