TRANSBILAYER DISTRIBUTION IN SMALL UNILAMELLAR PHOSPHATIDYLGLYCEROL-PHOSPHATIDYLCHOLINE VESICLES

The transbilayer distribution of the phospholipids in small unilamellar vesicles comprised of egg phosphatidylglycerol (PG) and egg phosphatidylcholine (PC) was ascertained by 31P NMR. These vesicles, containing 10-75 mol % PG, were formed by sonication (pH 7.6) and fractionated by centrifugation. Data from spectra accumulated in the presence and absence of a paramagnetic shift reagent, Mn2+, indicated that the phospholipids are randomly arranged across the bilayer. The absence of compositional asymmetry, which contradicts earlier reports, is also exhibited by small unilamellar vesicles (50 mol % PG) prepared by the rapid ethanol injection method. Control experiments showed that Mn2+ did not induce fusion, permeate the vesicles or cause the phospholipids to migrate across the bilayer. It has been proposed that the transbilayer distribution of charged phospholipids in membranes is a consequence of the different surface charge densities on the opposing sides of the membrane. It is probably the difference in the effective polar head-group volumes of the components rather than the net charge of 1 component that determines the packing constraints for mixtures of phospholipids with the same acyl chains, at least in highly curved bilayers.