The effect of surface charge on the hydrolysis kinetics of partially hydrogenated egg phosphatidylcholine and egg phosphatidylglycerol in aqueous liposome dispersions.

The hydrolysis kinetics of partially hydrogenated egg phosphatidylcholine (PHEPC) and egg phosphatidylglycerol (EPG) were investigated as a function of surface pH at 60 degrees C. The bulk pH of the liposome dispersions was varied from pH 3.0 to 8.0. Charge was introduced to the liposomal bilayers by addition of EPG. Different electrostatic profiles were obtained at the bilayer-water interface by varying the surface charge density and the ionic strength of the buffer solutions. With the Gouy-Chapman equation the surface potential and with the Boltzmann equation the resulting shift in the surface pH compared to the bulk pH was estimated. The hydrolysis rate of both phospholipids increased with surface charge density in acidic medium; in alkaline medium the opposite was true. The hydrolysis rate constants, however, plotted as a function of surface pH agreed reasonably well with the results found with neutral liposomes.