Charged Lipid Bilayers in Aqueous Surroundings with Low pH

Abstract The present chapter discusses our experimental results on morphology, stability, and the mechanical properties of two-component charged lipid bilayers in aqueous solutions with controlled ionic strength at low pH. Observations by phase-contrast and fluorescent microscopy revealed coexistence of two structural phases in membranes from stearoyl oleoyl phosphatidylcholine (SOPC), containing more than 10 mol% of the charged lipid dioleoyl phosphatidylserine. At temperatures lower than 29 °C, the existence of nonfluctuating “hard” domains was registered in fluid membranes at pH 5.0. The method of shape-fluctuation analysis of quasi-spherical vesicles was applied for the determination of the bending modulus of homogeneous liquid bilayers. The value obtained for this material constant of bilayers with 0.2 molar fraction of charged lipid is around 30% higher than the bending modulus of uncharged (SOPC) membranes in the same surrounding solution. The experimental findings, presented and discussed in the chapter, qualitatively agree with our previous results for the curvature rigidity of charged bilayers as measured by vesicle micromanipulation.