On the disproportionation of vitamin B 12r and the voltammetry of vitamin B 12 compounds

Abstract

Summary Polarographic, cyclic voltammetric and potentiometric measurements have been made on vitamin B12a and B12r in Na2SO4 (at adjusted pH), (NH4)2H citrate (pH=4.94) and KH2PO4−Na2HPO4 (pH=7.43). In the phosphate medium, polarographic E1/2 values for the vitamin B12a/B12r and B12r/B12s couples are E1/2=−0.002 V vs. SCE and E1/2=−0.891 V vs. SCE, respectively. These E1/2 values are very close to reversible values as shown by logarithmic plots of E vs. log(id−i)/i and by cyclic voltammetry. Analysis of cyclic voltammograms shows that the B12r+e→B12s electron transfer reaction at a mercury drop electrode is quasi-reversible with a ks=1.7±0.3×10−3 cm s−1 in 0.0667 M KH2PO4−0.0667 M Na2HPO4. Using the polarographic E1/2's given above, a disproportionation constant of 0.80×10−15 is calculated for vitamin B12r at pH=7.43. A steady state rate calculation using this value of disproportionation constant shows that a previously proposed mechanism for the methylation of vitamin B12r by methyl iodide involving the disproportionation of B12r would have a half-life of 360 years which does not correspond to the experimental rate.