NONEQUILIBRIUM PHENOMENA IN LIPID-MEMBRANE PHASE-TRANSITIONS

Time-resolved X-ray powder diffraction with synchrotron radiation has been applied to study the structural mechanisms of phase transitions in phospholipid model membrane systems at the millisecond time-scale. Transitions were triggered by single IR-pulses from an erbium glass laser. In those transitions where only the periodicity of the lamellar lattice changes, the transitions were found to be instantaneous, and interpreted as martensitic ''Umklapp''-mechanisms. For transitions, which involve also a change in symmetry type, most notably the lamellar-to-rippled phase transition, short-lived ordered intermediates have been detected, which do not occur under near-equilibrium conditions. By prolonged incubation at the transition temperatures, it was found that defect structures are created which are conserved upon further changes in thermal conditions, indicating a memory phenomenon.