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

JOSÉ NATERA; EDUARDO GATICA; CECILIA CHALLIER; DAVID POSSETTO; WALTER MASSAD; SANDRA MISKOSKI; ADRIANA PAJARES; NORMAN A. GARCÍA

REDOX REPORT

MANEY PUBLISHING

Lugar: London; Año: 2015

1351-0002

Objectives Themultifunctional drug Niclosamide (NSD), extensively employed in therapeutictreatments, is an usual surface waters? pollutant. Considering the environmental importance of photodegradative processes in this typeof contaminants, this research dealt on the kinetic and mechanistic aspects ofthe possible visible-light mediatedphotooxidation of NSD, under natural-like conditions. MethodsThevisible-light absorber Riboflavin (vitamin B2) was employed as aphotosensitizer. The vitamin can usually be found in natural waters andconstitutes the most common  endogenousphotosensitizer in mammals. Theinteraction of NSD with electronically excited states of Rf and withphotogenerated reactive oxygen species (ROS) was evaluated through conventionalUV spectroscopy, laser flash photolysis, time resolved phosphorescencedetection of singlet molecular oxygen (O2(1Dg) ) and polarographic dosage of dissolved oxygen.ResultsGround state NSDquenches the long-lived triplet excited state of Rf (3Rf*) and the photogenerated ROS (O2(1Dg)) and superoxide radical anion (O2·?). As a result, NSD is photooxidized. The rate constants for theinteraction NSD-O2(1Dg) are particularly low, in the order of 106 M-1s-1although the whole interaction is attributable to a pure reactive process. TheO2(1Dg) quenching is faster in alkaline medium, favored bythe ionization of the NSD phenolic group.In comparative terms and under Rf-photosensitization,NSD is degraded very much rapidly than phenol, the latter considered as aparadigmatic water-contaminant model compound. NSD may behave as an antioxidant inbio-environments. It was demonstrated employing the photooxidizable amino acidtryptophan as relevant biological target. DiscussionResults indicate thata O2·?-mediated is the mainroute for the Rf-sensitized photooxidation of NSD. Photodegradation of thebiocide in the presence and in the absence of phenol and tryptophan was quantitatively evaluated, discussed andinterpreted in terms of competitive quenching processes of 3Rf*, O2(1Dg) and O2·? by the substrates.