Photooxidation mechanism of fenthion by singlet oxygen: Evidence by ESR analysis with a selective spin trapping agent

Abstract

Our previous study suggested that UVB irradiation of the organophosphorus pesticide, fenthion and its photodegradation product, 3-methyl-4- methylthiophenol (MMTP) yielded fenthion sulfoxide and 3-methyl-4- methylsulfinylphenol (MMSP), and that the formation mechanism was related to generation of singlet oxygen (1O2). The objective of this study was to elucidate the 1O2-triggered photooxidation mechanism of fenthion in detail. Generation of 1O2 in the photooxidative reaction was directly detected by electron spin resonance (ESR) technique with 2,2,6,6-tetramethyl-4-piperidone (TMPD) as a selective 1O2 spin trapping agent which yields 2,2,6,6-tetramethyl- 4-piperidone-1-oxyl (TEMPONE). When fenthion and MMTP solutions were irradiated by UVA or UVB light, the TEMPONE signal was observed. However, no signal was detected after exposure of MMSP, dimethyl phosphorothioate or fenthion sulfoxide solutions to the UV light. The production of the signal in fenthion and MMTP solutions was more predominant under UVB irradiation than under UVA irradiation. When the signal was detected in these solutions, fenthion sulfoxide and MMSP were also formed in amounts proportional to the signal intensity. The TEMPONE signal intensity and the formation of these oxidative products were significantly inhibited by the addition of 1O2 scavenger, L-histidine or sodium azide to the reaction medium. The study provided the first direct evidence that 1O2 is generated during photolysis of fenthion and MMTP by UV irradiation. We also proposed its oxidation mechanism of fenthion and MMTP.