SIMULTANEOUS SMALL-ANGLE AND WIDE-ANGLE X-RAY-DIFFRACTION DURING THE MAIN TRANSITION OF DIMYRISTOYLPHOSPHATIDYLETHANOLAMINE

The main transition of dimyristoylphosphatidylethanolamine (DMPE) disp ersed in excess water has been investigated by simultaneous small- and wide-angle X-ray diffraction during heating and cooling scans. At sca n-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 c ooling. 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 (19 92)). It is suggested that the different components of the transition from the gel to the liquid crystalline phase and vice versa are attrib uted to different steps of a cluster growth mechanism in liposomes. In itially the growth proceeds primarily radially, most likely along defe cts, and not shell by shell. A model is presented to explain (1) the s equence of steps, (2) their relative width and cooperativity and (3) t he hysteresis of the transition.