This award in the Inorganic, Bioinorganic and Organometallic Chemistry program supports research by Professor Craig Grapperhaus at the University of Louisville that focuses on functional model complexes of nitrile hydratase (NHase) and the related oxygen sensitivity of iron thiolates. The project includes the synthesis and spectroscopic characterization of target complexes coupled with reactivity studies and computational methods to develop an understanding of the relationship between electronic structure and reactivity. The first specific aim is to test the hypothesis that functional NHase models mimic S-oxygenation effects including tuning of Lewis acidity and substrate activation/intermediate stabilization via a second coordination sphere. To meet this objective, substrate binding and activity assays with electronic mimics of NHase that lack S-oxygenated ligands will be investigated. Further, Ru model complexes that incorporate S-oxygenation will be developed to probe their second coordination sphere effects and new ligands with alternate second coordination sphere moieties will be prepared and evaluated. The second specific aim will probe the NHase oxygen-sensitivity of iron-thiolates. The hypothesis that oxygen sensitivity of Fe-thiolates is spin-state dependent with low-spin Fe yielding S-oxygenates and high-spin Fe forming disulfides and Fe-oxo clusters will be evaluated via reactivity studies on a series of Fe complexes with varying spin-states. Complimentary density functional theory investigations to elucidate the electronic structure effects will be conducted. Additionally, the hypothesis that chelate ring size can enforce S-coordination of Fe S-oxygenates will be evaluated through the design and synthesis of new ligand motifs.
The broader impacts of this study focus on promoting teaching, training and learning and broadening the participation of students in an underrepresented region. This project will provide training for post-doctoral fellows and graduate students working alongside undergraduate researchers and high school students in the fields of synthesis, spectroscopy, reactivity, and computation. Undergraduate students will be recruited locally and from regional undergraduate institutions. Additionally, collaborative efforts with a faculty member and students at a primarily undergraduate institution (PUI) are in place. The annual, regional undergraduate research symposium initiated by the PI as a mechanism for students and faculty at PUIs to present research will be continued with the aim of including industrial participation to bridge the gap between industry and undergraduate students. This symposium has been successful at attracting female and underrepresented minorities, including participants from a historically black university.
" in the Inorganic, Bioinorganic and Organometallic Chemistry program at the National Science Foundation supported research by Prof. Craig A. Grapperhaus (PI) and Robert M. Buchanan (Co-PI) of the University of Louisville (UofL). The project focused on the development of bio-inspired small molecule complexes to address the unusual post-translational sulfur-oxygenation at the nitrile hydratase (NHase) enzyme active sites and its influence on catalytic activity. The NHase-mediated hydrolysis of nitriles to amides offers significant advantages over traditional routes, exemplified by its status as the first industrial scale biotransformation of an organic product and the first biotechnology application in the petroleum industry. It is currently used for the large-scale production of acrylamide with potential additional utility for the bioremediation of toxic nitriles, including those in pharmaceutical waste-water, shale oil, and herbicide and other contaminated soils. The active site of NHase contains either an iron or cobalt center coordinated in part with S-oxygenated cysteine donors. With the support of this award, we performed a series of synthetic and reactivity studies to elucidate the key features necessary to develop artificial catalysts inspired by NHase. Early studies focused on iron complexes revealed the importance of establishing high metal-sulfur bond covalency by careful control of the type and arrangement of donor atoms to the metal center. With this knowledge in hand, we performed a series of competition studies revealing a binding preference of water > amide > nitrile suggesting hydrolytic activity was feasible within our system. However, complications arising from the formation of iron-oxo clusters limited success with the original design. With the knowledge gleaned from these earlier studies, we pursued the related ruthenium complexes leading to the development of an unprecedented and functional family of ruthenium S-oxygenate complexes. These complexes accurately mimic the enzyme's mixed S-oxygenate donor environment and catalytic function. The complexes also provide the unique opportunity to systematically evaluate the influence of S-oxygenation on substrate binding, ligand exchange, and hydrolytic activity. In addition, we developed an alternate approach for the design of bio-inspired functional complexes based on the design of active sites mimics with an extended functional groups that reproduce key features of the enzyme pocket. Highlights include the report of a highly variable synthon that can be employed in the design and synthesis new structures for the creation of artificial enzyme pockets. The broader impacts of this proposal include educational benefits, dissemination of results, and potential applications. One post-doctoral fellow and five graduate students participated on the project. One student completed his Ph.D. during this award and two additional students are anticipated to complete their Ph.D. in Spring 2012. Additionally, the project supported the work of two undergraduate research students leading to the publication of one manuscript with the undergraduate as the sole co-author with the PI. The grant also supported collaboration with Prof. Christopher Mullins and his students at Campbellsville University. In total the work conducted during the grant period has generated 10 peer reviewed manuscripts.
