ESR STUDY OF MAIN PHASE TRANSITION OF DMPC BILAYERS

这个子项目是许多研究子项目中利用 资源由NIH/NCRR资助的中心拨款提供。子项目和 调查员(PI)可能从NIH的另一个来源获得了主要资金， 并因此可以在其他清晰的条目中表示。列出的机构是 该中心不一定是调查人员的机构。 我们一直在研究脂质双层主要相变(凝胶向液晶相变)的分子机制。在此之前，我们观察到了在Tc附近的DOPC双层膜的准临界涨落。在过去的一年里，我们改进了头基团自旋标记物DPP-TEMPO和链自旋标记物5PC和14PC在温度为23℃的DMPC双层膜中的ESR谱的模拟。5pC和14pC的光谱需要两个组分才能令人满意地拟合，这与以前关于脂双层的主要相变的ESR和核磁共振研究是一致的。当DMPC分散体的温度从30℃降至15℃时，两组分的动态有序性变化大不相同。5pC和14pC谱第二组分的有序参数S0表明在T_c附近有很强的临界现象。例如，5PC的S0从24.0C时的0.591急剧下降到23.3C时的-0.457，然后在22C时跃升至0.903。这些变化是二级相变的特征。相反，对于第一组分，S0在Tc附近几乎没有变化或略有变化。我们还观察到了5pc和14pc谱线宽的发散，这也是二级相变的标志。 与5Pc的光谱不同，DPP-TEMPO在DMPC中的光谱只用一个组分就可以很好地拟合，并且S0随温度的变化在Tc附近呈现出尖点状的图案，类似于我们以前在DOPC中观察到的在Tc附近的DPP-Tempo。这也是一个关键现象。有趣的是，尖端位于24.4C，略有偏离T_c。5pC和14pC的第二组分的旋转扩散速率R垂直和平行，在23.3C处急剧下降(下降了100倍以上)。第一(第二)组分的布居随着温度的降低而减少(增加)，只是在T_c以下和T_c以上逐渐减少(增加)，但在T_c附近从~0.8(~0：2)到0.5(~0.5)下降(跳跃)。根据对脂双层主要相变的红外光谱研究，这两个群体可能代表了具有两种不同酰链构象的脂类，因此它们在双层中有两种不同的堆积模式。这种类型的两类脂族的温度变化表明在T_c处有一级相变。然而，总体来说，DMPC双层膜的主要相变是弱的一级相变，这是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. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. 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.