EFFECT OF VESICLE COMPOSITION AND CURVATURE ON THE DISSOCIATION OF PHOSPHATIDIC-ACID IN SMALL UNILAMELLAR VESICLES - A P-31-NMR STUDY

Sonicated small unilamellar vesicles (SUVs) containing phosphatidic ac id (PA) give two PA P-31-NMR resonances corresponding to PA molecules in the inner and outer leaflets of the bilayer. This NMR differentiati on between the two monolayers is not due to a pH gradient across the m embrane but instead reflects differential packing in the inner and out er leaflets imposed by the highly curved SUV surface. The apparent pK( a) of the outer-leaflet PA increases with decreasing surface curvature and with increasing PA content. The estimated relationship between th e apparent pK(a) of the outer-leaflet PA headgroup and vesicle curvatu re may provide a qualitative probe for effects related to surface curv ature in these model-membrane systems.