Vitamin B-sensitized photo-oxidation of dopamine.
Kinetics and mechanism of the photo-oxidation of the natural catecholamine-type neurotransmitter dopamine (DA) has been studied in aqueous solution, under aerobic conditions, in the presence of riboflavin (Rf, vitamin B(2)) as a photosensitizer. Results indicate the formation of a weak dark complex Rf-DA, with a mean apparent association constant K(ass) = 30 m(-1), only detectable at DA concentrations much higher than those employed in 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 x 10(9) and 2.2 x 10(9) m(-1) s(-1), respectively. With the catecholamine 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 (O(2)) and singlet molecular oxygen (O(2)((1)Delta(g))) by processes initiated by electron and energy-transfer mechanisms, respectively. Rate constants values of 1.9 x 10(8) and 6.6 x 10(6) m(-1) s(-1) have been obtained for the overall and reactive (chemical) interaction of DA with O(2)((1)Delta(g)). The presence of superoxide dismutase increases both the observed rates of aerobic DA photo-oxidation and oxygen uptake, due to its known catalytic scavenging of O(2), a species that could revert the overall photo-oxidation effect, according to the proposed reaction mechanism. As in most of the catecholamine 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.
Massad
Walter A
WA
Departamento de Química, Facultad de Ciencias Exactas, Físico-Químicas y Naturales, Universidad Nacional de Río Cuarto, Río Cuarto, Argentina. wmassad@exa.unrc.edu.ar
Barbieri
Yanina
Y
Romero
Mario
M
García
Norman A
NA
D004298:Dopamine / Q000737:chemistry* / Q000528:radiation effects*; D045563:Indolequinones / Q000138:chemical synthesis / Q000737:chemistry / Q000528:radiation effects; D007700:Kinetics; D008027:Light; D015394:Molecular Structure; D010084:Oxidation-Reduction / Q000187:drug effects; D010100:Oxygen / Q000737:chemistry / Q000528:radiation effects; D010777:Photochemistry; D010782:Photolysis; D017319:Photosensitizing Agents / Q000737:chemistry / Q000494:pharmacology*; D011789:Quantum Theory; D012256:Riboflavin / Q000737:chemistry / Q000494:pharmacology*; D013481:Superoxides / Q000737:chemistry / Q000528:radiation effects; D013997:Time Factors
D045563:Indolequinones; D017319:Photosensitizing Agents; D013481:Superoxides; C091584:aminochrome 1; D010100:Oxygen; D012256:Riboflavin; D004298:Dopamine