The Effect of Membrane-To-Domain Thickness Mismatch in Phase Separation Ternary Lipid Systems as a Function of Vesicle Size

Abstract

Cellular membranes are no longer viewed as a homogeneous mix of lipids and proteins, but rather as having distinct lipid domains, so-called “rafts”, which are key for many biological processes. Much work devoted to understanding the actual mechanisms that drives lateral organization in cell membranes has been done on model membrane systems. This approach has given great insight into the formation of lipid rafts because in simple ternary mixtures with a saturated lipid, an unsaturated lipid and cholesterol, a region of liquid-liquid coexistence was found, with one of the liquid phases rich in cholesterol and saturated lipids, the finger-print of rafts. However, there is still no clear understanding of the molecular parameters that drives phase separation. Recently a new curvature hypothesis proposed a correlation between composition, leaflet coupling and emulsion-like induced curvature to predict phase separation and the formation of domains. To explore these ideas, we recently studied the phase behavior of well-researched phase separating ternary mixtures: dDPPC-DOPC-Cholesterol (1:1:1) and dDMPC-DOPC-Cholesterol (1:1:1), in 30nm vesicles using Small Angle Neutron Scattering. dDMPC is a 14-carbon long lipid while dDPPC is 16-carbon long lipid. As temperature was lowered, the system with dDPPC showed excess scattering, whereas the system with dDMPC showed no scattering even below its melting temperature. These results are interesting when compared to the previously established work where both systems show phase separation in GUVs as seen by fluorescent microscopy. Phase separation behavior differences as a result of variations to the tail length of the saturated lipids in our vesicles with high curvature gives insight into the role of local curvature (emulsion-like effects) at the domain-membrane interface.