Measurement of Kinetics and Mechanisms of Phase Transitions in Lipid-Water Systems

The lipids from biological membranes, when dispersed in aqueous media, form a variety of phases that include bilayer and nonbilayer arrangements of the molecules. Such phases have been well characterized by conventional X-ray powder diffraction and other methods. Transitions between phases are believed to underlie a number of dynamic membrane processes such as membrane fusion. Studies of the kinetics and mechanisms of phase transitions in lipid-water mixtures require high-intensity synchrotron X-ray sources to monitor the associated structural changes. Facilities at four synchrotron sources providing facilities to measure kinetics and mechanisms of phase transitions in lipid-water systems are described. Some examples are given of how these facilities have been used to measure kinetics of transitions in the dipalmitoylphosphatidylcholine-water system and how different transition mechanisms are defined.