(775a) Extracting the Line Tension and Dipole Density Difference From the Domain Distributions of Model Cytoplasmic Myelin Lipid Monolayers

The existence of micron- and/or sub-micron domains in a membrane is an indication of multiple thermodynamic or transient states or phases of the lipid molecules. In this work, a thermodynamic equation expressing the domain size distribution was derived and fitted to measured lipid domain distributions from fluorescence images of both healthy (control) and experimental allergic encephalomyelitis (EAE) cytoplasmic model myelin monolayers. From these fits, we simultaneously extracted the line tension, λ, and headgroup dipole density difference, Δm, between the liquid-orderd (L_o) and liquid-disordered (L_d) phases.

Both λ and Δm decrease with increasing surface pressure, Π, although λ/Δm² remains roughly constant, as the monolayer approaches the miscibility surface pressure. The most probable domain radius, was significantly smaller than that predicted by the energy alone, showing that the entropy of mixing promotes a greater number of smaller domains. Monolayers based on the composition of myelin from animals with experimental allergic encephalomyelitis (EAE) had slightly lower values of λ and Δm, and a higher area fraction of domains, than control monolayers at all Π. Our results show that subtle changes in surface pressure and composition can alter the distribution of material in the monolayer, which will likely also alter the interactions between monolayers important to the adhesion of the myelin sheath.