Dipole potentials of monolayers of phosphatidylcholine, phosphatidylserine, and phosphatidic acid on mercury

A novel procedure for the measurement of the absolute value of the surface dipole potential chi of self-assembled lipid monolayers, which makes use of a phospholipid monolayer supported on mercury, is described. It consists o f increasing the tilt angle of the lipid molecules with respect to the mono layer normal by expanding progressively the supporting mercury drop and in measuring the charge on the mercury surface that accompanies the drop expan sion. Dipole potential values of +145 +/- 10 mV for neutral dioleoylphospha tidylcoline (DOPC) and dioleoylphosphatidylserine (DOPS) and off +30 +/- 3 mV for neutral dioleoylphosphatidic acid (DOPA) were determined. A gradual increase in the negative charge of the DOPS and DOPA headgroups due to an i ncrease in pH causes an increase in chi. This suggests that the dipole pote ntial of about +145 mV in neutral dioleoylphospholipids stems from the este r linkages to the glycerol backbone (which in DOPA monolayers are screened by the water molecules) and possibly, to a minor extent, from the orientati on of the hydration water molecules. The latter contribution is small in ne utral phospholipid monolayers, but becomes progressively more positive with an increase in the negative charge on the polar heads of the lipids.