Methylcobalamin, photochemistry

ESR studies 112,114-118a) are also consistent with the formation of free radicals upon photolysis of alkylcobalamin and coenzyme Bi2- For example, Lappert and co-workers 116) demonstrated that homolysis of the cobalt-alkyl bond occurs upon photolysis of coenzyme B12 and ethylcobalamin by trapping the S -deoxyadenosyl and ethyl radicals produced with (CH3)3CN0. They were able to detect the spin-trapped (CH3)3CN(0)R radicals by ESR spectroscopy. Homolysis of the cobalt-methyl bond was also shown to occur upon anaerobic photolysis of methylcobalamin 117). However, the presence of traces of oxygen in the methanol solvent was shown to affect signiflcantly the photochemistry of methylcobalamin 118). Indeed, under those conditions, the 5,S-dimethyl-1-pyrroline iV-oxide (DMPO) spin adducts of both the methyl and hydrogen radicals, 113 and 114, respectively, were detected by ESR spectro-... 

The photochemistry of the cobaloximes is similar to that of the cobala-mins except that the quantum yields are much lower (108). The nature of the primary photochemical processes is also less clear. Giannotti and co-workers (112c, 118a), for example, have proposed a mechanism different from that for methylcobalamin. They obtained evidence that photoaquation of the axial ligand B precedes oxygen insertion when... 

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