MODE SELECTION AND SHAPE TRANSITIONS OF PHOSPHOLIPID MONOLAYER DOMAINS

Above a critical surface pressure, lipid monolayers at the air/water interface form gellike domains which coexist with a less dense fluid phase. In this work, we employ the free energy theory of McConnell and co-workers to predict the shapes and associated shape transitions of regularly undulating lipid domains, i.e., domains with boundaries that undulate with a characteristic mode and relative amplitude. The equilibrium shape is determined by a dimensionless number, Γ, which relates the importance of repulsive electrostatic interactions arising from the dipole moments of the lipid molecules to the effect of line tension. Shape transitions are predicted between successive modes as Γ is varied. While the mode of the shape with the lowest free energy increases with Γ, the relative amplitude of the undulation asymptotes to a limit. We also consider mixed-mode shapes formed by combining two distinct modes. For the cases examined, we find no evidence of mode coupling. We also show how the analysis can be used to interpret experimental observations, at fixed Γ, of the shapes of lipid domains as their area is varied. Shape transitions between higher order modes are predicted as the area is increased; over the range of areas where a given mode is the most stable, the relative amplitude of the undulation increases with increasing area. © 1990 American Chemical Society.