ON THE GENERATION AND QUENCHING OF REACTIVE-OXYGEN-SPECIES BY AQUEOUS VITAMIN B2 AND SEROTONIN UNDER VISIBLE-LIGHT IRRADIATION

HAGGI ERNESTO; BLASICH NÉSTOR; GUTIÉRREZ LIHUEL; VÁZQUEZ GABRIELA; CRIADO SUSANA; MISKOSKI SANDRA; FERRARI, GABRIELA V. ; MONTAÑA M. PAULINA; GARCÍA NORMAN A.

JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY B-BIOLOGY

ELSEVIER SCIENCE SA

Lugar: Amsterdam; Año: 2012 vol. 113 p. 22 - 28

1011-1344

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 (3Rf*) by the In-der. It produces the species Rf-./RfH. and the In-der radical cation that could react to form phenoxy and alfa-amino radicals. In a further reaction step the species and OH could be produced. In parallel, energy transfer from 3Rf* to dissolved oxygen would generate singlet molecular oxigen. Within the frame of the proposed mechanism, results suggest that Rf-sensitized degradation of Sero occurs via the mentioned ROS and non-oxygenated radicalmediated processes. The indole compound quenches singlet molecular oxigen 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 singlet molecular oxigen and 3Rf*, 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.