Outer-sphere oxidation of Fe(II) in nitrosylmyoglobin by ferricyanide

• Published in: Journal of biological inorganic chemistry 2014-08, Vol.19 (6), p.805-812
• Main Authors: Møller, Jens K. S., Skibsted, Leif H.
• Format: Article
• Language: English
• Subjects: Biochemistry; Biomedical and Life Sciences; Ferricyanides - chemistry; Ferrous Compounds - chemistry; Kinetics; Life Sciences; Microbiology; Myoglobin - chemical synthesis; Myoglobin - chemistry; Original Paper; Oxidation-Reduction
• ISSN: 0949-8257; 1432-1327
• DOI: 10.1007/s00775-014-1112-y

Nitrosylmyoglobin, MbFe(II)NO, was found to be oxidized by [Fe(CN) 6 ] 3− and HClO/ClO − , but not by the semistable radical nitrosodisulfonate (anion of Frémy’s salt) or NO 2 − at ambient temperature in aqueous solution with pH 7.0. The oxidation by HClO/ClO − was significantly faster than that by [Fe(CN) 6 ] 3− . With excess [Fe(CN) 6 ] 3− , MbFe(II)NO was oxidized to metmyoglobin, MbFe(III), in a second-order reaction with k 2  = 1.67 ± 0.10 M −1  s −1 at 288 K without detectable intermediates as determined by stopped-flow spectroscopy. The activation parameters were Δ H ‡  = 43 ± 2 kJ mol −1 and Δ S ‡  = −93 ± 9 J −1  K −1  mol −1 . Outer-sphere electron-transfer to [Fe(CN) 6 ] 3− was assigned as rate determining rather than NO dissociation from iron(II) followed by electron transfer. Outer-sphere electron transfer from MbFe(II)NO to certain moderate oxidizing agents may thus have a role in labilizing NO association slowly through oxidation of iron(II) to iron(III). In contrast, hypochlorite oxidizes MbFe(II)NO much faster in a complex sequence of processes involving a rate-determining second-order (unidentified) reaction with k 2  = 2.6 ± 0.3 × 10 3  M −1  s −1 at 288 K and possibly involving protein degradation.