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

Abstract

The kinetics and mechanism of the barotropic lamellar gel (L beta')/lamellar liquid crystal (L alpha) phase transition in fully hydrated 1,2-dihexadecyl-sn-glycero-3-phosphoethanolamine (DHPE) has been studied using time-resolved x-ray diffraction (TRXRD). The phase transition was induced by pressure jumps of varying amplitudes in both the pressurization and depressurization directions at controlled temperature (78 degrees C). Both low- and wide-angle diffracted x rays were recorded simultaneously in live time using an x-ray-sensitive image intensifier coupled to a CCD camera and Super-VHS videotape recorder. Such an arrangement allowed for the direct and quantitative characterization of the long- (lamellar repeat spacing) and short-range order (chain packing) during a kinetic experiment. The image-processed live-time x-ray diffraction 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-state (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-to-L beta' phase transition decreased to a limiting value of approximately 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 depressurization-induced L beta'-to-L alpha phase transition was slower, taking approximately 1 s to complete, and occurred with no obvious dependence of the transit time on pressure jump amplitude. In the depressurization jump experiment, the lipid responded rapidly to the pressure 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 measurements.

Manipulating Mesophase Behavior of Hydrated DHPE: An X-ray Diffraction Study of Temperature and Pressure Effects

“…2,8 The powder diffraction pattern was radial averaged over a 10°arc centered on the vertical axis to produce an intensity-scattering vector (I-s) scan. The integrated intensities of the individual lamellar reflections were calculated from a Lorentzian function fitted to each diffraction peak.…”

“…1,6,7 Here, an investigation of pressure-and temperature-induced changes in equilibrium mesophase structure in the fully hydrated 1,2-dihexadecyl-sn-glycero-3-phosphoethanolamine (DHPE) system is described. The results of the study provide valuable insight into the structure-thermodynamic variable relationship and provide important quantitative mensurations for use in ongoing studies of the kinetics and mechanism of lipid phase transitions (see ref 8, for example).…”

Pressure-jump relaxation kinetics of a DNA triplex helix-coil equilibrium

“…Although the magnitude of the pressure jump we used is large, 49 MPa, it is within the range employed in other experiments reported in the literature. 5,7 The approximations inherent in relaxation kinetics are valid only if the size of the perturbation (pressure jump) is sufficiently small. For a single-step bimolecular reaction, this condition means that the results should be independent of how the equilibrium is approached.…”

“…[1][2][3][4][5][6][7][8][9] The influence of pressure on the kinetics of helix-coil transitions of nucleic acids remains largely an unexplored area. Due to the central role played by these transitions in a number of fundamental biochemical events, a thorough understanding of both the mechanism of the helix-coil equilibrium and the factors influencing the stability of the DNA is important.…”

Bright prospects for biological non-crystalline diffraction

“…Time resolved X-ray diffraction has also been used to study the physical properties of synthetic model membrane systems in order to understand such processes as membrane fusion using various kinetic techniques such as temperature-jumps induced with lasers or rapid pressure changes 9,10 .…”