Interactions Determining the Growth of Chiral Domains in Phospholipid Monolayers:  Experimental Results and Comparison with Theory

Results are presented from a quantitative analysis of liquid condensed domain shapes in monolayers of the phospholipid d -α-phosphatidylcholine, dipalmitoyl (dipalmitoylphosphatidylcholine or DPPC). We study the effect on liquid-condensed (LC) domain shape of varying in-plane dipole density by examining monolayers composed of different mixtures of the zwitterionic DPPC and anionic phospholipids at high and low electrolyte concentration. Comparison of our data and calculations with existing theories show that the dominant dependence of the shape factor (a measure of the noncompact geometry of LC domains) on in-plane density is described well by electric dipole−dipole interactions; however, the detailed chiral shapes of the liquid-condensed domains remains unexplained. Also unaccounted for is the noncompact growth of LC domains either at electrolyte concentrations high enough to screen out electric dipole interactions or in mixtures with anionic phospholipids. We have also tested the prediction of linear stability analyses that circular domain shapes should give way to a well-defined bilobe transition by computing the Fourier spectrum of our LC domains as a function of domain size and in-plane molecular density. We have performed Monte Carlo simulations of domain growth using an Eden model with energetic contributions from line tension and a novel short-range chiral interaction term. We show that these chiral interactions influence the geometry of the growing domains, yielding noncompact domain growth even in the absence of dipole−dipole interactions.