Visible-light-promoted degradation of the antioxidants propyl gallate and t-butylhydroquinone: mechanistic aspects.
The kinetic and mechanistic aspects of the visible-light-mediated photodegradation of the phenolic antioxidants (PA), propyl gallate (PG), and t-butylhydroquinone (TBHQ), employing riboflavin (Rf) as photosensitizer, have been studied by time-resolved and stationary techniques.  The photosensitizer Rose Bengal (RB) was used for auxiliary experiments. Results show the occurrence of chemical transformations on PA with the participation of electronically excited states of Rf and different reactive oxygen species (ROS) generated from these states. With 0.02 mM Rf  and 1.0 mM PA, the electronically excited triplet state of Rf is quenched by PA, in a competitive manner with the dissolved oxygen. As a consequence, a cascade of photoprocesses produces singlet oxygen (O(2)((1)Δ(g))) and H(2)O(2) in the  case of PG and, O(2)((1)Δ(g)), H(2)O(2) and HO(•) in the case of TBHQ. The participation of these species is supported by experiments of oxygen consumption carried out in the presence of specific ROS scavengers. TBHQ has a relatively  high capacity for O(2)((1)Δ(g)) physical deactivation and a low photodegradation efficiency by the oxidative species. Comparatively, it can be asserted that TBHQ has a higher antioxidant capacity than PG.
Departamento de Química, Universidad Nacional de Río Cuarto, Campus Universitario, 5800 Río Cuarto, Argentina. scriado@exa.unrc.edu.ar
Susana
S
Criado
Carolina
C
Allevi
Norman A
NA
García
D000818:Animals; D000975:Antioxidants / Q000737:chemistry*; D015200:Fluoroimmunoassay; D006861:Hydrogen Peroxide / Q000737:chemistry; D006873:Hydroquinones / Q000737:chemistry*; D017665:Hydroxyl Radical / Q000737:chemistry; D008027:Light; D008163:Luminescent Measurements / Q000379:methods; D010782:Photolysis; D011435:Propyl Gallate / Q000737:chemistry*; D012395:Rose Bengal / Q000737:chemistry; D026082:Singlet Oxygen / Q000737:chemistry
D000975:Antioxidants; D006873:Hydroquinones; D026082:Singlet Oxygen; D012395:Rose Bengal; D017665:Hydroxyl Radical; D011435:Propyl Gallate; D006861:Hydrogen Peroxide; C018855:2-tert-butylhydroquinone