Mossbauer Spectroscopy of Nitrogenase Components

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In this project, jointly funded by Molecular Biophysics in the Division of Molecular and Cellular Biosciences and the Inorganic, Bioinorganic and Organometallic Chemistry Program in the Chemistry Division, 57Fe Mossbauer spectroscopy, Electron Paramagnetic Resonance (EPR) and density functional theory will be used to study the active sites of iron-containing proteins and synthetic model complexes. Although [2Fe-2S] proteins have been investigated for more than 30 years, many issues regarding their electronic structure have remained unanswered. These include corrections to the observed EPR g-values and 57Fe magnetic hyperfine interactions. Moreover, as shown recently in this laboratory antisymmetric exchange can have a profound influence on the spectroscopic signatures of these proteins. This project will address these questions for a variety of [2Fe-2S] ferredoxins, including Rieske proteins. Attempts will be made to reconstitute the iron-sulfur centers with gallium and iron to obtain ferredoxins containing [GaFe-2S] clusters; such ferredoxins would allow determination of many parameters currently not directly accessible. Iron complexes involving diketiminates have properties resembling features of the cofactor of the biological nitrogen fixation system. Thus, diketiminate iron complexes have been shown to contain activated dinitrogen, and theoretical studies have shown that electronic properties of the diketiminate ligand are quite similar to those of sulfido groups found in the biological system. A series of structurally characterized diketiminate complexes containing iron in oxidation states ranging from Fe(III) to Fe(0) will be studied. In the biological system, dinitrogen and hydrogen are thought to bind to trigonal-pyramidal iron sites of the iron-molybdenum cofactor, and therefore studies of existing diketiminate complexes with bound nitrogen, hydride and ammonia will be carried out. Research on the A-clusters of CoA synthase/carbon monoxide dehydrogenase will also be continued.This project will lead to a more fundamental understanding of the magnetic properties and electronic structure of iron-sulfur clusters and synthetic complexes relevant for nitrogen fixation. The PI's prior research has led to the discovery of new iron sulfur clusters and cluster assemblies, pioneering Mossbauer studies of whole cells, and the introduction of double exchange into the chemical literature. This research is at the interface of chemistry, physics, biochemistry, and biology and thus offers a unique opportunity for students to develop an interdisciplinary perspective in research.