Charged Lipids Modulate the Phase Separation in Multicomponent Membranes

Phase separation in lipid membranescontrols the organizationofmembrane components and thus regulates membrane-mediated processes.Membrane phase behavior is influenced by the molecular propertiesof its components and their relative concentrations. Charged lipidspecies are among the most essential components of lipid membranes,and their impact on the membrane phase behavior is yet to be fullyunderstood. Aiming to provide insight into this impact, this paperinvestigates how the presence and amount of anionic and cationic lipidsaffect the phase behavior of multicomponent membranes. Membranes of ternary composition DOPC:DPPC:Chol with two distinct molar ratioswere used to test the hypothesis that inclusion of charged lipidswith saturated tails, beyond a certain concentration, would impede phase separation in an otherwise phase-separating membrane. Fluorescencemicroscopy examination of electroformed giant liposomes revealed thatwhen more than half of DOPC in the examined mixtures was replacedwith DOPA or DOTAP, phase separation in liposomes was somewhat suppressed,and this effect increased with increasing charged lipid content. Thiseffect depended on the membrane surface charge density as the half-maximaleffect was observed at around 0.0072 C & ANGS;(-2) inall examined cases. The phase-separation suppressing effect of DOPAwas neutralized when oppositely charged lipid DOTAP was included inthe mixture. Likewise, presence of divalent cation Ca2+ in the solution neutralized the impact of negatively charged DOPA.These results underline the detrimental influence of surface chargedensity on membrane phase behavior. More importantly, these findingssuggest that the charged lipid content in membranes may be a regulatorof their phase behavior and open new opportunities for the designof synthetic lipid membranes.