One-electron Reactions in Some Cobalamins

Abstract

Pulse radiolysis has been used to study the reaction of hydrated electrons and other agents with various cobalamins, viz. cyanocobalamin (vitamin B12), hydroxocobalamin (vitamin B12a), and cob(II)alamin (vitamin B12r). It appears that hydrated electrons react with the cobalt atom of cyanocobalamin with an efficiency of about two-thirds, the product being vitamin B12r. The rate constant for this reaction is 3.8 × 1010 dm3 mol–1 s–1. Hydroxyl radicals react with cyanocobalamin with a rate constant of 6.5 × 109 dm3 mol–1 s–1 to produce an uncharacterised yellow-brown compound.

The radical CO–2 does not reduce cyanocobalamin to vitamin B12r but reduces hydroxocobalamin with an efficiency of about two-thirds and with a rate constant of 1.45 × 109 dm3 mol–1 s–1. Vitamin B12r is reduced CO–2 with nearly 100% efficiency, the product being cob(I)alamin (vitamin B12s). The rate constant for the reaction is 8.2 × 108 dm3 mol–1 s–1.

Hydroxyisopropyl radicals do not react with cyanocobalamin but reduce vitamin B12r in a similar fashion to CO–2.