KINETICS AND MECHANISM OF THE BAROTROPIC LAMELLAR GEL LAMELLAR LIQUID-CRYSTAL PHASE-TRANSITION IN FULLY HYDRATED DIHEXADECYLPHOSPHATIDYLETHANOLAMINE - A TIME-RESOLVED X-RAY-DIFFRACTION STUDY USING PRESSURE JUMP

The kinetics and mechanism of the barotropic lamellar gel (L(beta),)/l amellar liquid crystal (L(alpha)) phase transition in fully hydrated 1 ,2-dihexadecyl-sn-glycero-3-phosphoethanolamine (DH PE) has been studi ed using time-resolved x-ray diffraction (TRXRD). The phase transition was induced by pressure jumps of varying amplitudes in both the press urization and depressurization directions at controlled temperature (7 8 degrees C). Both low- and wide-angle diffracted x rays were recorded simultaneously in live time using an x-ray-sensitive image intensifie r coupled to a CCD camera and Super-VHS videotape recorder. Such an ar rangement allowed for the direct and quantitative characterization of the long- (lamellar repeat spacing) and short-range order (chain packi ng) during a kinetic experiment. The image-processed live-time x-ray d iffraction data were fitted using a nonlinear least-squares model, and the parameters of the fits were monitored continuously throughout the transition. The pressure-induced transitions from the L(alpha) to the L(beta), phase and from the L(beta), to the L(alpha) phase was two-st ate (no formation of intermediates apparent during the transition) to within the sensitivity limits of the method. The corresponding transit time (the time during which both phases coexist) associated with the long- and short-range order of the pressurization-induced L(alpha) and L(beta), phase transition decreased to a limiting value of approximat ely 50 ms with increasing pressure jump amplitude. This limiting value was close to the response time of the detector/recording system. Thus , the intrinsic transit time of this transition in fully hydrated DHPE at 78 degrees C was less than or equal to 50 ms. In contrast, the dep ressurization-induced L(beta),-to-L(alpha) phase transition was slower , taking approximately 1 s to complete, and occurred with no obvious d ependence of the transit time on pressure jump amplitude. In the depre ssurization jump experiment, the liquid responded rapidly to the press ure jump in the L(beta), phase up to the rate-determining L(beta),-to- L(alpha) transition. Such behavior was examined carefully, as it could complicate the interpretation of phase transition kinetic measurement s.