title: Quenching of singlet molecular oxygen, O2(1Deltag), by dipyridamole and derivatives.
abstract: Dipyridamole (DIP) is known for its vasodilating and antiplatelet activity, exhibiting also a potent antioxidant effect, strongly inhibiting lipid peroxidation. This effect has been studied in mitochondria and a correlation between the DIP derivatives' structure, the ability to bind to micelles and biological activity has been suggested. In the present work, the quenching of singlet molecular oxygen, O(2)((1)Delta(g)), by DIP and RA47 and RA25 derivatives was analyzed in acetonitrile (ACN) and aqueous acid solutions. Laser flash photolysis excitation of methylene blue (MB) was made at 532 nm and monomol light emission of O(2)((1)Delta(g)) was monitored at 1270 nm. Bimolecular quenching constants in ACN are consistent with an efficient physical quenching, presenting values a bit lower than the diffusion limit (k(t) = 3.4-6.8 x 10(8) M(-1 )s(-1)). The quenching process probably occurs via reversible charge transfer with the formation of an exciplex. Calculation of DeltaG(et) associated with O(2)((1)Delta(g)) quenching corroborates with uncompleted electron transfer. In aqueous acid solutions (pH = 3.0), the k(t) values for DIP and derivatives are 20-fold smaller when compared with ACN. The electrochemical properties of DIP in ACN are characterized by two consecutive one-electron processes with half-wave oxidation potentials of 0.30 and 0.67 V vs saturated calomel electrode (SCE). However, in an aqueous acid medium, a single oxidation wave is observed involving a two-electron process (0.80 V vs SCE). Therefore, O(2)((1)Delta(g)) quenching is consistent with electrochemical data.
authors.affiliation: Departamento de Química e Física Molecular, Instituto de Química de São Carlos, Universidade de São Paulo, São Carlos, SP, Brazil.
authors.forename: Marilene Silva
authors.initials: MS
authors.lastname: Oliveira
authors.forename: Michelle
authors.initials: M
authors.lastname: Lima
authors.forename: Divinomar
authors.initials: D
authors.lastname: Severino
authors.forename: Maurício da Silva
authors.initials: Mda S
authors.lastname: Baptista
authors.forename: Paolo
authors.initials: P
authors.lastname: Di Mascio
authors.forename: Marcel
authors.initials: M
authors.lastname: Tabak
mesh_terms: D004176:Dipyridamole / Q000031:analogs & derivatives* / Q000737:chemistry; D004563:Electrochemistry; D015394:Molecular Structure; D010084:Oxidation-Reduction; D038761:Photobleaching; D026082:Singlet Oxygen / Q000737:chemistry*; D012997:Solvents; D013050:Spectrometry, Fluorescence
chemical_list: D012997:Solvents; D026082:Singlet Oxygen; D004176:Dipyridamole