ESR STUDY OF MAIN PHASE TRANSITION OF DMPC BILAYERS

这个子项目是许多利用资源的研究子项目之一 由NIH/NCRR资助的中心拨款提供。子项目的主要支持 而子项目的主要调查员可能是由其他来源提供的， 包括其它NIH来源。 列出的子项目总成本可能 代表子项目使用的中心基础设施的估计数量， 而不是由NCRR赠款提供给子项目或子项目工作人员的直接资金。 我们一直在研究脂质双层膜的主要相变（凝胶到液晶相变）的分子机制。在此之前，我们观察到DOPC双层膜在Tc附近的准临界涨落。 在过去的一年中，我们改进了23 ℃下DMPC双分子膜中头基自旋标记DPP-Tempo和链自旋标记5 PC和14 PC在Tc附近的ESR谱的模拟。需要两个组件的光谱从5 PC和14 PC，这是符合以前的ESR和NMR研究的主要相变的脂质双层的一个令人满意的拟合。当DMPC分散体的温度从30 ℃冷却到15 ℃时，两种组分的动态有序变化是完全不同的。5 PC和14 PC谱第二分量的序参量S 0在Tc附近表现出强烈的临界现象。例如，5 PC的S 0从24.0C时的0.591急剧下降到23.3C时的-0.457，然后在22 C时跳到0.903。这些变化是二阶相变的特征。相反，对于第一组分，在Tc附近S 0几乎没有变化或略有变化。我们还观察到在Tc附近的5 PC和14 PC谱的线宽的发散，这也是一个二级相变的迹象。 与5 PC的光谱不同，DMPC中的DPP-Tempo的光谱可以很好地与仅单个组分拟合，并且S 0随温度的变化在Tc附近表现出尖状图案，类似于我们先前在DOPC中观察到的DPP-Tempo在Tc附近的图案。这也是一个关键的现象。有趣的是，瓣尖的尖端位于24.4C，略微偏离Tc。5 PC和14 PC的第二组分的旋转扩散速率R垂直和R平行在23.3C急剧下降（超过100倍）。第一（第二）组分的布居数随着温度的降低而减少（增加），仅在Tc以下和以上逐渐减少（增加），但在Tc附近从~0.8（~ 0：2）下降（跳跃）到0.5（~0.5）。通过对脂双层膜主要相变的红外光谱研究，发现这两类分子可能代表了两种不同的脂酰链构象，因而在脂双层膜中具有两种不同的堆积模式。这种类型的温度变化的两个脂质群体表示在Tc的一级相变。然而，总的来说，DMPC双层膜的主要相变是一个弱的一级相变，因为伴随着强临界现象。

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. We have been studying the molecular mechanism of the main phase transition (the gel to liquid crystalline phase transition) of lipid bilayers. Previously, we observed the quasicritical fluctuation in DOPC bilayers around Tc. During the past year, we improved the simulations of ESR spectra of the head group spin label DPP-Tempo and chain spin labels 5PC and 14PC in DMPC bilayers around Tc at 23C. Two components are required for a satisfactory fit of the spectra from 5PC and 14PC, which is consistent with previous ESR and NMR studies on the main phase transition of lipid bilayers. When the temperature of a DMPC dispersion is cooled down from 30C to 15C, changes of the dynamic ordering of the two components are quite different. Order parameters S0 of the second component of 5PC and 14PC spectra show a strong critical phenomenon around the Tc. For example, S0 of 5PC sharply drops from 0.591at 24.0C to -0.457 at 23.3C, then jumps to 0.903 at 22C. These changes are characteristic of a second order phase transition. In contrast, there is nearly no change or a slight change in S0 around Tc for first component. We also observed a divergence in the linewidth of 5PC and 14PC spectra around the Tc, which is also a sign of a second order phase transition. Unlike the spectra of 5PC, the spectra of DPP-Tempo in DMPC can be fit very well with only a single component, and the variation of the S0 with temperature exhibits a cusp-like pattern around Tc, similar to that we previously observed for DPP-Tempo in DOPC around Tc. This is also a critical phenomenon. Interestingly, the tip of the cusp is located at 24.4C, which slightly deviates from Tc. The rotational diffusion rates R perpendicular and R parallel of the second component for both 5PC and 14PC drop precipitously at 23.3C (by factors of more than 100). The population of the first (second) component decreases (increases) with decreasing temperature only gradually below and above the Tc, but drops (jumps) around the Tc from ~0.8(~ 0:2) to 0.5(~0.5). According to IR studies of the main phase transition of lipid bilayers, the two populations may represent lipids which have two different acyl chain conformations, thus they have two different packing modes in the bilayers. This type of temperature variation of two lipid populations signifies a first order phase transition at Tc. However, overall the main phase transition of DMPC bilayers is a weak first order phase transition, because of concomitant strong critical phenomena.