Chemical Reactivity and Redox Behavior of Heme-Nitrogen Oxide Derivatives

University Of Oklahoma Norman Campus, Norman OK

Investigators

Abstract

Many factors contribute to changes in the weather and the environment, one of which is the increased accumulation of the nitrogen-containing gas nitrous oxide (N2O) in the atmosphere. Bacteria and fungi utilize complex metalloenzymes to generate N2O from the natural starting material nitric oxide (NO), but the fundamental chemical mechanisms for this process are not understood. Therefore, there is an urgent need to probe and understand this component of the global N-cycle, in particular, the role that metals play in mediating the production and consumption of such NOx gases of environmental and agricultural importance. Collaborators Dr. George Richter-Addo and Dr. Michael Shaw study non-protein chemical analogues of complex bacterial and fungal metalloenzymes to probe N2O formation and NOx utilization by these organisms. In particular, the team is preparing, isolating, and characterizing unstable intermediates along observed reaction pathways. These studies allow for a better understanding of how these gases are produced and controlled, and how nitrogen, an essential element in fertilizer required for crop growth, can be improved. The collaborative team combines a Ph.D. environment (at the University of Oklahoma) and a primarily undergraduate environment (at Southern Illinois University Edwardsville) to provide high-level training to a diverse group of students. The team is also active in introducing at-risk high school students and the general public to modern science. Dr. George Richter-Addo participates in a summer program designed for inner city high school kids at high risk for failure in academic programs. Dr. Michael Shaw produces on-line lecture videos suitable for individuals with disabilities, thus representing an avenue of science outreach to a traditionally hard-to-reach underrepresented group. Heme proteins are involved in the nitric oxide (NO) to nitrous oxide (N2O) conversion of relevance to global warming, and in the inorganic-NOx to organo-NOx conversions of relevance to agricultural N-assimilation and nitrosative stress. Both processes are important, but their fundamental chemical pathways are not well understood, thus preventing further development of the chemistry of these components of the global N-cycle. With funding from the Chemical Structure, Dynamics and Mechanisms-B Program of the NSF Chemistry Division, the collaborative research team of Dr. George Richter-Addo (University of Oklahoma) and Dr. Michael Shaw (Southern Illinois University Edwardsville) is determining the factors that lead to chemical reactivity of the bound NO ligand in synthetic porphyrin systems that model heme-containing enzymes in bacteria and fungi involved in N2O and/or organo-NOx generation. The collaborative team is determining the experimental conditions that favor nucleophilic attack of the bound NO ligand in ferric-NO porphyrins and promote N2O formation (via H-N bond formation; fungal N2O pathway) and organo-NOx generation (via C-N/N-N/S-N bond formation). They are also determining the requirements for activating the bound NO ligand in ferrous-NO porphyrins towards N-N bond formation and N2O production (bacterial N2O pathway). The team utilizes a combination of chemical synthesis, spectroscopy, and advanced spectroelectrochemistry, complemented by density functional theory calculations for this research. The collaborative team is active in introducing at-risk high school students and the general public to modern science. In addition to developing new educational materials at the freshman (introductory chemistry) and graduate (electrochemistry) levels, the team actively participates in a summer program designed for ethnically diverse and economically disadvantaged inner-city high school children. The team also produces on-line lecture videos in a rich multilayer flexible online environment suitable for individuals with disabilities, thus representing an avenue of science outreach to a traditionally hard-to-reach underrepresented group. The investigators also produce and curate several freely available electrochemistry Labview software programs for general worldwide use. 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.