Vesiculation of unsonicated phospholipid dispersions containing phosphatidic acid by pH adjustment: physicochemical properties of the resulting unilamellar vesicles.

Aqueous dispersions of phosphatidic acid and mixtures of phosphatidic acid with other phospholipids vesiculate when the pH is transiently increased to a pH near or above the second pK of the phosphatidic acid. Both small unilamellar vesicles (SUV) of a narrow size distribution (average diameter 25-30 nm) and large unilamellar vesicles (LUV) of a wide size distribution are formed. The fraction of SUV increases linearly with increasing pH from 6 to 12 and was also increased by increasing the rate of the pH change from 2 min to 1 s. For mixed phospholipid dispersions, the fraction of SUV appears to be linearly related to the phosphatidic acid content. Unilamellar vesicles formed by the transient pH increase are subsequently relatively stable to changes in pH and ionic strength. If, after vesiculation is induced, the dispersion (pH 7-8) is acidified to pH 3, the fraction of SUV decreases. However, the multilamellar structures present in the original dispersion do not re-form, and most of the reduction in the SUV fraction is reversible when the pH is returned to 7-8. The addition of NaCl to the dispersion after vesiculation has no effect on the fraction of SUV up to physiological NaCl concentrations. Subsequent addition of NaCl to a concentration in excess of 0.2 M reduces the SUV fraction; this disappears above 1 M NaCl due to aggregation or fusion. SUV of phosphatidic acid or mixed phospholipids containing phosphatidic acid can be stored at 4 degrees C for 14 days without detectable aggregation and/or fusion.