2015-12-02
10:15 at HCI J498

Line Active Lipids Promote Formation of Nanoscale Domains in Model Membranes

Sam Safran

Department of Materials and Interfaces, Weizmann Institute of Science, Israel

Finite-sized (10-100 nm), long-lived domains, are found in biological membranes where they are termed “lipid rafts” that selectively transport proteins and lipids. The existence of these domains has inspired studies of lipid phase separation in model, multicomponent membranes. A priori, one expects the size of phase separated domains to be on the order of the system size because the positive line tension of the interfaces between the domains of incompatible lipids drives domain coarsening. This expectation motivates the puzzle of the observed stability of nano-metric domains in some model systems. In this talk on possible theoretical resolutions of this puzzle, we first review the physics of phase separation in mixtures and mention several scenarios in which finite sized domains (akin to modulated structures) can be stable in equilibrium. We then introduce the relevant experiments that focus on domain formation in mixed membranes. The theory explains how nano-sized domains may be stabilized by line active, “hybrid” lipids (such as POPC) that can reside at the interfaces separating domains of saturated and unsaturated lipids; in the absence of these line active species, the domains are of the order of the system size. The line activity arises solely from the chain packing characteristics of the hybrid lipids near the saturated-unsaturated interface and not from hydrophobic-hydrophilic effects. In the higher temperature, mixed (single phase) regime, the theory predicts correlation lengths and correlation times that are, respectively, shorter and longer when hybrid lipids are added. Nanoscale fluctuation domains are more probable and have longer lifetimes than in two-component, saturated/unsaturated mixtures. In the lower temperature, phase separated (coexistence) regime, zero line tension between domains rich in saturated/unsaturated lipids may occur at physiologically relevant temperatures and large hybrid fractions. Under these conditions, the membrane can spontaneously form more interfaces and promote the formation of stable nanoscale domains. Collaborations: Benoit Palmieri, Tetsuya Yamomoto, Robert Brewster