Effect of the phase transition on the transbilayer movement of dimyristoyl phosphatidylcholine in unilamellar vesicles.

Dimyristoyl phosphatidylcholine rapidly exchanges between vesicles at 37 degrees C without vesicle fusion. The rate of the transbilayer movement of dimyristoyl phosphatidylcholine in sonicated vesicles has been measured employing 13C NMR using N-13CH3-labeled lipids which are introduced into the outer monolayer of non-labeled vesicles by a phosphatidylcholine exchange protein. The rate of transbilayer movement of dimyristoyl phosphatidylcholine shows a distinct maximum (half-time 4 h) in the temperature range at which the hydrocarbon phase transition occurs. The activation energy of the flip-flop rate above the phase transition is 23.7 +/- 2.0 kcal/mol.