Kinetics of phase transformations between lyotropic lipid mesophases of different topology: a time-resolved synchrotron X-ray diffraction study usingthe pressure-jump relaxation technique

By using the pressure-jump relaxation technique in combination with time-re solved synchrotron X-ray diffraction, the kinetics of different lipid phase transformations under conditions close to and far from equilibrium were in vestigated. The inter-lamellar gel-fluid [L-beta(P-beta)-L-alpha] main tran sition of different phosphatidylcholine systems, the lamellar to hexagonal L-alpha-H-II transition of dioleoylphosphatidylethanolamine and egg phospha tidylethanolamine and the lamellar to hexagonal-cubic phase transformation of fatty acid-phospholipid mixtures were studied. The time constants for co mpletion of the transitions vary from seconds to many minutes, depending on the direction of the transition, the symmetry of the lipid structures invo lved, the temperature and the pressure-jump amplitude. The technique also p roved to be a powerful tool to study mesophase transitions of lipids showin g structural intermediates under non-equilibrium conditions, which are not seen in transitions close to equilibrium, i.e., under slow scan conditions. In most cases the rate of the transition is probably limited by the transp ort and redistribution of water into and in the new phase, rather than bein g controlled by the time required for a rearrangement of the lipid molecule s. In addition, nucleation phenomena and domain size growth of the structur es evolving might play a significant role. The results are compared with da ta obtained from other relaxation techniques.