THE INTRINSIC PK(A), VALUES FOR PHOSPHATIDYLCHOLINE, PHOSPHATIDYLETHANOLAMINE, AND PHOSPHATIDYLSERINE IN MONOLAYERS DEPOSITED ON MERCURY-ELECTRODES

The intrinsic pK(a) values of the phosphate groups of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) and of the phosphate and carboxyl groups of phosphatidylserine (PS) in self-organized monolayer deposited on a hanging mercury drop electrode were determined by a novel procedure based on measurements of the differential capacity C of this lipid-coated electrode. In view of the Gouy-Chapman theory, plots of 1/C at constant bulk pH and variable KCl concentration against the reciprocal of the calculated diffuse-layer capacity C-d,C-0 atge exhibit slopes that decrease from an almost unit value to vanishingly low values as the absolute value of the charge density on the lipid increase from zero to approximate to 2 mu C cm-2. The intrinsic pK(a) values so determined are 0.5 for PE and 0.8 for PC. The plots of 1/C against 1/C-d,C-0 f exhibit slopes that pass from zero to a maximum value and then back to zero as pH is varied from 7.5 to 3, indicating that the charge density of the lipid film passes from slight negative to slight positive values over this pH range. An explanation for this anomalous behavior, which is ascribed to the phosphate group of PS, is provided. Interdispersion of PS and PC molecules in the film decreases the ''formal'' pK(a) value of the latter group by about three orders of magnitude.