Denis S. Salnikov

Cobalamin reduction by dithionite. Evidence for the formation of a six-coordinate cobalamin(II) complex.

Evidence for the formation of a unique, six-coordinate cobalamin(II) complex with the anion-radical SO(2)(-) during the reduction of aquacobalamin(III) by sodium dithionite, was obtained from spectrophotometric and EPR measurements. The pK(a) value of the weakly coordinated dimethylbenzimidazole group was found to be 4.8 ± 0.1 at 25 °C.

Comparative study of reaction of cobalamin and cobinamide with thiocyanate

The interaction of Co(III) and Co(II) cobalamin (Cbl) and cobinamide (Cbi) with thiocyanate was examined with UV-vis and EPR spectra. S/N-linkage isomerism was explored on Co(III) and Co(II) Cbl and Cbi models using density functional theory (DFT; BP86, B3LYP). Performed calculations suggest the prevalence of isothiocyanato isomers over thiocyanato complexes on both Co(III) and Co(II) centers. The formation of Cbl(II) complex with thiocyanate was observed at high ligand concentrations which was proposed to be hexacoordinated. DFT data maintain the possibility of hexacoordinated Co(II) complexes with thiocyanate in which one of extra-ligands is weakly coordinated. It is found that high thiocyanate concentrations could retard cyanide binding to cobalamin but not to cobinamide.

Kinetics and mechanism of oxidation of super-reduced cobalamin and cobinamide species by thiosulfate, sulfite and dithionite

We studied the kinetics of reactions of cob(I)alamin and cob(I)inamide with thiosulfate, sulfite, and dithionite by UV-Visible (UV-Vis) and stopped-flow spectroscopy. We found that the two Co(I) species were oxidized by these sulfur-containing compounds to Co(II) forms: oxidation by excess thiosulfate leads to penta-coordinate complexes and oxidation by excess sulfite or dithionite leads to hexa-coordinate Co(II)-SO2(-) complexes. The net scheme involves transfer of three electrons in the case of oxidation by thiosulfate and one electron for oxidation by sulfite and dithionite. On the basis of kinetic data, the nature of the reactive oxidants was suggested, i.e., HS2O3(-) (for oxidation by thiosulfate), S2O5(2-), HSO3(-), and aquated SO2 (for oxidation by sulfite), and S2O4(2-) and SO2(-) (for oxidation by dithionite). No difference was observed in kinetics with cob(i)alamin or cob(i)inamide as reductants.

Interaction of cyanocobalamin with sulfur-containing reducing agents in aqueous solutions

The kinetics and mechanism of cyanocobalamin reduction by sodium hydroxymethanesulfinate and dithionite in alkaline media are studied. It is established that the character of the rate-determining step depends on the concentration of the reducing agents: when they are in excess, it is a step of elimination of cyanocobalamin, at lower concentrations of reducing agents a rate-determining is a step of their addition to cobalamin.

Phosphorus(V) tetrapyrazinocorrolazines — first corrolazine derivatives with fused heterocyclic rings

Tetrapyrazinocorrolazines — first corrolazine derivatives with annulated heterocycles — have been prepared as phosphorus(V) complexes by the reaction of metal free tetrapyrazinoporphyrazines [(R8TPyzPA)H2] (R = Ph, Me) with phosphorus(III) chloride or bromide in pyridine. Oxophosphorus(V) complexes [(R8TPyzCA)P=O] were obtained when the reaction mixture was poured into water, while dilution with methanol leads to dimethoxyphosphorus(V) complexes [(R8TPyzCA)P(OMe)2]. The obtained compounds were characterized by MALDI-TOF mass-spectrometry and by IR, 1H, 31P NMR measurements. Their UV-vis spectral and basic properties in CH2Cl2–CF3COOH medium were studied in comparison to phosphorus(V) complex of octaphenylcorrolazine [(Ph8CA)P=O].

A new route to carbon monoxide adducts of heme proteins

Sulfoxylate SO2H−(SO22−), a strong reducing agent readily produced by hydrolysis of thiourea dioxide, reacts with ferric myoglobin (Mb) to reversibly produce Fe(II)-Mb, starting from either aerobic or anaerobic conditions. Exposure of Fe(II)-Mb to excess sulfoxylate further produces Fe(II)-CO-Mb. Fe(II)-Mb can be regenerated by reoxidation with ferricyanide at this stage; hemin, rubredoxin and cytochrome c show a similar reactivity towards sulfoxylate. The source of CO is not the protein moiety, nor is it the heme or the thiourea dioxide – but rather CO2, via its reaction with sulfoxylate when the latter is used in large excess. These findings provide a convenient single-step route to carbon monoxide heme adducts, without the need to manipulate toxic CO gas.

Effect of amino acids on the interaction between cobalamin(II) and dehydroascorbic acid

The kinetics of the reaction between one-electron-reduced cobalamin (cobalamin(II), Cb(II)) and the two-electron-oxidized form of vitamin C (dehydroascorbic acid, DHA) with amino acids in an acidic medium is studied by conventional UV–Vis spectroscopy. It is shown that the oxidation of Cbl(II) by dehydroascorbic acid proceeds only in the presence of sulfur-containing amino acids (cysteine, acetylcysteine). A proposed reaction mechanism includes the step of amino acid coordination on the Co(II)-center through the sulfur atom, along with that of the interaction between this complex and DHA molecules, which results in the formation of ascorbyl radical and the corresponding Co(III) thiolate complex.

Reactions of aquacobalamin and cob(II)alamin with chlorite and chlorine dioxide

Reactions of aquacobalamin (H2O–Cbl(III)) and its one-electron reduced form (cob(II)alamin, Cbl(II)) with chlorite (ClO2−) and chlorine dioxide (ClO2•) were studied by conventional and stopped-flow UV–Vis spectroscopies and matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS). ClO2− does not react with H2O–Cbl(III), but oxidizes Cbl(II) to H2O–Cbl(III) as a major product and corrin-modified species as minor products. The proposed mechanism of chlorite reduction involves formation of OCl− that modifies the corrin ring during the course of reaction with Cbl(II). H2O–Cbl(III) undergoes relatively slow destruction by ClO2• via transient formation of oxygenated species, whereas reaction between Cbl(II) and ClO2• proceeds extremely rapidly and leads to the oxidation of the Co(II)-center.

Studies on reaction of glutathionylcobalamin with hypochlorite. Evidence of protective action of glutathionyl-ligand against corrin modification by hypochlorite

Glutathionylcobalamin (GSCbl), a tight complex of glutathione (GSH) with cobalamin(III), is readily oxidized to aquacobalamin by hypochlorite. Corrin macrocycle remains unmodified in the presence of threefold excess of hypochlorite, whereas aqua- and cyanocobalamins are partially transformed to chlorinated species under the same conditions. The suggested mechanism of reaction between GSCbl and hypochlorite involves subsequent oxidation of thiol and amino groups and dissociation of oxidized glutathione from Co(III)-ion.

Comparative studies of reaction of cobalamin (II) and cobinamide (II) with sulfur dioxide

The kinetics of reactions of cobalamin (II) and cobinamide (II) with sulfur dioxide was studied by UV–visible (UV–vis) spectroscopy. Reaction results in oxidation of Co(II) center and involves two aquated SO2 moieties. The final product is suggested to be complex Co(III)–S2O4•–. The absence of corrin ring modifications during the reactions was proved.