Monolayer Domain Shapes - Equilbrium or Kinetically Controlled?

Lateral phase separation of lipid monolayers and bilayers into domains of different composition or local order is the basis of the “raft” hypothesis of cell membrane organization in which sub-micron domains, or rafts, of different local composition or order nucleate and grow from a continuous phase within the cell membrane. These physical and chemical inhomogeneities within the membrane provide sites for multiple different proteins to localize and carry out complex cell functions. Phase separation and domain formation is also important to the dynamic spreading and surface tension lowering ability of native and clinical lung surfactants (LS) used to treat neonatal respiratory distress syndrome (NRDS) in premature infants.<br/>The relative simplicity of two-dimensional lipid monolayer films makes them ideal systems to study the fundamental issues that govern lateral phase separation, the evolution of domain microstructure, and the effects of this microstructure on interfacial dynamics. Here we show that a myriad of domain morphologies that occur during monolayer compression are the result of the classic Mullins-Sekerka growth instability (1) that occurs during crystallization from a multicomponent melt. On compression of dipalmitoylphosphatidylcholine-hexadecanol (DPPC-HD) monolayers with 1-5 mol% cholesterol, originally circular domain develop finger-like growth patterns as the surface pressure is increased. The finger widths and number vary with compression rate and quench depth with semi-quantitative agreement with the predictions of the Mullins-Sekerka theory. The Mullins-Sekerka theory postulates a diffusion front instability that predicts that the finger widths are set by a balance between line tension, crystallization rate, and the local variation in chemical potential between crystal and melt. The fingers are purely kinetically driven but eventually evolve into equilibrium extended stripe domains whose width depend only on surface pressure and temperature. The stripe width in independent of the route by which the morphology is approached, suggesting thermodynamic equilibrium. These observations help explain the wide variety of domain shapes observed in lipid monolayers that depend on compression rate and monolayer history.