title: Probing the stereochemistry of successive sulfoxidation of the insecticide fenamiphos in soils.
abstract: Successive sulfoxidation is widely recognized as a general characteristic of the metabolism of chiral or prochiral thioethers, producing sulfoxides, and sulfones. However, information related to the stereochemistry of this process in soils is rare. In this study, the biotic transformation of the insecticide fenamiphos (a model thioether) was followed over two months in three soils, through separate incubations with fenamiphos parent, the sulfoxide intermediate (FSO), the sulfone intermediate (FSO2), and their respective stereoisomers. The results showed that the successive sulfoxidation involved oxidation of fenamiphos to FSO and subsequently to FSO2 as well as diastereomerization/enantiomerization of FSO, all of which were primarily biotic and stereoselective. The concomitant hydrolysis of fenamiphos, FSO, and FSO2 to phenols that occurred at lower rates was biotically favorable, but not stereoselective. The stereochemistry of this successive sulfoxidation transferred principally through two parallel systems, R(+)-fenamiphos → SRPR(+)-/SSPR(-)-FSO → R(+)-FSO2 and S(-)-fenamiphos → SRPS(+)-/SSPS(-)-FSO → S(-)-FSO2, between which unidirectional intersystem crossing occurred at FSO via isomeric conversions and created a system of S(-)-fenamiphos → SRPR(+)-/SSPR(-)-FSO → R(+)-FSO2. This pattern accounts for the enrichment of the intermediates SSPR(-)-/SSPS(-)-FSO and R(+)-FSO2 that are toxicologically close to the highly toxic S(-)-fenamiphos, associated with soil application of fenamiphos. Selective formation/depletion of these intermediate stereoisomers leads to dramatic variations in the ecotoxicological effects of the thioether insecticide.
authors.affiliation: Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education), School of Environmental Science and Technology, Dalian University of Technology , Dalian 116024, China.
authors.forename: Xiyun
authors.initials: X
authors.lastname: Cai
authors.forename: Weina
authors.initials: W
authors.lastname: Xiong
authors.forename: Tingting
authors.initials: T
authors.lastname: Xia
authors.forename: Jingwen
authors.initials: J
authors.lastname: Chen
mesh_terms: D006868:Hydrolysis; D007306:Insecticides / Q000737:chemistry / Q000378:metabolism; D009943:Organophosphorus Compounds / Q000737:chemistry* / Q000378:metabolism; D010084:Oxidation-Reduction; D010636:Phenols / Q000737:chemistry; D012451:Safrole / Q000031:analogs & derivatives / Q000737:chemistry; D012989:Soil Pollutants / Q000737:chemistry* / Q000378:metabolism; D013450:Sulfones / Q000737:chemistry
chemical_list: D007306:Insecticides; D009943:Organophosphorus Compounds; D010636:Phenols; D012989:Soil Pollutants; D013450:Sulfones; C002698:fenamiphos; D012451:Safrole; C005746:sulfoxide