Studies on Phosphatidylcholine Vesicles

The partial specific volume of phospholipid vesicles was determined by the sedimentation velocity method in various concentrations of hydrogen and deuterium oxide mixture. Since the density of D20 is very much greater than the reciprocal of the specific volume of phospholipids, the density of DzO-Hz0 medium corresponding to zero redistribution of phospholipids during sedimentation can be obtained with high precision by inferpolafion of the data. This method is therefore direct and accurate for phospholipids. Similar measurements on phospholipid vesicles in various concentrations of potassium chloride and sodium chloride solutions have also been made. The apparent isodensity point in the KC1 solution was found to be approximately equal to that obtained in NaCl solution but smaller than that obtained in D,O-Hz0 medium. This difference can be attributed to the influence of preferential interaction of the vesicle with water. The partial specific volume and effective specific volumes of phosphatidylcholine vesicles are c = 0.9814 f 0.0004 ml per g9 ~‘KCI = 0.9883 f 0.0002, and #N&1 = 0.9886 f 0.0006 ml per g.