(350b) Model Myelin Membranes: Asymmetry, Domains, Miscibility and Equilibrium

This myelin membrane bilayer is asymmetric, with different composition and phase behavior on either side of the bilayer.  Both cytoplasmic and extracellular sides of the myelin phase separate into liquid ordered and liquid disordered domains that are miscible at low surface pressures.  With increasing surface pressure the domains mix; for the cytoplasmic side, the transition is nearly immediate, the extracellular side transitions via a critical point at which the domains coalesce, elongate and composition fluctuations persist into the homogenous phase.  We show that the domain distribution on the cytoplasmic side is consistent with the equilibrium distribution predicted through a balance of domain line tension, electostatic self-energy and the entropy of mixing.  From the size distribution we extract the line tension and dipole density difference and show that the line tension is extremely low in complex lipid mixtures compared to single component monolayers. Subtle differences between normal myelin and myelin from animals with multiple sclerosis may lead to differences in the adhesion and fluidity of the membranes and correlate with the origination of this disease.