Abstract
The previous studies of the electrochemical reduction of vitamin B 12r by polarography and cyclic voltammetry (CV) have been restricted to a few pH values. In CV, the sweep rates used were limited to very low values owing to adsorption phenomena, hampering a detailed analysis of the reduction mechanism. The aim of the present communication is to report and discuss the results obtained by CV at several pH values ranging from 0 to 8. The adsorption phenomena were minimized by addition of a solubilizing salt at pH above 3 enabling a large range of sweep rates to the explored. It is shown, onder these conditions, that the base-off/base-on tautomerism of vitamin B 12r, i.e. the cleavage and binding of the 5,8-dimethyl-benzimidazole of the nucleotide side-chain, plays a major role in the oxido-reduction mechanism of the B 12 r-B 12s couple. The analysis of the CV results leads to the description of the mechanism and to the evaluation of various kinetic and thermodynamic characteristic factors of the B 12r-B 125 couple.
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