Energy transduction in membranes involves proton translocation which is coupled to electron transport through bound electron carriers. One group of electron transport proteins involved in this process is the iron-sulfur proteins. An integral protein complex involved in energy transduction in mitochondrial, chromatophore and chloroplast membranes is the cytochrome b-c complex which contains an iron-sulfur protein known as the Rieske iron-sulfur center. Previous studies have shown this center interacts with quinone molecules but details of this interaction are not understood and these are critical in understanding electron transfer through this energy transducing site. Factors which are important in semiquinone stabilization remain to be defined in molecular terms. We will study the interaction of inhibitory quinone analogs which bind to the Rieske center in order to determine structure-function relationships of this interaction. This will involve EPR techniques in conjunction with redox potentiometry. The techniques of resolution and reconstitution will be used to characterize the interaction of physiologically active quinones with the Rieske center. We shall determine the organizational structure of a cytochrome complex from photosynthetic membranes which has many functional similarities to the mitochondrial complex. The possible role of quinone binding proteins and quinone free-radical species in electron transport processes of this complex will also be considered. Finally, we plan a characterization of another class of iron-sulfur proteins, NADH dehydrogenases, of simpler composition than their mitochondrial counterpart in order to assess the role of iron-sulfur centers in energy transduction associated with these enzymes. These experiments will involve both the bound and soluble enzyme and will examine EPR properties of the associated iron-sulfur center(s). These studies should provide a description in molecular terms of the function of bound iron-sulfur centers in the basic cellular process of energy transduction which results in the synthesis of ATP.