The Role of Secondary Interactions Relevant to Biological Reductions of Small Molecules

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Abstract: The conversion of dinitrogen to ammonia is required for the global nitrogen cycle and is accomplished biologically by nitrogenase enzymes. Although highly inert, dinitrogen is “fixed” by nitrogenase enzymes, and made biologically available, allowing uptake to form key nutrients necessary to sustain life. The nitrogenase enzyme active site features a multi-metallic core contained within a complex network of amino acids, which are necessary to orchestrate a series of multi-proton, multi-electron transfers to small molecule substrates during the reduction process. Although crucial for dinitrogen reduction, the precise molecular role that these secondary interactions serve to promote reduction is not well known. More explicitly, the scientific community does not precisely know where and how substrates bind, how electrons are delivered, and products released. Thus, there is an inherent gap in our knowledge underlying key contributors to nitrogenase reactivity. To address this gap, this proposal targets the design and study of small molecular constructs that contain highly directed and variable secondary coordination sphere interactions. We will use a rational design approach to prepare synthetic analogues that feature modifiable appended functionality (hydrogen-bond donors, Lewis acids/bases) in the secondary coordination sphere environment to evaluate cooperative reactivity. We will use these molecular structures to test key mechanistic hypotheses regarding the molecular-level reduction of substrates using secondary-sphere cooperativity. We propose that the same type of interactions evaluated in our synthetic systems that promote nitrogenase-type activity can be, by extension, adapted to describe biological systems. The knowledge we acquire will provide key needed contributions to mechanistic studies of nitrogenase function and also synthetic nitrogenases. Substrate activation promoted by highly directed secondary sphere interactions is a broad theme among many biocatalytic cycles, and thus, we envision that the results of our studies will have broad utility to elucidate meaningful contributors to enzymatic reactivity.