Photodehalogenation of 7- and 8-halogen-substituted flavins. Photochemistry of the reduced flavin chromophore.

Flavodoxin was reconstituted with 8-chloro- and 7-bromo-FMN and p-hydroxybenzoate hydroxylase with the analogous FAD derivatives. In all cases, the spectral properties of the artificial enzymes changed as a result of photoreduction in the presence of ethylenediaminetetraacetate or oxalate as sources of reducing equivalents. The same changes were found to occur on irradiation of the enzymes which had been reduced previously in the dark under anaerobic conditions with dithionite. Using analogous 7- and 8-chlorolumiflavins, the observed changes were shown to be due to a novel photoreaction of the reduced flavin chromophore, in which either the 7- or 8-halogen substituent is eliminated and replaced by a proton derived from the solvent. The same reaction was shown to occur with 7,8-bis-norlumiflavin where 1 deuterium atom was incorporated into the molecule as a result of photoirradiation of the reduced flavin in deuterated medium. In the case of p-hydroxybenzoate hydroxylase, both the 8-chloro-FAD and 7-bromo-FAD enzymes, as well as their 8-nor-FAD and 7-nor-FAD photoproducts, possessed catalytic activity comparable to that of the native enzyme.