Appearance of Modulated Bilayer Morphology for Coexisting LD and LO Phases is Correlated with Line Tension

Giant Unilamelar Vesicles (GUVs) appear uniform for the brain-SM/POPC/Chol bilayer mixture, whereas GUVs exhibit macroscopic domains of coexisting liquid-disordered + liquid-ordered phases for the brain-SM/DOPC/Chol mixture. We can travel through composition space in order to study this transition from nanodomains with POPC to macrodomains with DOPC. Using ρ defined as the ratio [DOPC]/[DOPC]+[POPC], the domain morphology undergoes a transition regime where “modulated phases” appear as a function of ρ. The formation of these different morphologies on a GUV can be understood as a competition between line tension, which favors large domains, and bending energy, which favors small domains (Amazon et al). We measured the ρ values where modulated phases appear for different 4-component mixtures, using brain-SM, or egg-SM, or palmitoyl-SM as the high-melting lipid, and DOPC/POPC or DOPC/SOPC as the low melting lipids, for a total of six different 4-component mixtures. We then measured the line tension of the macroscopic domains vs ρ, and found the same line tension at ρ values at the transition between modulated phases and macroscopic phases. This finding implies that line tension has major control over domain morphology. We are currently studying how the transmembrane peptide, GWALP-23, changes the morphology by changing the values of the competing interactions.