ESR: STUDY OF DYNAMIC STRUCTURE OF HEADGROUPS IN DOPC MULTILAMELLAR MEMBRANES

这个子项目是利用由NIH/NCRR资助的中心拨款提供的资源的许多研究子项目之一。子项目和调查员(PI)可能从另一个NIH来源获得了主要资金，因此可能会出现在其他CRISE条目中。列出的机构是针对中心的，而不一定是针对调查员的机构。脂双层主相变的分子机制是膜生物物理学的一个基本问题。由于这一机制尚未被很好地理解，围绕主要相变的脂双层的异常物理性质仍然令人费解。我们已经完成了DOPC多层囊泡头部基团结构的ESR研究。通过将我们的ESR自旋标记结果与已发表的X射线衍射和2H核磁共振测量结果相比较，我们发现：(1)头部基团的有序性与层间水的有序性相关；(2)头部基团有序性的变化与层间水相与体水相之间的水运动相关。有人认为，双层区域有序性的增加，意味着熵的降低，本身将导致双层间区域的进一步脱水，直到其降低的压力阻止进一步的流动，即渗透现象。PC双层膜在主相变附近的反常膨胀也可以用这个渗透模型来解释。关于胆固醇对这些观察结果的影响，已经进行了进一步的研究。加入10mol%的胆固醇后，囊泡的冻结温度略有降低，DOPC双层膜的主相变也略有降低。然而，加入10mol%胆固醇后，DOPC头部基团的有序参数随温度的变化不变，跃迁也没有变宽。由于渗透模型涉及细胞质膜表面结构的变化，即头部基团的有序化和水化，以及头部基团与可能结合到细胞表面的其他基团之间的分子相互作用的变化，因此它具有重要的生物学意义。膜融合尤其如此，膜脱水很可能促进膜融合。这部作品已经出版了。

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. The molecular mechanism of the main phase transition of lipid bilayers is a fundamental problem of membrane biophysics. Since this mechanism is not well understood, anomalous physical properties of lipid bilayers around the main phase transition are still puzzling. We have completed an ESR study of the structure of head groups in DOPC multilamellar vesicles. By comparing our ESR spin labeling results with published x-ray diffraction and 2H NMR measurements we found (1) that the ordering of the head group is correlated with the ordering of the interbilayer water; (2) that the change of ordering of head groups is correlated with water movement between the interbilayer and the bulk water phases. It is suggested that increased ordering in the bilayer region, implying a lowering of entropy, will itself lead to further dehydration of the interbilayer region until its lowered pressure resists further flow, i.e. an osmotic phenomenon. The anomalous swelling of PC bilayers around the main phase transition can also be interpreted in terms of this osmosis model. Further studies have been conducted on the effect of cholesterol on these observations. Addition of 10 mol% of cholesterol slightly reduces both the freezing temperature of the vesicles, and the main phase transition of DOPC bilayers. However, the variation of the order parameter of DOPC head groups with temperature does not change after addition 10 mol% cholesterol nor is the transition broadened. Since the osmosis model relates the change in the structure on the surface of cell plasma membrane, i.e., the ordering and the hydration of head groups, wit h t he change in the molecular interactions between the head groups and other groups which may bind to the surface of the cell, its biological significance could be significant. This is especially the case for membrane fusion, which may well be facilitated by membrane dehydration. This work has been published.