Structural rearrangements during crystal-liquid-crystal and gel-liquid-crystal phase transitions in aqueous dispersions of dipalmitoylphosphatidylethanolamine. A time-resolved X-ray diffraction study.

The mechanism and kinetics of the crystal-liquid-crystal (Lc----L alpha) and gel-liquid-crystal (L beta----L alpha) transitions of the L-enantiomer and racemic dipalmitoylphosphatidylethanolamine have been examined in temperature scans and jumps using time-resolved X-ray diffraction methods. The Lc----L alpha transformations (at 66 degrees C for L-dipalmitoylphosphatidylethanolamine and 82 degrees C for DL-dipalmitoylphosphatidylethanolamine) were found to be two-state (first-order) processes characterised by co-existence of the initial Lc and final L alpha states during the transition with the absence of any detectable intermediates states. The transition mechanism involves firstly, disordering of the hydrocarbon chains which makes a major contribution to the transition enthalpy and secondly by a transition in the lamellar repeat spacing. The overall relaxation time of the Lc----L alpha transition of L-dipalmitoylphosphatidylethanolamine during temperature jumps of 4.5 degrees C/s was about 10 s. A gradual increase in the gel-state interchain spacing during the L beta----L alpha transitions of L- and DL-dipalmitoylphosphatidylethanolamine preceded a broadening of the wide-angle diffraction peak. There was a concomitant and continuous increase of the lamellar repeat spacing to values typical of the L alpha phase with increasing temperature. This sequence of events is completely reversible on cooling with a temperature hysteresis of 5-6 degrees C. The relaxation times of the L beta---L alpha transitions during jumps of 4.5 degrees C/s were about 2 s in both the heating and cooling directions.