Free radical reactions in the coenzyme B12 system
Pulse radiolysis has been used to study the reaction of hydrated electrons and other agents with coenzyme B12 and cob(II)alamin (Vitamin B12r). The efficiency of reaction of hydrated electrons with coenzyme B12 to produce vitamin B12r is about 80 %. The rate constant for vitamin B12r formation is approximately 3.0 × 1010 dm3 mol–1 s–1 while that for disappearance of the hydrated electron is 3.2 × 1010 dm3 mol–1 s–1. Carboxyl radicals (CO–2) appear to be very inefficient at reducing coenzyme B12 to B12r and several other free radicals, viz. hydroxy-2-methylpropyl, hydroxyisopropyl and 1,2-dihydroxyethyl do not reduce coenzyme B12 in neutral solution. However, in those systems where these radicals are present with the hydrated electron, the latter reduces coenzyme B12 to B12r which subsequently can react with the organic radicals either by reduction to cob(I)alamin (vitamin B12s), by oxidation to cob(III)alamin (vitamin B12a), or by addition. The oxidation reaction is slower than that of reduction, the rate constant being about 2.4 × 108 dm3 mol–1 s–1 for radicals derived from 2-methylpropan-2-ol and ethane-1,2-diol. The addition products have optical absorption spectra very similar to that of coenzyme B12.