The Photochemistry of Riboflavin—v. The Photodegradation of Isoalloxazines in Alcoholic Solvents

Riboflavin and other N ‐10 substituted isoalloxazines have been irradiated anaerobically in alcohols and alcohol‐water mixtures. Deuterium substitution was used in several different ways to determine the hydrogen kinetic isotope effect. Riboflavin and 10‐(3′‐hydroxypropyl)‐isoalloxazine were photobleached in 2‐propanol, 2‐propanol‐0‐d, 2‐propanol‐2‐d, t ‐butyl alcohol, and t ‐butyl alcohol‐0‐d. No isotope effect significantly different from 1.0 was found for any of the nondeuterated to deuterated solvent comparisons. When the anaerobic irradiation of 10‐(3′‐hydroxypropyl‐3′,3′‐d 2 )‐isoalloxazine and 10‐(3′‐hydroxypropyl)‐isoalloxazine in either 2‐propanol or t‐butyl alcohol were compared, kinetic isotope effect ratios for H to D of 2.8 were obtained. Thin layer chromatography in conjunction with color return at 445 nm during oxidation showed that lumichrome and formylmethylflavin, which are irreversible photodegradation products, were formed from the photoreaction in alcohols. The photochemistry of isoalloxazines in alcohols, while somewhat different from the aqueous photochemistry, does not involve a primary photoreduction with the solvent, and photo degradation still dominates. This was an unexpected result and it illustrates the unusual photochemical behavior of isoalloxazines. The anaerobic irradiation of riboflavin in 2‐propanol‐water mixtures showed that the efficiency of photodegradation is linked to the physical properties of the solvent.