Model Complexes for Reactive Intermediates in NOx Interconversions in Nature and Applications in Medicine
Manickas, Elizabeth
2024
Abstract
Fungi consume nitric oxide (NO) using Cytochrome P450 nitric oxide reductase (Cyt P450nor) by reducing two molecules of NO to nitrous oxide (N2O) – a potent greenhouse gas. The efficiency of this enzyme makes it difficult to study its reactive Intermediate I. For this reason, a bulky bis-picket fence porphyrin was synthesized to prevent the decomposition of an Intermediate I model complex for full vibrational characterization and reactivity studies with NO. Experimental conditions were designed analogous to the reaction in Cyt P450nor with hydride transfer to a Fe(II)-NO+ species to form a ferrous heme nitroxyl species, [Fe(3,5-Me-BAFP)(MI)(HNO)]. This model was determined to be the most stable heme nitroxyl complex in organic solvents, with a half-life of 56 min at -30 C by UV-vis spectroscopy. This stability allowed me to perform the first detailed spectroscopic characterization of a heme-HNO model complex. In addition, this complex demonstrates high reactivity toward NO, exhibiting a 91% N2O by IR spectroscopy that resulted in the formation of a ferric heme species. The obtained results indicate that heme-HNO complexes are catalytically competent intermediates for NO reduction to N2O. The proximal cysteinate in the Cyt P450nor active site is proposed to either stabilize the FeIII-NHO*- valence tautomer of the heme-HNO complex or increase the pKa of the bound nitroxyl in Intermediate I. Previous work has also shown that the presence of the axial thiolate makes the Fe(II)-NO+ species less stable in synthetic model complexes. In this work, the bulky bis-picket fence porphyrin was used to stabilize a thiolate bound heme Fe(II)-NO+ complex, [Fe(3,5-Me-BAFP)(NO)(SPhF4CF3)] at low temperatures. The formation of this complex was verified by UV-vis and NMR spectroscopy. Vibrational characterization of this complex by IR and rRaman spectroscopy demonstrated similar Fe-NO and N-O stretching frequencies in comparison to Cyt P450nor of 1853 and 543, and 1851 and 530 cm-1, respectively. For the first time, hydride transfer reactions to a thiolate-bound ferrous nitrosonium synthetic model complex were conducted in this work, and followed by IR and NMR spectroscopy, to ultimately obtain a thiolate-bound Intermediate I model with direct relevance to the active site of Cyt P450nor. Biomedical devices like IV catheters come with risks to patients, such as bacterial infections and blood clots. NO is known to prevent these risks. This part of my thesis focuses on catalyst development, inspired by copper nitrite reductase, for the electrochemical reduction of nitrite (NO2-) to NO for biomedical applications. The current limitation of these copper catalysts is their oxygen sensitivity, which reduces NO generation. The presence of O2 cannot be avoided in IV catheters, which are in contact with oxygen rich blood. This issue is addressed by synthesizing two second coordination sphere (SCS) derivatives of the BMPA-Pr- (BMPA-Pr- = bis-(2-methylpyridyl)amine)propionate) ligands, Amide-BMPA-Pr- and Amine-BMPA-Pr-, to shuttle protons necessary for nitrite reduction and therefore decrease oxygen sensitivity. It was determined that [Cu(Amide-BMPA-Pr)(OAc)] is not capable of performing nitrite reduction due to poor substrate binding and low electrochemical stability of the complex. In contrast, [Cu(Amine-BMPA-Pr)(OAc)] can outperform the initially reported catalyst [Cu(BMPA-Pr)(OAc)] under inert atmosphere, showing that SCS H-bonding residues are able to accelerate nitrite reduction. However, the presence of a hydrogen bond donor does not reduce O2 sensitivity as previously hypothesized.Deep Blue DOI
Subjects
Cyt P450nor heme nitroxyl, HNO nitric oxide, NO copper catalysts
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