Lateral Heterogeneity of Cholesterol on Binary Lipid Mixtures of POPC/Chol Imaged with AFM

Abstract

Phase diagrams on ternary and quaternary lipid mixtures showing the existence of liquid-ordered (lo), liquid-disordered (ld) and a mixed phase (lo+ld) are now well established. However the existence of such a phase diagram on binary mixtures is still in debate despite reports of a phase diagram on the mixtures POPC/Chol and POPC/Erg and nystatin activity co-related to this purported phase diagram; with a maximal activity appearing in the mixed phase. Recent Molecular Dynamics simulations of POPC/Chol at different temperatures and cholesterol concentrations along this phase diagram, suggest the existence of cholesterol enriched nano-sites in this mixed phase, indicating the presence of cholesterol lateral heterogeneity. Here we present atomic force microscopy images of supported lipid bilayers made up of POPC and different cholesterol concentrations along the phase diagram. We observe structures that are consistent with the Molecular Dynamics predictions of order parameters and membrane thickness, supporting the idea of cholesterol lateral heterogeneity in binary mixtures, which can enrich the discussion on the existence of lipid rafts and domains in biological membranes. Funding: DGAPA-PAPIIT-RG100416.