Dissociation constants of lipid ionizable groups II. Changes in surface pK at low ionic strengths

Abstract Surface dissociation constants (p K s ) for the PO 4 − group in dioleoylphosphatidylglycerol (DOPG) and the COO − group in phosphatidylserine (PS) bilayer membranes were found by fitting theoretical curves to the experimental dependences of electrophoretic mobilities (EPMs) on ionic strength ( C ) and pH in solutions of monovalent salts. The measured EPMs were expressed in potential units through the Smoluchowsky equation; the corresponding ξ sm values were used as experimental points. The ξ sm vs. log C curves were obtained for DOPG in KCl and for PS in NaCl solutions at pH 7 in the range 10 −3 −1 M; the ξ sm vs. pH curve was measured for PS in 2 × 10 −2 M NaCl in the pH range 2–7. Theoretical analogues of ξ sm were calculated from the Dukhin-Deryaguin equation, which takes into account the mobility decrease due to the relaxation effect. For both lipids, fitting to the ξ sm vs. log C curves, with due regard for cation adsorption, allowed us to find changes in th p K s values at low ionic strengths (below 10 −1 M). For DOPG, changes in p K s were verified by comparison of the related surface potential ( ψ s ) vs. log C curve with Δψ s vs. log C curves measured on DOPG planar lipid membranes. Analysis of the ξ sm vs. pH curve for PS showed that a single p K s value cannot be obtained from the fitting procedure; this is interpreted as an indication of a change in p K s with pH, in agreement with predictions made in Part I of this paper.