Kinetics of the lamellar-inverse hexagonal phase transition determined by time-resolved X-ray diffraction.
Kinetics of the lamellar-inverse hexagonal phase transition determined by time-resolved X-ray diffraction.
复制标题
由时间分辨 X 射线衍射测定的层状反六方相变动力学。
DOI：
10.1021/bi00119a017
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发表时间：
1992
期刊：
影响因子：
2.9
通讯作者：
D'Amico,KL
中科院分区：
文献类型：
--
作者：
Tate,MW;Shyamsunder,E;Gruner,SM;D'Amico,KL
摘要：用时间分辨x射线衍射研究了二酰基-磷脂酰乙醇胺(PE)-水体系的层状(La)-逆六方（Hn）相变动力学。通过50 ms电流脉冲通过脂质-盐水分散，导致样品的欧姆加热，从而引发大的温度跳变（高达30℃），研究了快速时间（-100 ms）内的转变动力学。在国家同步加速器光源上获得了时间分辨率为10毫秒的衍射。l， 2-二酰基-sn-甘油-3-磷酸乙醇胺（DOPE）的相变时间常数为100 ms左右。1,2 -二烯二酰-s -甘油-3- pe混合物的转变行为更快。两种体系的Hn晶格参数都从与片层晶格相称的初始值膨胀到最终的平衡值。膨胀的速度与温度上升的幅度无关。对于小的温度跳跃（< 10℃），相变动力学急剧减慢，并且可以很容易地在传统的旋转阳极x射线源上进行转变研究。在4℃时，观察到DOPE样品在一周内缓慢地转变为六边形相，片层强度的衰减在四十年的时间内符合幂律行为。这种幂律行为对用量热法等常规方法测定脂-水分散体的相变温度有有趣的影响。细胞膜由多种磷脂组成，其中约10-50%与其他膜组分分离后会形成非双层相（Cullis et al., 1985）。将这些非双层脂质纳入双层膜的一个作用是促进在各种细胞功能过程中发生的瞬时非双层结构的形成（Ellens等，1989）。然而，人们对这些结构形成的动力学知之甚少。希望通过研究定义良好的系统中的结构相变，可以建立理解伴随各种细胞过程的膜形态变化的基础。脂质分子的两亲性决定了脂质与水混合时的基本组织原理。脂质具有一个极性头基，它更喜欢由水提供的极性环境。相反，非极性烃链聚合体与极性溶剂的接触减少，导致首基团在烃链和水之间形成连续界面的结构。满足这些条件的两种常见的中间相是层状（La） 1和反六方（Hn）相（图1）。L '相由脂质和水相间的平面区域组成。Hn相由排列在六边形晶格上的圆柱形水棒组成。水区被极性头基包围，碳氢化合物尾部填充晶格的间隙区域。
Revised Manuscript Received October 21, 1991 abstract: The kinetics of the lamellar (La)-inverse hexagonal (Hn) phase transition in diacyl-phosphatidylethanolamine (PE)-water systems were probed with time-resolved X-ray diffraction. Transition kinetics in the fast time regime (—100 ms) were studied by initiating large temperature jumps (up to 30 C) with a 50-ms electrical current pulse passed through a lipid-salt water dispersion, resulting in ohmic heating of the sample. Diffraction with a time resolution to 10 ms was acquired at the National Synchrotron Light Source. The time constant for the phase transition for l, 2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) was on the order of 100 ms for the largest temperature jumps recorded. Faster transition behavior was found for a 1, 2-dielaidoyl-s «-glycero-3-PE mixture. The Hn lattice parameters for both systems were seen to swell from an initial value commensurate with the lamellar lattice to the final equilibrium value. The rate of swelling was seen to be independent of the magnitude of the temperature jump. For small temperature jumps (< 10 C), the phase transition kinetics slow dramatically, and transition studies can readily be performed on a conventional rotating anode X-ray source. At 4 C, a DOPE sample was observed to slowly convert to the hexagonal phase over the course of a week, with the decay in the lamellar intensity fitting a power law behavior over four decades of time. This power law behavior is shown to have interesting consequences tothe determination of the phase transition temperature of lipid-water dispersions by con-ventional methods such as calorimetry. e cell membrane is composed of a large variety of phos-pholipid species, roughly 10-50% of which will form a non-bilayer phase when isolated from the other membrane con-stituents (Cullis et al., 1985). One role proposed for the inclusion of these nonbilayer lipid species into the bilayer membrane is to facilitate the formation of the transient non-bilayer structures which occur during various cellular functions (Ellens et al., 1989). Little is known about the dynamics of formation of these structures, however. Hopefully, the basis for understanding thechanges in membrane morphology that accompany various cell processes can be established by stud-ying structural phase transitions in well-defined systems. The amphiphilic nature of the lipid molecule dictates the basic organizationalprinciple of the lipid when mixed with water. The lipid has a polar headgroup which prefers the polar environment provided by water. The nonpolar hydrocarbon chains, by contrast, aggregate tominimize contact with the polar solvent, resulting in structures in which the headgroups form a continuous interface between the hydrocarbon chains and water. Two commonly seen mesophases which satisfy these conditions are the lamellar (La) 1 and the inverse hexagonal (Hn) phases (Figure 1). The L „phase consists of alternating planar regions of lipid and water. The Hn phase consists of cylindrical rods of water arranged on a hexagonal lattice. The water region is surroundedby the polar headgroup with the hydrocarbon tails filling the interstitial regions of the lattice.