Scaffold-based Biomimetics of Fe-Hydrogenase and Nitrogenase (FeMoco): Interrogating Dynamics, Protein Matrix Effects, and Carbide Motifs

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With the support of the Chemistry of Life Processes Program in the Chemistry Division, Dr. Michael Rose from University of Texas at Austin investigates the role of scaffold-based ligands and carbides in supporting biomimetic models of the enzymes hydrogenase and nitrogenase. These enzymes catalyze fundamental reactions that underpin living systems in numerous environments. Hydrogenases use dihydrogen (H2) produced by microbiological fermenters to reduce carbon dioxide (CO2) in organisms called methanogens. Nitrogenases convert atmospheric dinitrogen (N2, which makes up 80% of the air) into ammonia (NH3), which is essential for plant growth. These two chemical reactions are critical to environmental and green chemistry. Studies of the molecular structures and mechanisms of the active sites of these enzymes are critical to the design, synthesis and understanding of earth abundant and sustainable catalysts that will carry out these reactions. This research will be implemented by a diverse pool of undergraduate researchers from UT Austin and REU programs. These students will learn laboratory and computational skills that will prepare them for graduate study and/or STEM careers in. Dr. Rose will continue to focus on raising and improving safety awareness in the Chemistry Department including through his support of the Chemistry Student Safety Organization (CSSO), which promotes best practices in laboratory safety across the Department. The H2fromH2O educational program will continue to operate in local schools and at local events; a hands-on water-splitting experiment delivered through the program emphasizes the importance of renewable fuels and the potential of sunlight-to-hydrogen conversion as an energy paradigm. More specifically, this research will investigate the use of supramolecular scaffold ligands based on anthracene and related units that support the chemically complex structure of [Fe]-hydrogenase. This enzyme uses a low-spin Fe(II)-dicarbonyl bound to thiolate, pyridone and organometallic acyl donor to perform H2 activation and hydride transfer. The proposed research aims to discover how molecular flexibility in the scaffold can accelerate H2 activation and catalysis. This will be achieved by installing flexible ‘anthranoids’ such as thianthrene and selenthrene in the scaffold to (i) enable more facile access to strained and reactive ground state geometries, and/or (ii) lower the energy of strained transition states to accelerate catalysis. Secondly, the research will aim to install molecular Fe complexes inside a well-characterized protein scaffold — namely, β-lactoglobulin (βLG). While it has been demonstrated that Fe complexes are catalytically competent, the research will endeavor to enhance catalysis by building structurally well-defined interactions (H-bonding, ion pairs, molecular motion) between the metal site and proteinaceous units. This research also aims to synthesize carbide-based Fe clusters relevant to the nitrogenase active site. Presently there are no known synthetic methods to access iron-sulfur-carbide clusters. The project will utilize ‘historical’ iron-carbide-carbonyl clusters as synthons for sulfur and thiolate incorporation in new Fe model complexes. Both electrophilic (S2Cl2, RS–Cl) and nucleophilic (Na2S, RS–) addition mechanisms will be explored, with an emphasis on using under-coordinated versions (non-Wade-Mingos rules) of the iron-carbide-carbonyl clusters for reactive sulfur addition.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.