Binding of 13C-labelled 2-aminobenzothiazoles to humic acid as derived from 13C NMR spectroscopy

The metabolites 2-aminobenzothiazole (ABT) and 2-(methylamino)benzothiazole (MABT) have been reported to represent a major fraction of the soil-bound residue of the urea herbicide methabenzthiazuron, which is preferentially bound to the humic fraction of soil. To better understand the fate of this pesticide in the environment, the binding behaviour of ABT and MABT towards humic acids (HA) was studied by the use of 13C-labelled metabolites in combination with solid-state 13C NMR spectroscopy. ABT and MABT 13C-labelled at the C-2 position were synthesized and subjected to model reactions with humic acid extracted from Rhenish brown coal. The uptake of both metabolites by humic acid was higher in an oxygen atmosphere than in an argon atmosphere, indicating the importance of oxidative reactions in the binding process. A significantly lower molar amount of bound MABT-2-13C compared to that of ABT-2-13C suggests a considerable steric hindrance to the interaction by the methyl group of MABT. The 13C CP/MAS NMR spectra of the humic acid-metabolite adducts obtained after 30 days exhibited C-2 signals of bound ABT-2-13C or MABT-2-13C at about 170 and 157 ppm. Subtracting the 13C CP/MAS NMR spectra of the corresponding metabolite-free humic acid samples from these spectra, difference spectra (adduct-HA) were calculated and compared to the 13C NMR spectra of the pure 13C-labelled metabolites. The chemical shift of the main C-2 peak ( 170 ppm) in the difference spectra is almost unaffected by the reaction, indicating that ABT and MABT are partly bound by non-covalent interactions such as hydrogen bonding, charge-transfer interactions or trapping of metabolites into the macromolecular matrix of the humic acid. The small peak at about 157 ppm is attributed to covalent bonding and possibly arises from amide bond formation which was found to shift the C-2 signal in the 13C NMR spectra of amide model compounds of 2-aminobenzothiazoles to 155–161 ppm.