Simultaneous small- and wide-angle x-ray diffraction during the main transition of dimyristoylphosphatidylethanolamine

The main transition of dimyristoylphosphatidylethanolamine (DMPE) dispersed in excess water has been investigated by simultaneous small- and wide-angle X-ray diffraction during heating and cooling scans. At scan-rates of 0.6 degrees C/min and 0.1 degrees C/min the transition has at least three components during heating, and four components during cooling. Amplitude and thermal width of these components are similar to those found in calorimetric experiments (B. Z. Chowdhry, G. Lipka, A. W. Dalziel, and J. M. Sturtevant, Biophys. J. 45, 901-904 (1984); H. Yao, I. Hatta, R. Koynova, and B. Tenchov, Biophys. J. 61, 683-693 (1992)). It is suggested that the different components of the transition from the gel to the liquid crystalline phase and vice versa are attributed to different steps of a cluster growth mechanism in liposomes. Initially the growth proceeds primarily radially, most likely along defects, and not shell by shell. A model is presented to explain (1) the sequence of steps, (2) their relative width and cooperativity and (3) the hysteresis of the transition.