Research Projects

Enzyme catalyzed multi-electron and multi-proton transformations of oxygen (O₂) and nitric oxide (NO) are paramount processes in numerous biological/physiological activities, ranging from global cycles of life-essential components to cellular respiration. To accomplish this daunting task, Nature has utilized hetero-bimetallic cofactors that possess tunable thermodynamic and electronic properties to catalyze the reduction of O₂ (by Heme-copper oxidases, HCO) and NO (by Nitric oxide reductases, NOR). However, mechanistic subtleties of both HCO and NOR enzymes are enigmatic to date (which is in sharp contrast to the mono/homo-nuclear metalloenzymes) possibly owing to their inherent structural intricacy. Synthetic model systems have long-served as powerful tools in addressing such mechanistic ambiguities; these model systems are therefore highly desirable due to their ability to (1) withstand a variety of experimental conditions (e.g. pH, organic solvents, temperatures etc.), (2) isolated in high to excellent yields, and (3) straightforwardness in introducing specific structural alterations compared to their biological counterparts. Herein this proposal focuses on the rational design and generation of such bio-inspired synthetic models of different intermediates of HCO and NOR to develop a clear mechanistic understanding relevant to (i) the binding, activation, and reduction chemistry of O₂ and NO, (ii) the effects of protonation and/or addition of electrons (as in the enzyme), and (iii) the hydrogen atom abstraction reactivity of the final oxidized states of the enzymes, FeIII−O−CuII/FeIII species. To accomplish these objectives, we will (a) utilize a series of heme and other non-heme ligands (having electronically divergent environment, secondary coordination sphere interaction, tethered axial bases etc.) allowing formation of appropriate Fe or Cu complexes, and (b) cryogenic generation of relevant intermediates (e.g. heme-copper peroxo, hydroperoxo, hyponitrite etc.), (c) elucidation of structure and physical properties of such intermediates using UV-visible, nuclear magnetic resonance (NMR), resonance Raman and electron paramagnetic resonance (EPR) spectroscopies, X-ray crystallography, magnetic susceptibility and cyclic voltammetry, and (d) investigate their reactivity, mechanistic insights, kinetic and thermodynamic studies (involving rate constants, kinetic isotope effect (KIE), Eyring and bond dissociation free energy (BDFE) analyses) using suitable bio-relevant substrates. Comprehension of mechanistic precision of such model systems can establish relevant structure-function relationships of heme-Cu(Fe) systems to provide a sound basis for deducing biological active site structures, the nature of reactive intermediates, and mechanism of HCO/NOR. These studies will also provide a rationale for the design of practical O₂-carriers, organic oxidation, nitrogen oxides (NOx) reduction catalysts, therapeutics, and alternative energy applications.