Transbilayer Asymmetry and Surface Homogeneity of Mixed Phospholipids in Cosonicated Vesicles

Cosonication of equimolar quantities of phosphatidylglycerol (PG) and phosphatidylcholine (PC) results in bilayered vesicles the outer surface of which contain, on the average, twice as many PG as PC molecules. Within the surface these two lipid classes are not spatially segregated into “patches.” These results were obtained by exploiting the effects of paramagnetic ions on the proton and phosphorus nuclear magnetic resonances. The 31P resonances of PG and PC sonicated separately have different chemical shifts and broaden differently upon addition of Mn2+. At Mn2+ concentrations less than 10–4 M, these ions do not permeate the vesicles, permitting a distinction of the signals originating on the outer surface from those on the inner surface. For pure dispersions of PG and of PC, Mn2+ and Eu3+ reside closer to the phosphate than to the choline N(CH₃)₃+ protons and the residence time of Mn2+ is short, less than 10–4 sec. The integrated and asymmetric arrangement of the phospholipid molecules in the cosonicated dispersions is discussed in the context of the structure and biosynthesis of biological membranes.