Model Studies of Coenzyme B12 Dependent Diol Dehydratase. 2. A Kinetic and Mechanistic Study Focusing upon the Cobalt Participation or Nonparticipation Question

Article abstract of DOI:10.1021/ja00364a023

In the preceding paper ¹ model analogues of the putative diol dehydratase intermediates -CH(OH)CH2OH (1) and -CH2CHO (2) ( = coenzyme B₁₂) were synthesized and characterized by using the Co B₁₂ model system, with the unstable, postulated intermediate 1 being prepared in a carbonate-protected form (3), .The CH3O^-/CH3OH catalyzed deprotection of 3 and subsequent formation of 100percent Co^IICl, 95percent CH3CHO, and 50percent each of CH3OCO2CH3 and CH3OCO2^- were also described, as were experiments suggesting the noninvolvement of OHCCH2CoCl in this reaction.In the present manuscript, a kinetic and mechanistic study of this methanolysis reaction , 3 + CH3O^-/CH3OH, is presented and evidence for the formation of α-hydroxy, Co-CH(OH)CH2OR, complexes such as 1 is provided.Nitroxide radical trapping and added 1,5,6-trimethylbenzimidazole experiments are described, and the possible intermediates consistent with these experiments are presented.Cyclic voltammetry, literature precedent, and Meyerstein's related observations with coenzyme B₁₂ are provided as interpretation for the CH3CHO-inhibiting, but Co(I)- and HOCH2CHO-producing, side reaction induced by the added axial ligand.Evidence against cobalt participation pathways involving Co(I)- and Co(III) is described, and additional mechanistic experiments probing the apparent, nonformation of Co-CH2CHO (6) are presented.The results obtained (1) provide good evidence against cobalt participation in the rearrangement step, (2) rule out the often cited, but unverified, Co(III) ?-complex mechanism, (3) demonstrate that base-on cobalt participation is not only unnecessary, it introduces a CH3CHO-inhibiting side reaction, and (4) provide evidence that, when combined with literature stereochemical and other studies, suggest that it is the protein and not the cofactor which has the more significant role in diol dehydratase.