Nitrosyl hydride adduct of deoxymyoglobin: Structure, reactivity and biological importance of Mb-nitrosyl hydride

In the past decade, investigations into the physiological roles of nitric oxide, NO, have expanded to nitroxyl, its one-electron reduced form. The study of nitroxyl, NO-/HNO, is complicated by its short lifetime in solution due to rapid dimerization and reactivity with other small molecules. Nitroxyl can be stabilized by coordination to a metal ion. The HNO adduct of myoglobin (Mb-HNO) is one of about a dozen known transition metal HNO complexes. Mb-HNO is the only long-lived nitroxyl adduct (t1/2 ∼ 6 months), which is stable in aqueous solutions.; The first chapter offers a detailed overview of the biological importance and reactivity of nitroxyl and a description, characterization, and electrochemistry of stable nitroxyl adducts. The synthesis and interaction of Mb with HNO is explored in the second chapter; a free HNO, generated in an aqueous solution is rapidly trapped by ferrous myoglobin as first demonstrated by time course NMR. The concentration profiles of the myoglobin species were analyzed by a unique spectral method from time course absorbance experiments and used to model competitive and noncompetitive reactions of Mb with HNO. The following order of increasing reaction rates with HNO was determined: Mb-FeII > Glutathione (GS) > Mb-FeIII ∼ Mb-HNO ∼ GS-NO. Based on the 1H NMR, EXAFS, and Resonance Raman data, the third chapter explores the unusual stability of the Mb-HNO adduct, which is found to be influenced by sterics of the active site, hydrogen bonding with His64 and pi-backbonding with the iron. The fourth chapter looks at reactivity of Mb-HNO which is followed by reactions with small molecules and by electrochemistry. In contrast to the reactivity of free HNO that reacts readily with nucleophiles, Mb-HNO does not undergo such reactions. Chapter five describes HNO adducts in other heme-HNO models, which include an oligopeptide (MP-11), chemically substituted myoglobin (Mb-Ru), and an oxygen binding protein, LegHb, along with some complications associated with their isolation. Finally, deuterium exchange of the myoglobin HNO adduct and its pKa determinations by 1H NMR and UV-Vis spectroscopy are illustrated in Chapter 6.