Effect of pH and monovalent cations on the ionization state of phosphatidylglycerol in monolayers. An experimental (surface potential) and theoretical (Gouy-Chapman) approach.

Ionization of the acidic phospholipid phosphatidylglycerol has been studied by measuring the surface potential of monomolecular films of the lipid as a function of the aqueous subphase pH and the concentration of monovalent cations (Li, Na, Cs). It is shown that the experimental data can be interpreted by means of the Gouy-Chapman theory in its simplest formulation, provided an adsorption of cations at the membrane surface is accounted for. This allows us to predict the ionization state of the lipid for given ionic conditions in the subphase. Above pH 4, for subphase ion concentration higher than 10 mM, or for ion concentrations above 0.1 mM at pH 5.6, phosphatidylglycerol is fully deprotonated. Within the limits of our theoretical approach, association constants of the cations to the lipid lie around 0.1-0.6 M-1.