Photolysis of alkylcobalamins and coenzyme B12 : an electron spin resonance study

Abstract

Exposure of methylcobalamin in CD₃OD–D₂O glasses at 77 K to a high-pressure mercury arc gave low yields of methyl and CD₂OD radicals and high yields of a radical-Co^II pair species (triplet state), which on annealing was converted into normal Co^IIB_12^r. The mean separation between cobalt and the radical was ca. 8.3 Å. The ethyl derivative gave broad features for Co^IIB_12^r and alkyl radicals, suggesting weak spin–spin interaction and hence greater mean separation. In contrast, the acetyl derivatives gave no detectable features at 77 K, but intense Co^IIB_12^r features grew in on annealing above 77 K. We suggest that close random pair-trapping broadened all features beyond the detection limit at 77 K.

For the coenzyme, in agreement with Lowe et al., no detectable reaction occurred on photolysis at 77 K, but normal photolysis was detected above ca. 200 K. Again, some random pair-trapping was evident.