| Several organocobalamins relevant to the mechanisms of the carbon skeleton rearrangements catalysed by coenzyme B(,12) - dependent enzymes have been synthesized. Reactions of these model compounds have been investigated in order to determine the mechanism by which the carbon skeletons of the organic ligands rearrange after Co-C bond cleavage. In addition, simpler reactions of alkylcorrins and related alkylcobalt complexes in base have been examined.; Base-induced decompositions of simple alkylcobalamins and alkylcobinamides with alkyl groups containing (beta)-hydrogen produce alkenes. Alkylcobaloximes and related model compounds produce both alkenes and alkanes under similar conditions. The ratio of alkene to alkane is dependent on both the base concentration and on the structure of the alkyl group. Alkenes are formed by a (beta)-elimination reaction, whereas alkanes are formed by initial homolysis of the Co-C bond. Homolysis also occurs in the decompositions of alkylcorrins, but the radicals do not terminate by hydrogen abstraction to form alkanes.; Methylmalonyl-CoA mutase, (alpha)-methyleneglutarate mutase, and glutamate mutase catalyse the reversible interconversions of methylmalonyl-CoA and succinyl-CoA, methylitaconic acid and (alpha)-methyleneglutaric acid, and (beta)-methylaspartic acid and glutamic acid, respectively. These enzymes require coenzyme B(,12) as a cofactor, and organocobalamins with the substrate covalently attached to cobalt are postulated to be intermediates. Organocobalamins with methylitaconic acid and (beta)-methylaspartic acid attached to cobalt via their methyl groups have been prepared. Methylmalonyl-CoA has not been attached to cobalt, but several model compounds with methylmalonate or dimethylmalonate esters attached to cobalt have been prepared. The latter compounds undergo spontaneous decomposition in aqueous solutions by (beta)-elimination if a (beta)-hydrogen is present, or by Co-C bond homolysis if no such hydrogen is available. The model compounds for the rearrangements of methylitaconic acid and (beta)-methylaspartic acid are stable in neutral aqueous solution. Co-C bond cleavage reactions of the model compounds for the rearrangements of methylmalonyl-CoA and methylitaconic acid under various controlled conditions demonstrate that the rearrangements of the carbon skeletons occur via intermediate carbanions. |
