Our research program focuses on understanding how thiolate ligands promote N-O and O-O (O2- and O2) bond activation. Nitric oxide (NO) is frequently used to probe O2 binding sites, and form stable analogues of key metastable Fe-O2 intermediates. The ?NO stretching frequency provides a measure of N-O, and thus O-O bond activating properties. A growing number of metalloenzymes use Mn and Fe interchangeably without altering function, and a majority of these promote O-O bond activation and cleavage, and involve M-dioxygen, -superoxo, -peroxo, and -oxo (M= Mn, Fe) species as key intermediates. These enzymes function to remove toxic O2- radicals implicated in cancer, Alzheimer's, Parkinson's disease, and synthesize key biomolecules such as DNA, neurotransmitters, fatty acids, and steroids. The spectroscopic and reactivity properties of thiolate-ligated Fe- and Mn-peroxo, superoxo, and oxo species remain largely unexplored, and the O2 and O2- chemistry of Mn and Fe is complementary with respect to the spectroscopic visibility and stability of reactive intermediates. For the proposed project period we will design and synthesize new, readily derivatized thiolate ligands, and their corresponding Mn and Fe complexes. By systematically altering substituents we will fine-tune the electronic and steric properties of the molecule, and adjust the redox potential, metal ion Lewis acidity, and relative pKa of the distal versus proximal peroxo oxygens. Using a systematic approach we will attempt to determine which properties are key to promoting reactivity and function. By probing redox potentials and ?NO of structurally analogous Mn/Fe pairs we will examine the tunable nature of the M- SR bond, and establish whether cysteinates can facilitate the interchangeability of Mn and Fe while maintaining function. We will fully characterize our new pyridine/thiolate-ligated Fe-peroxo intermediates formed via O2- reduction, and continue to explore the O2- reactivity of additional pyridine- substituted derivatives. Ultimately we will be looking for correlations between Hammett parameters and reactivity, and determining which properties are key to promoting Fe-O versus O-O bond cleavage. We will continue to look for M3+ (M= Mn, Fe) promoted O2- oxidation activity by monitoring for O2 evolution. Ultimately we will be looking for a catalytic SOD mimic. We will fully characterize our new Mn-dioxygen and Mn-OOR (R= tBu, Cm) intermediates using XAS, MCD, resonance Raman, parallel- mode EPR, and DFT, and examine the kinetics and mechanism of the Mn-dioxygen species formation. We wil attempt to stabilize these intermediates, as well as our new Mn=O, by incorporating steric bulk into the ligand. We will continue to explore the reactivity of our new Mn-dioxygen, Mn-oxo, and Mn- OOR intermediates with oxidizable substrates.
Our research program focuses on understanding how thiolate ligands promote O-O bond activation in Mn- and Fe-containing enzymes that function to remove toxic O2- radicals (implicated in cancer, Alzheimer's, Parkinson's disease), and synthesize key biomolecules (e.g., DNA, and neurotransmitters). The proposed project will determine the metal ion properties that are key to promoting function, and establish whether the tunable nature of M-SR bonds can facilitate the interchangeable use of Mn and Fe without altering function. The spectroscopic and reactivity properties of thiolate-ligated Fe- and Mn-peroxo, superoxo, and oxo species remain largely unexplored, and the O2 and O2- chemistry of Mn and Fe is complementary with respect to the spectroscopic visibility and stability of reactive intermediates.
|Coggins, Michael K; Martin-Diaconescu, Vlad; DeBeer, Serena et al. (2013) Correlation between structural, spectroscopic, and reactivity properties within a series of structurally analogous metastable manganese(III)-alkylperoxo complexes. J Am Chem Soc 135:4260-72|
|Coggins, Michael K; Sun, Xianru; Kwak, Yeonju et al. (2013) Characterization of metastable intermediates formed in the reaction between a Mn(II) complex and dioxygen, including a crystallographic structure of a binuclear Mn(III)-peroxo species. J Am Chem Soc 135:5631-40|
|Coggins, Michael K; Brines, Lisa M; Kovacs, Julie A (2013) Synthesis and structural characterization of a series of Mn(III)OR complexes, including a water-soluble Mn(III)OH that promotes aerobic hydrogen-atom transfer. Inorg Chem 52:12383-93|
|Coggins, Michael K; Toledo, Santiago; Kovacs, Julie A (2013) Isolation and characterization of a dihydroxo-bridged iron(III,III)(?-OH)2 diamond core derived from dioxygen. Inorg Chem 52:13325-31|
|Coggins, Michael K; Kovacs, Julie A (2011) Structural and spectroscopic characterization of metastable thiolate-ligated manganese(III)-alkylperoxo species. J Am Chem Soc 133:12470-3|
|Nam, Elaine; Alokolaro, Pauline E; Swartz, Rodney D et al. (2011) Investigation of the mechanism of formation of a thiolate-ligated Fe(III)-OOH. Inorg Chem 50:1592-602|
|Villar-Acevedo, Gloria; Nam, Elaine; Fitch, Sarah et al. (2011) Influence of thiolate ligands on reductive N-O bond activation. Probing the O2(-) binding site of a biomimetic superoxide reductase analogue and examining the proton-dependent reduction of nitrite. J Am Chem Soc 133:1419-27|
|Swartz, Rodney D; Coggins, Michael K; Kaminsky, Werner et al. (2011) Nitrile hydration by thiolate- and alkoxide-ligated Co-NHase analogues. Isolation of Co(III)-amidate and Co(III)-iminol intermediates. J Am Chem Soc 133:3954-63|
|Lugo-Mas, Priscilla; Taylor, Wendy; Schweitzer, Dirk et al. (2008) Properties of square-pyramidal alkyl-thiolate Fe(III) complexes, including an analogue of the unmodified form of nitrile hydratase. Inorg Chem 47:11228-36|
|Brines, Lisa M; Shearer, Jason; Fender, Jessica K et al. (2007) Periodic trends within a series of five-coordinate thiolate-ligated [MII(SMe2N4(tren))]+ (M = Mn, Fe, Co, Ni, Cu, Zn) complexes, including a rare example of a stable CuII-thiolate. Inorg Chem 46:9267-77|
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