The structure of nitrosylcobalamin (NOCbl) in solution has been studied by NMR spectroscopy and the 1 H and 13 C NMR spectra have been assigned. 13 C and 31 P NMR chemical shifts, the UV-vis spectrum of NOCbl and the observed pK base-off value of ~5.1 for NOCbl provide evidence that a significant fraction of NOCbl is present in the base-off, 5,6-dimethylbenzimidazole (DMB) deprotonated, form in solution. NOE-restrained molecular mechanics modelling of base-on NOCbl gave annealed structures with minor conformational differences in the flexible side chains and the nucleotide loop position compared with the X-ray structure. A molecular dynamics simulation at 300 K showed that DMB remains in close proximity to the α face of the corrin in the base-off form of NOCbl. Simulated annealing calculations produced two major conformations of base-off NOCbl. In the first, the DMB is perpendicular to the corrin and its B3 nitrogen is about 3.1 Å away from and pointing directly at the metal ion; in the second the DMB is parallel to and tucked beneath the D ring of the corrin.

The X-ray structures of three new crystals of nitroxylcobalamin (NOCbl) have been determined. Unlike our earlier reported structure in which NOCbl was partially oxidized , the O atom of the nitroxyl ligand is located in a single position with a N=O bond distance of 1.12-1.14 Å, consistent with a double bond. The Co-N-O angle is in the 118.9-120.3 Å range. The α-axial Co-N(dimethylbenzimidazole) (Co-NB3) bond distance is a remarkable 2.32-2.35 Å in length, ~0.1 Å longer than that reported for all other cobalamin structures. The change in the Gibbs free energy for the base-on/base-off equilibrium now correlates extremely well with the Co-NB3 bond distance, as observed for other cobalamins.

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We report the first studies on the reaction between an HNO donor compound and vitamin B12 complexes. Kinetic and mechanistic studies have been carried out on the reaction between the vitamin B12 derivative aquacobalamin (H2OCbl(+)/HOCbl; pKa = 7.8) and the HNO donor Angeli's salt. Studies were carried out with aquacobalamin in excess, since nitrite also reacts with aquacobalamin to form nitrocobalamin (NO2Cbl). At pH <9.90 aquacobalamin reacts directly with the monoprotonated form of Angeli's salt, HN2O3(-), to form nitroxylcobalamin (NO(-)-Cbl(III); NOCbl) and nitrite. At pH >10.80 the reaction instead switches predominantly to a mechanism in which spontaneous decomposition of Angeli's salt to give HNO and nitrite becomes the rate-determining step, followed by the rapid reaction between aquacobalamin and HNO/NO(-) to again give NOCbl. Both reactions proceed with a 1:1 stoichiometry and formation of nitrite is confirmed using the Griess assay.

Unglaubliche Geschichten: Das Übertragen einer Nitroxylgruppe von R2N‐NONOaten auf Aquacobalamin, bei dem Nitroxylcobalamin entsteht, verläuft nicht über eine H+‐katalysierte R2N‐NONOat‐Zersetzung, sondern wahrscheinlich über ein NONOat‐Cobalamin‐Intermediat (siehe Schema; r.d.s.=geschwindigkeitsbestimmender Schritt).

The efficiency of singlet oxygen photosensitized by some ruthenium(ii) bipyridyl complex ions in aqueous media is reported in this study. Measurements were carried out in H2O and D2O. The effect of the deuterium isotope on the lifetime of (3)MLCT excited states of these complexes is studied in H2O and D2O. The deuterium isotope effect was discussed in terms of the vibronic coupling to the solvent in addition to the charge transfer to the solvent mechanism due to their dependence on the oxidation potential of the sensitizer. Quenching rate constants, kq, for quenching of the (3)MLCT states of these ruthenium complex ions by molecular oxygen were found to be in the range of (2.08-3.84) × 10(9) M(-1) s(-1) in H2O and (1.69-3.48) × 10(9) M(-1) s(-1) in D2O. The efficiency of singlet oxygen, O2((1)Δg), production as a result of the (3)MLCT quenching by oxygen, f, is reported in D2O and found to be in the range 0.25-0.56. It has been found that the lifetime of the excited state is longer in D2O, τ, than in H2O, τ, which was related to partial charge transfer to the solvent in addition to the vibronic coupling mechanism. Mechanisms by which the excited states of these ruthenium complexes are quenched by molecular oxygen that shows the competition between charge transfer, non-charge transfer deactivation channels or energy transfer assisted charge transfer deactivation mechanisms are reported.

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

The kinetics of the reactions between cob(I)alamin [Cbl(I)] and nitrite and nitrate have been studied by UV/Vis and stopped‐flow spectroscopy. Enzyme‐bound Cbl(I) is an important transient species in several B12‐catalyzed enzyme reactions. Levels of nitrite and nitrate are elevated during oxidative stress, as a consequence of elevated nitric oxide levels. Although nitrite and nitrate are generally considered to be benign species, our studies show that nitrate and especially nitrite react rapidly with Cbl(I) at neutral pH conditions (kapp = 6.5 × 10–3 and 1.7 × 103 M–1 s–1, respectively, at pH 7, 25.0 °C). A reaction pathway is postulated for the reaction between Cbl(I) and (H)NO2 involving a 2e– rate‐determining step to form Cbl(III) and HNO. The latter species reacts further with Cbl(I), ultimately resulting in the oxidation of 4Cbl(I) by HNO2 to yield 4Cbl(II) and NH2OH. The reaction between Cbl(I) and (H)NO3 results in the oxidation of 8Cbl(I) by (H)NO3 to give 8Cbl(II) and NH4+ (pH 5–7).