Riboflavin and rose bengal sensitised photooxidation of sulfathiazole and succinylsulfathiazoleKinetic study and microbiological implications

Abstract In the presence of the synthetic xanthenic dye rose bengal or the pigment riboflavin, the bacteriostatics sulfathiazole ( STZ ) and succinylsulfathiazole ( SCSTZ ) suffer visible-light promoted degradation in dilute aqueous and aqueous-ethanolic solutions. The events occur to a different extent, depending on the chemical structures of the thiazoles and pH of the medium. The photo-oxidation process is accompanied by a partial loss of the antibacterial activity of the thiazoles. The extent of photodegradation occurs quickly under alkaline conditions and is less aggressive in the physiological pH region, although not negligible in kinetic terms. Photo-oxidation quantum efficiencies, evaluated through singlet molecular oxygen [O 2 1 Δ g ] phosphorescence detection, spectrophotometric and polarographic methods, range from 0.002 to 0.31 as the upper limit in alkaline media. With rose bengal as a sensitiser in aqueous medium, photo-oxidation essentially proceeds via singlet molecular oxygen. The results indicate the occurrence of several photoprocesses that involve a series of competitive reactions, including singlet molecular oxygen photo-oxidation, when the vitamin riboflavin was employed as a sensitiser, in a 1:1 water: ethanol medium. Their relative importance depends on the concentration of the thiazoles and riboflavin. The bacteriostatics efficiently quench both the excited singlet and triplet states of the pigment. The final result of the interactions is the phototransformation of the thiazoles and the partial photostabilization of riboflavin. The O 2 1 Δ g -mediated photo-oxidation of STZ produces a decrease in its bacteriostatic properties.