A study of the photodegradation of 5
×
10−5M riboflavin (RF) in 0.2–1.0 M phosphate buffer in the presence
and absence of 2.50
×
10−4M caffeine at pH 6.0–8.0 has been carried out. RF in phosphate buffer is
photodegraded simultaneously by normal photolysis (photoreduction) and photoaddition reactions giving
rise to lumichrome (LC) and cyclodehydroriboflavin (CDRF) as the main final products, respectively.
RF and its photoproducts, formylmethylflavin (FMF), lumiflavin (LF), LC and CDRF in degraded solution
have been determined by a specific multicomponent spectrophotometric method with an accuracy of
±5%. The apparent first-order rate constants for the photodegradation of RF and for the formation of LC
and CDRF are 5.47–15.05
×
10−3 min−1, 1.06–8.30
×
10−3 min−1 and 4.31–8.05
×
10−3 min−1, respectively.
An increase in phosphate concentration leads to an increase in the rate of formation of CDRF and alters
the photodegradation of RF in favor of the photoaddition reaction. This photoaddition reaction is further
enhanced in the presence of caffeine which results in a further decrease of the fluorescence of RF in phosphate
buffer. Caffeine may facilitate the photoaddition reaction by suppression of the photoreduction
pathway of RF.