31P NMR analysis of the surface homogeneity of mixed sphingomyelin-phosphatidylcholine vesicles

The surface compositional symmetry of a mixed sphingomyelin-phosphatidylcholine vesicle has been studied by ³¹P NMR spectrometry. The molecules on the outer vesicle surface could be distinguished from the molecules on the inner vesicle surface by utilization of the shift reagent, Eu³⁺. This polyvalent cation interacts with the outer surface phosphate groups and, as a result, shifts these molecules upfield. Analysis of the data obtained indicates that a slight excess of sphingomyelin is present on the outer surface of the vesicle, compared to phosphatidylcholine, which appears to have a slight preference for the inner surface.