Spectroscopy and DFT calculations of Bimetallic complexes and Biomolecules involved in small molecule activation

Metalloproteins are essential for the functioning of living organisms including photosynthesis, DNA synthesis, metabolism, respiration, and the transformation of carbon, nitrogen, and oxygen molecule required for life. Metalloenzymes containing transition-metal active sites perform important functions by catalyzing important reactions that involve transforming the most stable chemical bonds in nature. Many diseases are caused due to the metal imbalance or inactivity of critical metalloenzymes.

Transition metals, due to their high abundance on earth and ability to access various oxidation states, have been unitized by nature to serve as key cofactors in metalloenzymes. These active sites have been extensively modeled by synthetic chemists to mimic their biological functions. There is still a dearth of understanding on how the metal ions interact with inactivated small molecules. My work hereby is to elucidate the electronic and geometric structures of key reactive intermediates in metalloenzymes and model complexes using a range of spectroscopic tools. The experimental methods used in this dissertation primarily include electron paramagnetic resonance spectroscopy (EPR), ⁵⁷Fe Mossbauer spectroscopy, and density functional theory (DFT). We have utilized rapid freeze quench technique (RFQ) to trap these transient intermediates. Overall, this dissertation focuses on utilizing EPR and Mӧssbauer spectroscopy, combined with DFT analysis to characterizing high-valent intermediates formed at bimetallic sites in different proteins, artificial metalloenzymes, and model complexes, with emphasis on Fe centers. Studying the properties of these intermediates provide insight into the mechanistic pathway of enzymes catalysis for chemical reactions such as O₂, NO activation found in Nature.

This Dissertation focuses on the bimetallic cores found in metalloenzymes, artificial metalloproteins, and synthetic model systems. We first investigate the unusual Mössbauer parameters for a diheme-containing protein called BthA using Density function theory (DFT). In the next chapter, we look into the diiron-containing enzyme called nitric oxide reductase (NOR) cycle of flavin-diiron proteins (FDPs). The chapter will focus on the reaction path connecting the last two intermediates on which the nitrous oxide product is released. In chapter 4, characterize the properties of artificial metalloproteins (ArMs) that utilize a protein-assisted assembly process to generate unusual di-nuclear Fe cores. Finally, in Chapter 5 we investigate heterobimetallic complexes with M²⁺Fe³⁺ core (M = Mn, Ni, and Cu) and analyze the exchange interaction J for this series of complex with the help of density functional theory (DFT)

calculations. In the final chapter, drawing inspiration from RNR, we characterize the structure and electronic properties of the diiron containing unsymmetric bimetallic core, starting from the [TMTACNFe^II(OH)Fe^IIIpoat]+ complex and explore the stepwise oxidation and reduction of the [Fe^II(OH)Fe^IIIpoat] core to form distinct species using spectroscopy and DFT Methods.