Research Projects

Multi-electron and multi-proton processes are key to energy storage and transfer both in nature and in artificial systems. These include reduction of oxygen to water by 4e-/4H+, reduction of nitrite to NH3 by 6e- and 6H+, reduction of sulfite to hydrogen sulfide by 6e- and 6H+ and 8e-/8H+ reduction of carbon dioxide to methane which, in the natural world, represent respiration or organisms that use oxygen, nitrate or sulfate for energy and, in the chemical industry ecosystem, helps store clean electrical energy in the form of chemical energy as well as convert industrial waste products and pollutants to chemicals of commercial value. Thus, these reactions are important for clean energy and environment. One of the major challenges involved in efficiently driving these reactions in the possibility of releasing partially reduced products e.g. hydrogen peroxide during oxygen reduction, hydroxylamine on the way to ammonia, CO or HCOOH before reduction of CO2 to CH4. In nature, these processes are catalyzed by heme based metalloenzymes. In addition to the heme cofactors, these metalloenzymes have several amino acids in the active site which do not bind the heme centre but are present in the distal site of the heme and are proposed to hydrogen bond to the partially reduced species and facilitate rapid proton transfer to them enabling selective reduction of oxygen, nitrite and sulphite to water, ammonia and hydrogen sulphide, respectively. These residues are basic in nature, i.e. they are protonated at physiological pHs, and they include lysine (with ammine head group), histidine (with imidazole head group) and arginine (with a guanidine head group). Mutation of these residues, inhibit the reaction and result in the formation of partially reduced species. In addition to these residues, tyrosine (with a phenol head group) is also found in some of these active sites. The proposal aims at understanding the roles played by these 2nd sphere residues in the active sites of these proteins in determining the rates and selectivity of these transformations using synthetic iron porphyrins which are covalently modified to include similar functional groups in the distal site. These iron porphyrins will be used to investigate oxygen, nitrite and carbon dioxide reduction under both homogeneous and heterogeneous conditions. The mechanism of the reaction will be investigated by trapping and characterizing the intermediates involved in the reaction using a combination of EPR, resonance Raman and Mossbauer spectroscopies along with rapid kinetic analysis. DFT calculations will be used to elucidate the electronic structures of these species specifically to understand how these 2nd sphere residues attenuate the reactivity of these iron porphyrins. Successful completion of the project will result in catalysts which are efficient and selective for the reduction of oxygen, nitrite and carbon dioxide which are currently in great demand.