title: On the generation and quenching of reactive-oxygen-species by aqueous vitamin B2 and serotonin under visible-light irradiation.
abstract: It is well known that endogenous daylight-absorbing compounds produce the sensitized photodegradation of biologically relevant substrates. In this context the photostability of a mixture of the indole neurotransmitter serotonin (Sero) and vitamin B2 (riboflavin, Rf) upon visible-light irradiation and the possible role of Sero and related compounds as generators or deactivators of reactive oxygen species (ROS) was investigated through a kinetic and mechanistic study. The work was done at pH 7 and under experimental conditions in which only the vitamin absorbs photoirradiation. Tryptamine (Trpa) and 5-hydroxyindole (OHIn) were included in the study as model compounds for the neurotransmitter. The visible light irradiation of aqueous Rf in the individual presence of Sero, Trpa and 5-OHIn, under aerobic conditions, induce degradative processes on the indole derivatives (In-der). At least two different mechanisms operate. Our analysis shows that the main reaction pathway is an electron-transfer-mediated quenching of triplet excited Rf ((3)Rf(*)) by the In-der. It produces the species Rf(-)/RfH() and the In-der radical cation that could react to form phenoxy and α-amino radicals. In a further reaction step the species O(2)(-) and OH() could be produced. In parallel, energy transfer from (3)Rf(*) to dissolved oxygen would generate O(2)((1)Δ(g)). Within the frame of the proposed mechanism, results suggest that Rf-sensitized degradation of Sero occurs via the mentioned ROS and non-oxygenated radical-mediated processes. The indole compound quenches O(2)((1)Δ(g)) in a dominant physical fashion. This fact constitutes a desirable property in antioxidants, provided that the quenching process practically does not eliminate the scavenger. Sero exerts a photoprotective effect towards tryptophan through the combined quenching of O(2)((1)Δ(g)) and (3)Rf(*), the latter excited species responsible for the generation of ROS. The amino acid can be taken as a target model of oxidizable biological substrates, particularly proteins.
authors.affiliation: Unidad Académica Río Gallegos, Universidad Nacional de la Patagonia Austral, 9400 Río Gallegos, Argentina. haggies@infovia.com.ar
authors.forename: Ernesto
authors.initials: E
authors.lastname: Haggi
authors.forename: Néstor
authors.initials: N
authors.lastname: Blasich
authors.forename: Lihuel
authors.initials: L
authors.lastname: Gutiérrez
authors.forename: Gabriela
authors.initials: G
authors.lastname: Vázquez
authors.forename: Susana
authors.initials: S
authors.lastname: Criado
authors.forename: Sandra
authors.initials: S
authors.lastname: Miskoski
authors.forename: Gabriela
authors.initials: G
authors.lastname: Ferrari
authors.forename: M
authors.initials: M
authors.lastname: Paulina Montaña
authors.forename: Norman A
authors.initials: NA
authors.lastname: García
mesh_terms: D008027:Light; D017382:Reactive Oxygen Species / Q000737:chemistry / Q000528:radiation effects; D012256:Riboflavin / Q000737:chemistry* / Q000528:radiation effects; D012701:Serotonin / Q000737:chemistry* / Q000528:radiation effects; D014867:Water / Q000737:chemistry
chemical_list: D017382:Reactive Oxygen Species; D014867:Water; D012701:Serotonin; D012256:Riboflavin