Quantum yields (4) have been determined for the photolysis of methylcobalamin in aqueous solution in the presence of air as follows : A = 490 nm, 4 = 0.35 f 0.04; A = 520 nm, 4 = 0.32 f 0.06 ; A = 550 nm, 9 = 0.24 f 0.05. These results show that photolysis is initiated by the absorption of a quantum of radiation by the intense bands due to n -~* transitions localised within the conjugated corrin ring. Comparison of published data on the relative rates of photolysis under different conditions shows that the initial photolytic step is reversible, i.e., (i), light dark Co-CH, @ CoII + CH,* (i)

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