Bending rigidity of phosphatidylserine-containing lipid bilayers in acidic aqueous solutions

The elastic properties of anion lipid-containing bilayers were studied in aqueous solutions with fixed ionic strength at pH ≤5. Giant vesicles from stearoyl-oleoyl-phosphatidylcholine (SOPC) with various contents of dioleoyl-phosphatidylserine (DOPS) were obtained via electroformation and observed in phase contrast and fluorescence. Two structural phases were found to coexist in vesicle membranes. Thermal fluctuation spectroscopy of quasispherical vesicles with homogeneous liquid membranes was applied for the quantification of their bending rigidity. The white-noise contribution, significantly influencing the calculated value of the bending modulus, was taken into account in the analysis of the thermal shape fluctuations of vesicles. Electrokinetic measurements with SOPC/DOPS liposomes provided information about their surface potential dependence on pH and also as a function of the DOPS molar content in bilayers. The experimentally obtained values for the bending rigidity of SOPC bilayers containing various amounts of DOPS are reported and discussed in the light of the existing theory for the electrostatic contribution to the bending modulus of charged lipid membranes.