This work is aimed at a careful and accurate study of model membrane systems using electron-spin relaxation techiques with nitroxide and vanadyl probes. We use ESR linewidth and lineshape studies as well as spin echoes ENDOR, ELDOR and saturation transfer. In our initial work we are concentrating on hydrated dipalmitoyl phosphatidylcholine bilayers and we focus on moleculr dynamics in the various phases exhibited by this system, which we study as a function of sample orientation, percent water, temperature and pressure. The spin-relaxation analyses are based on the regorous theoris of Freed and coworkers for motional and slow-tumbling effects, which have previously been extensively applied to (thermotropic) liquid cyrstals. While previous ESR studies on model membranes have been interpreted approximately, revealing important properties such as membrane fluidity, segmental motion, polarity, probe location, and rates of rational and translational diffusion, our studies are refining and extending this approximate work. Our studies are also revealing possible cooperative modes of lipid chain distortions, magnetic-field induced headgroup cooperativity, as well as effects of local structure and interbilayer cooperativity in the biaxial phase. These properties are also to be studied at the lipid pahse transitions. These studies will be exteneded to protein-lipid interactions using reconstituted membrane systes, e.g. sonicated vesicles containing biologically active enzyme Ca++ ATP-ase. A new method will be used to prepare well-oriented multilayers from reconstituted and natural membrane fragments. The accuracy of the model of immobilized boundary-lipid plus fluid lipid is to be carefully tested by rigorous spin-relaxation experiment and theory, and the dependence of enzyme activity on the state of molecular motion of the membrane lipids is to be considered.
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