Cheng A, Hummel B, Mencke A, Caffrey M
Department of Chemistry, Ohio State University Columbus 43210.
Biophys J. 1994 Jul;67(1):293-303. doi: 10.1016/S0006-3495(94)80480-1.
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.
利用时间分辨X射线衍射（TRXRD）研究了完全水合的1,2 - 二十六烷基 - sn - 甘油 - 3 - 磷酸乙醇胺（DHPE）中，正压层状凝胶（Lβ'）/层状液晶（Lα）相变的动力学和机制。在控制温度（78℃）下，通过在增压和减压方向上施加不同幅度的压力跃变来诱导相变。使用与CCD相机和超级VHS录像机耦合的X射线敏感图像增强器，实时同时记录低角度和广角衍射X射线。这种设置允许在动力学实验期间直接和定量地表征长程（层状重复间距）和短程有序（链堆积）。使用非线性最小二乘模型对图像处理后的实时X射线衍射数据进行拟合，并在整个转变过程中连续监测拟合参数。在该方法的灵敏度极限范围内，压力诱导的从Lα到Lβ'相以及从Lβ'到Lα相的转变是双态的（在转变过程中没有明显的中间体形成）。随着压力跃变幅度的增加，与增压诱导的Lα到Lβ'相变的长程和短程有序相关的相应转变时间（两相共存的时间）降低到约50毫秒的极限值。该极限值接近探测器/记录系统的响应时间。因此，在78℃下完全水合的DHPE中该转变的固有转变时间小于或等于50毫秒。相比之下，减压诱导的Lβ'到Lα相变较慢，大约需要1秒完成，并且转变时间对压力跃变幅度没有明显依赖性。在减压跃变实验中，脂质在Lβ'相中对压力跃变迅速响应，直至速率决定的Lβ'到Lα转变。对这种行为进行了仔细研究，因为它可能会使相变动力学测量的解释复杂化。
