Structures of aqueous dispersions of phosphatidylserine.

Dilute ( ~10% w/v) unsonicated phosphatidylserine dispersions in Hz0 contain spherical particles of a wide size distribution (0.1 to 15 q). The majority have diameters between 0.1 to 2 pm and lack the multilamellar arrangement of bilayers typical of unsonicated lecithin dispersions. Sonication uroduces small thermodynamically unstable vesicles surrounded by a single closed bilayer with an average vesicle diameter of 160 A (range 125 to 250 A). The vesicles are more homogeneous with respect to size and their curvature is significantly greater than that of vesicles prepared from lecithins of similar chain length. While phosphatidylserine forms inverted micelles in organic solvent, no micelles were detected in sonicated, aqueous dispersions. Increasing the ionic strength leads to an increase in particle size and to the formation of multilamellar structures both with sonicated and unsonicated dispersions and eventually flocculation of a metal-phosphatidylserine complex occurs. Unsonicated phosphatidylserine dispersions give no high resolution spectrum. Proton nuclear magnetic resonance measurements on sonicated dispersions in 2HZ0 show that the molecular motion in the polar group of the molecule is sufficient for all protons, including those of the glycerol group, to contribute to the high resolution spectrum. Increasing the ionic strength leads to a closer packing of the polar groups and of the hydrocarbon chain CH:, groups close to the glycerol group without significantly affecting the correlation times of motion of most of the protons along the hydrocarbon chains. However, the anisotropy of the hydrocarbon chain motion and thus the degree of order within the bilayer increases significantly.