Mechanism of spontaneous vesiculation.

Both naturally occurring and synthetic phosphatidic acid (PtdOH) molecules show the phenomenon of spontaneous vesiculation on jump in pH value. This method involves a transient increase in pH of smectic PtdOH dispersions to values between 10 and 12. Such a pH increase induces spontaneous vesiculation with the formation of small unilamellar vesicles of diameter less than 50 nm as shown by 31P NMR. Both high-resolution and broad-line 31P NMR were used to study the mechanism of this process. When the pH of unsonicated PtdOH dispersions is raised to pH 10-12, lipid molecules on the outer monolayer of the bilayer become fully ionized. The second pK value of PtdOH in bilayers is 8.6 +/- 0.3, determined by 31P NMR. PtdOH molecules on the inner monolayer remain partially protonated. 31P NMR provides unambiguous evidence that the "pH-jump" treatment produces a pH gradient across the PtdOH bilayer. The orientation of the pH gradient is such that the pH in the external medium is 3-5 pH units higher than the internal pH. Associated with the pH gradient is a transverse packing asymmetry: partially protonated PtdOH molecules in the inner layer of the bilayer are more tightly packed than fully ionized molecules present in the outer layer. The pH gradient generated by the pH jump is proposed as the energy source that drives the spontaneous formation of highly curved vesicles.