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.