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

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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.