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Title: Transformation kinetics and mechanism of the sulfonylurea herbicides pyrazosulfuron ethyl and halosulfuron methyl in aqueous solutions

item Yates, Scott
item Schneider, Sharon

Submitted to: Journal of Agriculture and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/16/2008
Publication Date: 7/24/2008
Citation: Zheng, W., Yates, S.R., Papiernik, S.K. 2008. Transformation kinetics and mechanism of the sulfonylurea herbicides pyrazosulfuron ethyl and halosulfuron methyl in aqueous solutions. Journal of Agriculture and Food Chemistry. 56:7367-7372.

Interpretive Summary: Sulfonylureas are a new class of herbicide extensively used to control a wide range of weeds in many agricultural crops. They have high herbicidal activity and are effective at very low application rates. Sulfonylureas also exhibit extremely low acute and chronic mammalian toxicities compared to most other herbicides, therefore their use is increasing steadily worldwide. Because of their high herbicidal activity, however, there is potential for crop injury from trace-level residues in soil. More information on the fate and transport of this class of herbicide is needed to effectively utilize these important crop protection chemicals, without harming subsequent crops. To address this need, the stability of the two sulfonylurea herbicides was studied in aqueous solution for several pHs and temperatures. Also, the hydrolysis mechanisms and transformation pathways were characterized. This information should help growers efficiently utilize these herbicides without injury to subsequent crops.

Technical Abstract: Pyrazosulfuron ethyl (PE) and halosulfuron methyl (HM) are two new highly active sulfonylurea herbicides which have been widely used for weed control in a variety of vegetables and other crops. These two herbicides have similar molecular structure, differing only in the substitutions on the pyrazole ring. Chemical hydrolysis is a primary process affecting the environmental fate of sulfonylurea pesticides. The hydrolytic transformation kinetics of PE and HM were investigated as a function of pH and temperature. For both herbicides, the hydrolysis rate was pH dependent and increased with increasing temperature. The hydrolysis of both sulfonylureas was much faster in acidic or basic media than under neutral conditions. Identification of hydrolytic products by liquid chromatography-mass spectrometry (LC-MS) suggested that both PE and HM were subject to cleavage and contraction of the sulfonylurea bridge. The hydrolysis rate of HM was significantly higher than that of PE in alkaline solutions, despite their structural similarity. A chlorine substitution on HM’s pyrazole ring makes HM more susceptible to bridge contraction than PE under basic conditions. The hydrolysis of HM and PE was relatively unaffected by the presence of cyclic oligosaccharides (cyclodextrins), indicating that natural OH-containing organic compounds occurring in aquatic environments may have little impact on the transformation of these sulfonylurea herbicides.