Photosensitized Amino Acid Degradation in the Presence of Riboflavin and Its Derivatives

The addition of photosensitizers to water can accelerate disinfection in sunlight-based systems by enhancing oxidation of target compounds through direct reaction with the excited sensitizer or through production of another oxidant, such as singlet oxygen (O-1(2)). The kinetics of the oxidation of selected amino acids in the presence of the sensitizer riboflavin (Vitamin B2), its primary photoproduct lumichrome, and its derivative riboflavin tetraacetate (2',3',4',5'-tetraacetylriboflavin; RTA) were quantified and the mechanisms of reaction were determined during exposure to 365 +/- 9 nm light. O-1(2)-mediated reactions contributed to the rapid photodegradation of the four amino acids, but its contribution was sensitizer-dependent and varied from 5.4-10.2% for tyrosine, 7.1-12.4% for tryptophan, 18.7-69.0% for methionine, and 64.7-100.2% for histidine. Riboflavin was subject to rapid photodegradation (t(1/2) <8 min), while the half-lives of lumichrome and RTA were 100 and 30 times longer, respectively. Lumichrome and RTA also were more efficient O-1(2) sensitizers (quantum yield (Phi) = 0.63 and 0.66) compared to riboflavin (Phi = 0.48). Of the three flavin-based compounds, RTA shows the most promise as a sensitizer in sunlight-based disinfection systems because it absorbs both visible and UV light, is an efficient O-1(2) sensitizer, is a strong oxidant in its triplet state, and exhibits greater photostability.