ENZYMATIC COUPLING OF THE HERBICIDE BENTAZON WITH HUMUS MONOMERS AND CHARACTERIZATION OF REACTION-PRODUCTS

To elucidate the binding mechanism of the herbicide bentazon (3-isopro pyl-1H-2,1,3-benzothiadiazine-4(3H)-one 2,2-dioxide) with humic monome rs in the presence of an oxidative enzyme, the reaction of bentazon wi th catechol, caffeic acid, protocatechuic acid, and syringaldehyde was investigated. In the presence of a laccase from the fungus Polyporus pinsitus, catechol was the most reactive humic monomer; bentazon with catechol in the presence of the laccase was completely transformed in 30 min at pH 4.0. The reactivity of bentazon decreased with increasing pH, but reactivity of bentazon decreased with increasing pH, but comp lete transformation of bentazon could be achieved even at high pH if t he concentration of catechol was increased. When bentazon was incubate d with humic acid (extract of leonardite) in the presence of the lacca se, a reaction of the two substrates was observed, as indicated by ben tazon disappearance. Two metabolites that result from the reaction of bentazon with catechol were isolated by TLC and HPLC and identified by mass spectrometry and NMR spectroscopy. A product with a molecular we ight of 348 was characterized by 1-D, 2-D H-1-, and C-13-NMR spectrosc opy and identified as a dimer composed of one catechol and one bentazo n molecule. A second reaction product with a molecular weight of 586 a ppeared to be a trimer, consisting of one catechol molecule and two be ntazon molecules. The analyses also showed that catechol was bound to the protonated nitrogen of the heterocyclic ring and not to a carbon o f the aromatic ring of bentazon; this incorporation results from nucle ophilic addition of the o-quinone to the nitrogen.