Importance of glycerol and fatty acid residues on the ionic properties of phosphatidylglycerols at the air-water interface.

Ionic properties of didodecanoylphosphatidylglycerol (C12PG), didodecanolyphosphatidyl-l'-propanol (C12PP), di-(12-methyl, 13-methyl)-pentadecanoylphosphatidylglycerols (C15PG) and dihexadecanoylphosphatidylglycerol (C16PG) have been studied at the air-water interface using titration experiments at constant ionic strength and film expansion experiments at constant pH, with Li+, Na+, K+ and Cs+ in the subphase. For each lipid, the apparent pK in the surface is strongly dependent on the subphase salt concentration and differs from expected intrinsic pK in the bulk. Discrimination between alkaline cations is observed. These results can be accounted for by strong surface potentials, which are satisfactorily calculated by using the Gouy and Chapman theory of the diffuse double layer. The comparison of C12PP and PG expansion data shows the importance of the glycerol residue of PG ionic properties, favouring penetration of cations in the films. Lipids in the liquid-crystalline state, such as C12-and C15PG, do not interact with alkaline cations as does C16PG in the gel phase. In particular, film condensations bring about a clear-cut discrimination between Na+ and K+. Results are discussed with regard to cation penetration and the structure of water at the interface. The importance on membrane functions of these strong surface potentials generated by PG monolayers is suggested.