Hydrolytic degradation of azimsulfuron, a sulfonylurea herbicide Giovanna Boschin * , Alessandra D’Agostina, Cristina Antonioni, Daniela Locati, Anna Arnoldi Laboratory of Food Chemistry and Mass Spectrometry, Department of Agri-Food Molecular Sciences, University of Milan, Via Celoria 2, I-20133, Milan, Italy Received 4 October 2006; received in revised form 16 January 2007; accepted 17 January 2007 Available online 27 February 2007 Abstract The chemical degradation of the herbicide azimsulfuron was investigated in aqueous solutions at different pH values. The hydrolysis rate, determined by HPLC analyses, was pH dependent and was much faster in acidic than in neutral or weakly basic conditions. The metabolites formed at different pH values were compared with standards when possible or isolated and identified using ESI-LC-MS/MS, 1 H NMR and 13 C NMR. The two main products of hydrolysis in mild acidic solution were identified as 2-amino-4,6-dimethoxy-pyri- midine and 2-methyl-4-(2-methyl-2H-tetrazol-5-yl)-2H-pyrazole-3-sulfonamide, both produced as a result of the sulfonylurea bridge cleavage. Under basic conditions, a new product, a substituted 2-pyrimidinamine, deriving from the contraction of the sulfonylurea bridge, was isolated and completely characterized for the first time. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Azimsulfuron; Sulfonylurea herbicides; Chemical hydrolysis; LC-ESI-MS 1. Introduction Sulfonylurea herbicides are a group of pesticides widely used all over the world for controlling weeds in several crops, e.g., rice, wheat, maize, barley, sugar beet, and tomato. Their rapid and good acceptance was due to the high efficacy at low application rates (10–50 g/ha) and very low acute and chronic mammalian toxicities (the LD 50 in rats are generally >5000 mg/kg) (Beyer et al., 1988; Brown, 1990). Their mechanism is related to the inhibition of acetolactate synthase (ALS), a key enzyme involved in the biosynthesis of branched-chain amino acids in plants, but absent in animals and humans. The general structure of sulfonylureas is R 1 –SO 2 –NH–CO–NH–R 2 , where R 1 can either be an aliphatic, aromatic, or heterocyclic group connected by a sulfonylurea bridge to a R 2 group that can either be a triazine or a pyrimidine ring (Hay, 1990). In the environment, sulfonylurea herbicides are mainly decomposed by chemical hydrolysis (Sarmah and Sabadie, 2002) and microbial degradation (Beyer et al., 1988). Some recent reports suggest that photolysis may be an alternative pathway to hydrolysis (Pusino et al., 1999; Braschi et al., 2000; Vulliet et al., 2004). Chemical hydrolysis is pH and temperature dependent: in most cases the degradation is faster in acidic rather than in neutral or in weakly basic conditions, and at high tem- perature. The main effect of the hydrolysis is the cleavage of the sulfonylurea bridge to give the corresponding sulfon- amide and heterocyclic amine (Beyer et al., 1988; Sarmah and Sabadie, 2002). Nevertheless, alternative pathways have been observed: the O- and N-dealkylation of the groups on the triazine or on the aromatic ring, the triazine ring opening to form a triuret (Cambon and Bastide, 1996; Bray et al., 1997), the hydrolysis of esters (Sarmah and Sabadie, 2002). Another transformation pathway is the contraction of the bridge that occurs with some pyridyl- sulfonylureas, such as rimsulfuron (Schneiders et al., 1993), especially in alkaline conditions. 0045-6535/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2007.01.036 * Corresponding author. Tel.: +39 02 50316806; fax: +39 02 50316801. E-mail address: giovanna.boschin@unimi.it (G. Boschin). www.elsevier.com/locate/chemosphere Chemosphere 68 (2007) 1312–1317