Kinetics and mechanism of oxidation of pyridoxine by enneamolybdomanganate(IV)
Oxidation of pyridoxine (vitamin B6) by enneamolybdomanganate (IV) in aqueous perchloric acid medium has been studied spectrophotometrically at 25°C under pseudo-first-order conditions. The mechanism involves formation of a precursor complex between the reactants which decomposes in a subsequent slow step to give pyridoxal as the intermediate product. The pyridoxal is further oxidized to a final product, 4-pyridoxic acid, by another oxidant molecule in a fast step. The precursor complex formation is supported both kinetically and spectrophotometrically. The accelerating effect of hydrogen ions on the reaction is due to the formation of active hexaprotonated oxidant species. The protonated enneamolybdomanganate(IV) and the pyridoxine cation are found to be the active species of the oxidant and the substrate respectively. The reaction involves direct two-electron transfer step without any free radical intervention. The effects of ionic strength, solvent polarity and the activation parameters were also in support of the mechanism proposed.
Pyridoxamine has been found to inhibit protein glycation and to avoid the formation of advanced glycation end-products (AGEs). One of the mechanisms by which pyridoxamine can inhibit glycation involves the scavenger of carbonyl groups with glycation capacity. In this work, we conducted a kinetic study of the reactions of pyridoxamine with various carbohydrates under physiological pH and temperature. The reactions involving hexoses were found to give a tricyclic compound (5) in addition to pyridoxal and pyridoxine. Such a tricyclic compound inhibits the Amadori rearrangement and the formation of other carbonyl compounds with glycating properties. The reactions involving pentoses gave compound 7 and pyridoxal - by transamination of the Schiff base. The transamination reaction enhances the inhibitory action of pyridoxamine. The formation rate constants for the Schiff base, k3, were found to be similar to those for the reactions of D-glucose with amino acids, which suggests competition between pyridoxamine and terminal amino residues in proteins for glycating sites in sugars. These constants are dependent on the electrophilic character of the carbonyl carbon in the carbohydrate.
Adrover, Miquel,Vilanova, Bartolome,Munoz, Francisco,Donoso, Josefa
p. 154 - 167
(2008/09/17)
Studies on kinetics and mechanism of oxidation of pyridoxal by dichromates in aqueous HClO4 solutions
Oxidation of pyridoxal (PL) by Cr2O2-7 ion has been studied using an excess of the aldehyde under air or argon atmospheres. The reaction leads mainly to two Cr(III) complexes: [Cr(PA)(H2O)4]2+ and [Cr(H2O)6]3+ and to uncoordinated pyridoxic acid (PA). The rate of PL oxidation follows a mixed third order rate law: first order in concentrations of Cr(VI), PL and H3O+. The reduction of Cr(VI) to Cr(III) proceeds through chromium(V) intermediate complex, which has been detected by the EPR method. Mechanism of the reaction has been discussed.
Kita,Kita,Pietkiewicz,Wrzeszcz,Rozploch
p. 573 - 581
(2007/10/03)
Kinetics and mechanism of oxidation of pyridoxine by Cr2O72- ion in aqueous HClO4 solutions
Oxidation of pyridoxine by Cr2O72- ions in aqueous solutions (pH 1-0, HClO4) was studied under an excess of the vitamin B6 in air, oxygen and argon atmosphere. Inorganic and organic products of the reaction were analysed. Kinetics of the reaction has been followed spectrophotometrically at the ionic strength 1.0 M. The rate law and apparent activation parameters have been determined. Mechanism of the reaction is discussed.