Environmental photodegradation of pyrimidine fungicides -: Kinetics of the visible-light-promoted interactions between riboflavin and 2-amino-4-hydroxy-6-methylpyrimidine

To clarify the evolution of pyrimidine fungicides under natural aquatic environmental conditions, the visible-light-promoted degradation of the model fungicide 2-amino-4-hydroxy-6-methylpyrimidine (AHMPD) has been studied in air-equilibrated pH 6 aqueous solutions in the presence of riboflavin (Rf), employing time-resolved and stationary kinetic-spectroscopic methods. AHMPD, a compound practically inert towards the attack of singlet molecular oxygen (O-2((1)Delta(g))), quenches excited singlet and triplet states of Rf with rate constants of 2.7 x 10(9) M-1 s(-1) and 2.7 x 10(7) M-1 s(-1), respectively. In the presence of AHMPD, the photodecomposition of Rf, which occurs from the excited triplet state of the pigment, depends on the concentration of the fungicide: at greater than or equal to40-50 mM a limited photochemistry occurs due to the quenching of excited singlet Rf, while at ca. 5-10 mM triplet Rf is largely photogenerated and subsequently quenched either by oxygen, giving rise to O-2((1)Delta(g)), or by AHMPD, yielding semireduced Rf through an electron transfer process. Flash photolysis experiments and indirect auxiliary tests confirm the presence of superoxide anion generated by the reaction of Rf anion radical with the dissolved oxygen. The neat result of this intricate scheme of competitive reactions is the photodegradation of both AHMPD and Rf, mainly through a superoxide anion-mediated oxidation, although some contribution of O-2((1)Delta(g))-mediated photooxidations cannot be disregarded.