PHOTOCHEMISTRY OF RIBOFLAVIN .4. PHOTOBLEACHING OF SOME NITROGEN-9 SUBSTITUTED ISOALLOXAZINES AND FLAVINS

A variety of alcoholic type side chains attached at the N-9 position of the isoalloxazine nucleus have been photobleached under anaerobic and aerobic conditions. Anaerobic photolysis in neutral aqueous solution gave primarily two products as ascertained by polarography. One product was alloxazine and the other was an oxygen-sensitive intermediate which must have a ring formed by attachment of the side chain to the isoalloxazine nucleus. All isoalloxazines that underwent the photobleaching reaction produced the cyclic intermediate. The product ratio of alloxazine to intermediate is dependent on the length of the side chain. Linear ω-hydroxylic chains of three or more carbon atoms gave predominately the intermediate, whereas two carbon-atom side chains gave a mixture. Primary, secondary, and tertiary alcoholic groups on the side chain under anaerobic irradiation gave aldehydes, ketones, and the regenerated alcohol, respectively, from the dark oxidation of the cyclic intermediate. Quantum efficiencies varied from 0.2 to 2.0% for the various isoalloxazines and flavins. Aerobic photobleaching of the isoalloxazines gave similar products to the anaerobic photobleaching as determined by thin layer chromatography, but the rate processes were complicated by the competitive physical and chemical steps involving oxygen. Deuterium substitution at various locations on the ω-hydroxyalkyl side chain has conclusively shown that the carbon to hydrogen bond at the hydroxyl containing carbon controls the rate of the photobleaching process if such an a hydrogen is present. If an α hydrogen is not present on the hydroxyalkyl side chain, the photobleaching is initiated by abstraction of the hydroxyl hydrogen. Other alkyl hydrogens on the side chain are unreactive in the photobleaching process. These results substantiate a mechanism of intramolecular hydrogen abstraction from the N-9 substituted hydroxyalkyl side chain to the isoalloxazine nucleus during the primary photoprocess of photobleaching. © 1969, American Chemical Society. All rights reserved.