On the photooxidation of the multifunctional drug niclosamide. A kinetic study in the presence of vitamin B2 and visible light

Abstract

The multifunctional drug niclosamide (NSD), extensively employed therapeutically, is a frequent pollutant of surface waters. Considering the environmental importance of photodegradative processes for this type of contaminant, the kinetic and mechanistic aspects of the possible visible-light-mediated photooxidation of NSD were studied under naturalistic conditions. Methods: The visible-light absorber riboflavin (vitamin B2) was employed as a photosensitizer. The vitamin can usually be found in natural waters and is the most common endogenous photosensitizer in mammals. The interaction of NSD with electronically excited states of Rf and with photogenerated reactive oxygen species (ROS) was evaluated through conventional UV spectroscopy, laser flash photolysis, time-resolved phosphorescence detection of singlet molecular oxygen (O2(1Δg)), and polarographic dosage of dissolved oxygen. Results: Ground state NSD quenched the long-lived triplet excited state of Rf (3Rf*) and the photogenerated ROS (O2(1Δg)) and superoxide radical anion (O2•−). As a result, NSD was photooxidized. The rate constants for the interaction NSD–O2(1Δg) are particularly low, in the order of 106/M/s, although the whole interaction is attributable to a pure reactive process. The O2(1Δg) quenching was faster in alkaline medium, favored by the ionization of the NSD phenolic group. Under Rf-photosensitization, NSD was degraded very much more rapidly than phenol, the latter being considered a paradigmatic water-contaminant model compound. NSD may behave as an antioxidant in bio-environments, as demonstrated employing the photooxidizable amino acid tryptophan as a relevant biological target. Discussion: The results indicate that a O2•−-mediated process is the main route for the Rf-sensitized photooxidation of NSD. Photodegradation of the biocide in the presence and absence of phenol and tryptophan was quantitatively evaluated, discussed, and interpreted in terms of competitive quenching processes of3Rf*, O2(1Δg), and O2•− by the substrates.