Hydrolysis and Photolysis of Pyraoxystrobin Based on Isotope Labeling and Mass Spectrometric Analysis of Its Degradation Products

Pyraoxystrobin is a methoxyacrylic fungicide with low toxicity, high efficiency and broad spectrum. It has good control effect on rice blast, rice sheath blight, cucumber downy mildew and cucumber powdery mildew. In order to explore the degradation characteristics of pyraoxystrobin in water, the hydrolysis dynamics of pyraoxystrobin at different temperatures, pH values and the photolysis dynamics under different illumination were studied, the hydrolysis and photolysis products and pathways of pyraoxystrobin were studied by mass spectrometry with ¹³C-pyraoxystrobin as control. The test results showed that the Pyraoxystrobin was difficult to hydrolyze in acidic and alkalescent aqueous solution (pH 8.0). In alkaline aqueous solution (pH 10.0-13.0), the hydrolysis dynamics of pyraoxystrobin was affected by temperature. With the increase of temperature and pH values, the degradation rate was accelerated. The hydrolysis rate constants were linearly positively correlated with the pH values. Hydrolysis reaction was base-catalyzed reaction, and the higher temperatures, was the higher rate constants of base catalytic reaction were. Under indoor simulated sunlight and 30 ℃, pyraoxystrobin was easy to photolysis, and the photolysis reaction rate was increased with light intensity and ultraviolet intensity. Pyraoxystrobin was difficult to photolyze under the outdoor natural light at 13-19 ℃. It was speculated that the ester bond was broken on the unlabeled atom C and an alkyl group (CH3) was removed in the hydrolysis process of pyraoxystrobin. The product might be (E)-2-2-3-(4-chlorophenyl)-1-methyl-1H-pyrazol-5-oxymethylphenyl-3-methoxyacrylic acid（C₂₁H₁₉ClN₂O₄）. The photolysis products was presumed to be 3-(4-chlorophenyl)-5-((2-(4-methoxyfuran-3-yl) benzyl) oxy)-1-methyl-1H-pyrazole after carbonyl hydrogenation on unlabeled atom C, dehydration and ring formation. The hydrolysis and photolysis products of zolexin ester and ¹³C-pyraoxystrobin ester were analyzed by mass spectrometry. It was speculated that the hydrolysis process of zolexin ester may be the breaking of ester bond on unmarked C atom and the dehydration of an alkyl group to form C₂₁H₁₉ClN₂O₄. The photolysis process may be carbonyl hydrogenation and dehydration on unmarked C atom. After cyclization, 3-(4-chlorophenyl)-5-2-(4-methoxyfuran-3-phenyl methoxy-1-methyl-1H-pyrazole (C₂₂H₁₉ClN₂O₃) was formed. In this study, ¹³C-labelled pyraoxystrobin was used as internal standard, the degradation dynamics of zoxomycin in water were researched at different pH values and temperatures, and the photolysis of zoxomycin in water were researched under different light conditions. The hydrolysis and photolysis products and pathways of zoxomycin were analyzed and speculated in order to provide data for rational drug use, water source protection, ecological balance maintenance and food safety.