Besides its use in medicine, vitamin B12 (cobalamin) and its derivatives have found in numerous applications as catalysts. However, studies related to the activation of oxidants via cobalamin are scant. In this work, we showed how the addition of aquacobalamin (H2OCbl) accelerates the destruction of azo-dye Orange II by peroxymonosulfate (HSO5−) in aqueous solutions. In neutral and weakly alkaline media, the process is initiated by the modification of the corrin macrocycle with HSO5−, which requires the preliminary deprotonation of the aqua-ligand in H2OCbl to give hydroxocobalamin, producing 5,6-dioxo-5,6-secocobalamin or its isomer (14,15-dioxo-14,15-secocobalamin). In acidic solutions, where the concentration of hydroxocobalamin is negligible, the formation of dioxo-seco-species is not observed, and the reaction between H2OCbl and HSO5− results in slow chromophore bleaching. Using terephthalic acid, we demonstrated the formation of hydroxyl radicals in the mixture of H2OCbl with HSO5-, whereas the generation of sulfate radicals was proved by comparing the effects of ethanol and nitrobenzene on Orange II destruction using the H2OCbl/HSO5− system. The reaction mechanism includes the binding of HSO5− to the Co(III) ion of dioxo-secocobalamin, which results in its deprotonation and the labilization of the O-O bond, leading to the formation of sulfate and hydroxyl radicals which further react with Orange II.
Medicinal effects of hydrophobic derivatives of vitamin [Formula: see text] have been insignificantly investigated primarily due to their low solubility in aqueous solutions. One of the ways to increase their water solubility is the complexation with proteins. Here, we report the results of the studies of the reactions between aquacyano cobyrinic acid or monocyano cobesters (i.e., heptamethyl, heptaethyl, heptapropyl, and heptabutyl cobyrinates) and bovine serum albumin (BSA). The weakest binding is observed between aquacyano cobyrinic acid and BSA. In the case of monocyano heptaethyl cobyrinate, one BSA molecule is capable of binding up to six corrinoid molecules preventing its precipitation. Moreover, the pronounced effect of BSA on the solubility in water was observed in the case of monocyano heptapropyl cobyrinate. The absence of the precipitation of monocyano heptabutyl cobyrinate was observed only in the presence of high excess of BSA. BSA modification by diethyl pyrocarbonate, a chemical predominantly reactive toward imidazole motifs, indicated that a major fraction of BSA is bound with monocyano heptaethyl, heptapropyl, and heptabutyl cobyrinates via histidine residues. We showed that nitrosyl complexes of heptaethyl and heptapropyl cobyrinates can be stabilized from precipitation in aqueous solutions using BSA.
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