Geometry of domains and domain boundaries in monolayers and bilayers

Domains in lipid bilayers may exist when two or more phases are allowed. However, the geometry of domains are governed by additional constraints. The most direct way to study the geometry of domains is by microscopy. Electron diffraction patterns that are characteristic of particular molecular packings are used to enhance the contrast of domains by electron microscopy. Surface features associated with lipid phases are utilized in freeze-fracture electron microscopy to define domains. X-ray microanalysis helps to identify the composition of lipids in domains. Phase-specific fluorescence dyes such as merocyanine 540 (MC540) are useful in delineating domains in monolayers and bilayers. MC540 is also sensitive in detecting molecular packing stress below the onset of the bilayer-hexagonal phase transition. Domains in monolayers and bilayers, mapped by different microscopic techniques, are compared.