Surface dipole moments of lipids at the argon-water interface. Similarities among glycerol-ester-based lipids

Abstract

Surface potential-surface pressure-area isotherms at the argon-buffer interface have been determined for 38 lipid species comprising 19 chemical classes. These lipids all exhibited a finite range of liquid-expanded surface pressure-area behavior. For most species, the linearity of surface potential with reciprocal area was excellent, but nonzero intercepts were obtained. This suggests a lipid-induced reorganization of interfacial water molecules which is area independent. The linearity of the data permits calculation of the surface dipole moment, mu perpendicular, for each lipid. The values of mu perpendicular for a series of oleoyl-containing acylglycerols, dioleoyl phosphatidylcholine, and dioleoyl phosphatidylethanolamine exhibit acylglycerol ester group mu perpendicular's which are generally consistent with known conformational properties of such lipids. The values are 132 mD for the perpendicular oleoyl glycerol-ester group and 252 mD for that in the kinked-chain conformation. Comparison of mu perpendicular's calculated using these values with homologues confirms the approximate independence of mu perpendicular from aliphatic chain length and permits identification of exceptions with possible conformational or orientational differences. Notably, diphytanoyl phosphatidylcholine shows a 45% larger mu perpendicular than predicted. Differences in mu perpendicular among lipid classes allow estimation of the electrical consequences of lipid metabolism and exchange. Calculations show that reactions such as the generation of 1,2-diacylglycerol from diacyl glycerophosphocholine or diacyl glycerophosphoinositol should produce surface potential changes of -127 and +42 mV, respectively. Thus, the two phospholipids are not simply alternative sources of diacylglycerol with respect to processes dependent on surface potential.