Vitamin B2-sensitised-photooxidation of Dopamine

WALTER A. MASSAD; YANINA BARBIERI; MARIO ROMERO; NORMAN A. GARCÍA

PHOTOCHEMISTRY AND PHOTOBIOLOGY

WILEY-BLACKWELL PUBLISHING, INC

Año: 2008 vol. 84 p. 1201 - 1208

0031-8655

Kinetics and mechanism of the photooxidation of the natural cathecholamine-type neurotransmitter Dopamine (DA) has been studied in aqueous solution, under aerobic conditions, in the presence of Riboflavin (Rf, vitamin B2) as a photosensitizer. Results indicate the formation of a weak dark complex Rf-DA, with a mean apparent association constant Kass = 30 M-1, only detectable at DA concentrations much higher than those employed in the photochemical experiments. An intricate mechanism of competitive reactions operates, upon photoirradiation. DA quenches excited singlet and triplet states of Rf, with rate constants of 4.2 109 M-1s-1 and 2.2 109 M-1s-1, respectively. With the cathecholamine in a concentration similar to that of dissolved molecular oxygen in air-saturated water, DA and oxygen competitively quench the triplet excited state of Rf, generating superoxide radical anion (O2•) and singlet molecular oxygen (O2(1g)) by processes initiated by electron and energy-transfer mechanisms respectively. Rate constants values of 1.9 x 108 M-1s-1 and 6.6 x 106 M-1s-1 have been obtained for the overall and reactive (chemical) interaction of DA with O2(1g). The presence of superoxide dismutase increases both the observed rates of aerobic DA photooxidation and oxygen uptake, due to its known catalytic scavenging of O2•, a species that could revert the overall photooxidation effect, according to the proposed reaction mechanism. As in most of the oxidative processes described in the literature, aminochrome is the DA oxidation product upon visible light-irradiation in the presence of Rf. It is generated with a quantum yield of 0.05.