title: On the photooxidation of the multifunctional drug niclosamide. A kinetic study in the presence of vitamin B2 and visible light.
abstract: OBJECTIVE
  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((1)Δg)), and polarographic dosage of dissolved oxygen.
  
  
  RESULTS
  Ground state NSD quenched the long-lived triplet excited state of Rf ((3)Rf*) and the photogenerated ROS (O2((1)Δg)) and superoxide radical anion (O2•−). As a result, NSD was photooxidized. The rate constants for the interaction NSD-O2((1)Δg) are particularly low, in the order of 10(6)/M/s, although the whole interaction is attributable to a pure reactive process. The O2((1)Δg) 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.
  
  
  CONCLUSIONS
  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 of (3)Rf*, O2((1)Δg), and O2•− by the substrates.
authors.forename: José
authors.initials: J
authors.lastname: Natera
authors.forename: Eduardo
authors.initials: E
authors.lastname: Gatica
authors.forename: Cecilia
authors.initials: C
authors.lastname: Challier
authors.forename: David
authors.initials: D
authors.lastname: Possetto
authors.forename: Walter
authors.initials: W
authors.lastname: Massad
authors.forename: Sandra
authors.initials: S
authors.lastname: Miskoski
authors.forename: Adriana
authors.initials: A
authors.lastname: Pajares
authors.forename: Norman A
authors.initials: NA
authors.lastname: García
mesh_terms: D000838:Anions; D000975:Antioxidants / Q000737:chemistry; D007700:Kinetics; D007834:Lasers; D008027:Light; D000432:Methanol / Q000737:chemistry; D009534:Niclosamide / Q000627:therapeutic use*; D010100:Oxygen / Q000737:chemistry; D019800:Phenol / Q000737:chemistry; D010636:Phenols / Q000737:chemistry; D055668:Photochemical Processes; D010782:Photolysis; D017319:Photosensitizing Agents / Q000737:chemistry; D017382:Reactive Oxygen Species / Q000737:chemistry; D012256:Riboflavin / Q000737:chemistry*; D026082:Singlet Oxygen; D013056:Spectrophotometry, Ultraviolet; D013481:Superoxides / Q000737:chemistry; D014364:Tryptophan / Q000737:chemistry; D014867:Water / Q000737:chemistry; D014874:Water Pollutants, Chemical / Q000737:chemistry
chemical_list: D000838:Anions; D000975:Antioxidants; D010636:Phenols; D017319:Photosensitizing Agents; D017382:Reactive Oxygen Species; D014874:Water Pollutants, Chemical; D014867:Water; D013481:Superoxides; D026082:Singlet Oxygen; D019800:Phenol; D014364:Tryptophan; D009534:Niclosamide; D010100:Oxygen; D012256:Riboflavin; D000432:Methanol
keywords: Niclosamide; Photodegradation; Photooxidation; Reactive oxygen species; Riboflavin