Kinetic and Mechanistic Studies on the Reactions of the Reduced Vitamin B₁₂ Complex Cob(I)alamin with Nitrite and Nitrate (Eur. J. Inorg. Chem. 6/2012)

Abstract

The cover picture shows that nitrate and especially nitrite rapidly oxidize cob(I)alamin to cob(II)alamin. Enzyme‐bound cob(I)alamin is a short‐lived species whose formation is essential for the activity of both mammalian vitamin B12 dependent enzymes. Nitrite and nitrate, intracellular levels of which are elevated during oxidative/nitrosative stress as a consequence of elevated nitric oxide levels, are generally considered to be benign species. Details are discussed in the article by N. E. Brasch et al. on p. 913 ff. The cover picture was designed by Rohan S. Dassanayake. magnified image

Studies on the Reaction of Reduced Vitamin B12 Derivatives with the Nitrosyl ­Hydride (HNO) Donor Angeli's Salt: HNO Oxidizes the Transition‐Metal Center of Cob(I)alamin

“…Since Cbl(I) – is in excess, one would expect that Cbl(I) – is oxidized cleanly to Cbl(II) and that the amount of AS is insufficient for NOCbl formation. Figure 6a shows the clean conversion of Cbl(I) – to Cbl(II) with isosbestic points occurring at 417 and 542 nm, which are in good agreement with literature values for Cbl(I) – /Cbl(II) conversion 31a,32. The absorbance at 475 nm vs. time data for the reaction of Cbl(I) – (2.00 × 10 –4 M ) with 0.25 mol‐equiv.…”

“…It was also of interest to see if N 2 O [the product of HNO dimerization; Equation (1)] reacts with Cbl(II). The UV/Vis spectrum of a solution of Cbl(II) (200 µ M ) exposed to excess N 2 O (20.0 m M ) was identical to that for Cbl(II) ( λ max = 405 and 475 nm31a,32). There is, therefore, no reaction between Cbl(II) and N 2 O, as expected based on work of others who propose that Cbl(II) and N 2 O are the products of oxidation of Cbl(I) – by NO [Cbl(I) – + NO + H + → Cbl(II) + ${1 \over 2}$ N 2 O + ${1 \over 2}$ H 2 O] 33.…”

“…AS in the presence of excess nitrite were determined. Previous experiments from our laboratory have established that Cbl(I) – is rapidly oxidized to Cbl(II) by nitrite;31a hence, in the presence of excess nitrite it was anticipated that nitrite could replace the second HNO molecule as the Cbl(I) – oxidant (Scheme ). This would mean that only 1.0 mol‐equiv.…”

A Hybrid Catalyst for Light‐Driven Green Molecular Transformations

“…B 12s is recognized as a powerful nucleophile. Using the reactivity of the B 12 , a variety of molecular transformations was achieved, not only the B 12 ‐mimicking reactions, but also the biology‐inspired reactions, such as cyclopropane ring cleavage, reduction of a variety of substrates, including nitrite, nitrate, hydroxylamine, oxyhalogens, organic disulfides, unsaturated esters, thiosulfate, and sulfite, reductive radical cyclization, photoreduction of CO 2 , CH‐alkylation of terminal olefins, and reductive dehalogenation of organic halides . To perform these reactions, the method for generating reactive Co I species during the reaction is important.…”

Pulse Radiolysis and Ultra‐High‐Performance Liquid Chromatography/High‐Resolution Mass Spectrometry Studies on the Reactions of the Carbonate Radical with Vitamin B12 Derivatives

“…Vitamin B 12 derivatives, also known as cobalamins (Cbls), Figure 1, are essential cofactors for mammalian methionine synthase and mitochondrial methylmalonyl‐CoA mutase 15. Like porphyrins, the metal center of reduced cobalamins is rapidly oxidized by ROS/RNS, including superoxide,16, 17 nitric oxide,18 peroxynitrite,19 nitrogen dioxide,20 and nitrite 21. Furthermore ROS such as NO inactivate methionine synthase and methylmalonyl‐CoA mutase 22.…”