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

By using the pressure-jump relaxation technique in combination with time-resolved synchrotron X-ray diffraction, the kinetics of different lipid phase transformations under conditions close to and far from equilibrium were investigated. The inter-lamellar gel-fluid [L(beta)(P(beta))-L(alpha)] main transition of different phosphatidylcholine systems, the lamellar to hexagonal L(alpha)-H(II) transition of dioleoylphosphatidylethanolamine and egg phosphatidylethanolamine and the lamellar to hexagonal-cubic phase transformation of fatty acid-phospholipid mixtures were studied. The time constants for completion of the transitions vary from seconds to many minutes, depending on the direction of the transition, the symmetry of the lipid structures involved, the temperature and the pressure-jump amplitude. The technique also proved to be a powerful tool to study mesophase transitions of lipids showing 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 transport and redistribution of water into and in the new phase, rather than being controlled by the time required for a rearrangement of the lipid molecules. In addition, nucleation phenomena and domain size growth of the structures evolving might play a significant role. The results are compared with data obtained from other relaxation techniques.