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 i n phospholipid model membrane systems at the millisecond time-scale. T ransitions were triggered by single IR-pulses from an erbium glass las er. In those transitions where only the periodicity of the lamellar la ttice changes, the transitions were found to be instantaneous, and int erpreted 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 pro longed incubation at the transition temperatures, it was found that de fect structures are created which are conserved upon further changes i n thermal conditions, indicating a memory phenomenon.