Activation of N-O Bonds in Metal-NOx Complexes Related to Denitrifying Enzymes: Elucidation of Bioinorganic Pathways to Generate NO and HNO

University Of Georgia Research Foundation Inc, Athens GA

Investigators

Abstract

Denitrifying metalloenzymes are proteins that catalyze the interconversion of reactive oxidized forms of nitrogen (NOx), which are sources of chemical signals in cells and nitrate contaminants in groundwater. For example, the overuse of nitrate-based fertilizers has led to a saturation of aquatic systems as well as drinking water reservoirs with NOx. The result is ecological imbalance, with the associated overstimulated growth of phytoplankton and algae leading to low oxygen levels in water and ultimately death to larger aquatic organisms. With this award, the Chemistry of Life Processes Program in the Chemistry Division is funding Dr. Todd Harrop from the University of Georgia to study iron and cobalt complexes as synthetic analogues of denitrifying metalloenzymes as sources, converters, and/or scavengers of reactive NOx species. This project elucidates the interactions of these metalloenzyme analogues with environmentally and physiologically relevant NOx, and the potential interconversions of these species. Students, including those from underrepresented minority groups, are trained in the understanding of metal-NOx chemistry, which establishes design principles and fundamental structural, electronic, mechanistic benchmarks to guide the discovery of new ways to convert NOx through biology. The project brings together collaborators from across several fields, including the biochemical, biomedical, environmental, spectroscopic/physical, and theoretical sciences. This project is integrated with the UGA Young Dawgs program to provide research internships for high school students in the state and the Skype a Scientist program as an online general science discussion with K-12 classrooms around the world. The chemistry of nitrogen oxides (NOx) is critical to a variety of biological and environmental processes. Indeed, the NOx species nitrite (NO2-) and nitroxyl (HNO) have emerged as potential regulators associated with hypoxic signaling events, i.e., sources of nitic oxide (NO). Furthermore, both species have been shown to be critical in ischemic preconditioning. However, the link between NO2- and HNO is not entirely clear, but it involves metal sites and several studies suggest that hemes may perform this task. The question is how does heme-Fe perform the nitrite-to-NO (or HNO) conversion to result in an otherwise stable and highly covalent Fe-NO bond, and what (if any) other biological signaling agents assist in this task? To improve our understanding of M-NOx chemistry as it relates to biological signaling, low-molecular-weight coordination complexes (inspired by nitrite reductase) are synthesized and spectroscopically characterized to: (i) unravel features critical for M-N(O)/N-O bond activation, (ii) gain access to M-NOx intermediates that are challenging to experimentally achieve in the protein, (iii) interrogate their reactions with other critical small molecule signaling agents (thiols), and (iv) define potential NOx connections/conversions that serve as models for analogous transformations in biology. The synthesis and reactivity of the proposed M-NOx complexes represent significant advances toward the discovery of new/stable redox catalysts for NOx transformations that have applications in environmental remediation and mammalian physiology. 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.