Energy transduction in respiration and photosynthesis involves the coupling of electron-transfer reactions to the generation of an electrochemical gradient across a bilayer membrane. Although the sequence and kinetics of the electron-transfer reactions are by and large known in these systems, much less is known about the spatial arrangement of the electron carriers and the distances between them. Ideally, the electron carriers could be used as probes in order to unravel their spatial organization and the distances of long-range electron transfer in the biological electron-transport processes. The long-term goals of this project are to use electron spin-relaxation enhancement measurements to determine distances between redox-active sites in proteins and to probe the magnetic properties of multinuclear metal ion clusters. The first objective is to characterize the effects of pairwise magnetic interactions on the electron spin-relaxation rates in samples in which the distance between interacting centers and the magnetic properties of each center are known. In this study, the reaction center protein from Rb. sphaeroides will be used as a model system. This protein can be prepared in a state that contains two paramagnetic sites at a fixed distance of 28 Angstroms. The electron spin-relaxation rate enhancement of the bacteriochlorophyll special pair cation radical due to a magnetic interaction with the nonheme Fe2+, or Mn2+ or Cu2+ in the Fe2+ site, will be systematically investigated by varying the metal ions and the temperature in both non- oriented and oriented samples. These data will be used to determine the limits of applicability of, and to provide data from which to extend, the theories for spin-relaxation enhancement. The second objective is to use measurements of electron spin-lattice relaxation rate enhancements to determine the spatial organization and magnetic properties of redox sites in cytochrome c oxidase, photosystem II, and nitric oxide synthase, three systems for which structural information is not available. Two types of applications will be pursued in order to obtain structural and magnetic information in these three systems and also to establish the general applicability of this method to determine distances and magnetic data in other systems. The first application is to measure spin-spin interactions between endogenous sites in order to determine the distances between paramagnetic centers. The second is to use measurements of the temperature dependence of the spin-lattice relaxation rate of the stable tyrosine radical in photosystem II to probe the magnetic properties of the Mn4 cluster. These experiments will provide a basis for using electron spin-lattice relaxation measurements to determine distances of electron transfer and magnetic properties of metal ion clusters in other redox protein complexes. A better understanding of the mechanisms of electron spin relaxation in proteins is also essential in order to apply EPR spectroscopy to study dynamics in electron-transfer proteins and for applications of pulsed EPR to metalloproteins.
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