Coenzyme B12 and Cobaloxime Chemistry

The cobalt(ii) form of coenzyme Bi2a Bi2r, has been detected in the coenzyme 

Bi2 dependent ethanolamine ammonialyase catalysed reaction. Binding of adenosylcobalamin to the protein causes a slow conformational change and the reactions of 2-amino-alcohols proceed by formation of an enzyme-substrate complex followed by radical and cobalt(ii) formation and finally disappearance of the cobait(ii). Rate differences with 2-aminoethanol and L-2-aminopropanol arise mainly in this latter step. 

Photo-homolytic cleavage of Co—C bonds has been studied for Vitamin 812 and cobalt(m) complexes with tetra-aza-14-membered macrocyclic ligands. In the latter study, the rate of recombination of the radical with cobalt(ii) is ca. 10 M s at 25 °C. The cobalt(iii) complexes have a low energy threshold to photolysis, 540nm. 

Different decomposition mechanisms for the products of the reaction between Co Mea[14]-diene-N4, Co L, and carbon-centred aliphatic alcohol radicals suggest an explanation for the inhibition of diol dehydratase by NjO. 

If process (1) occurs in parallel with process (2) in the dehydration of ethylene glycol, then the 8123 form of the enzyme, containing Co, which reacts rapidly with N2O, is formed. 

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