Combined effects of an oxidative enzyme and dissolved humic substances on 13C-labelled 2,4-D herbicide as revealed by high-resolution 13C NMR spectroscopy.
Phenoxyalkanoic acids are a widely used class of herbicides. This work employed high-resolution 13C NMR to study the structural changes induced by humic substances and horseradish perodixase on 2,4-dichlorophenoxyacetic acid (2,4-D) 13C-labelled in the side chain. NMR spectra showed that humic substances chemically catalyze abiotic splitting of [13C]2,4-D into 2,4-dichlorophenol and [13C]acetic acid at pH 7 but not at pH 4.7. Peroxidase did not catalyze the oxidative degradation of [13C]2,4-D at any pH tested and inhibited the effect of humic substances. Catalytic degradation by humic substances was attributed to free-radical reactions enhanced by the stereochemical contribution of large conformational structures formed by heterogeneous humic molecules at neutral pH. Inhibition of 2,4-D degradation when humic substances were combined with peroxidase was explained by modification of both chemical and conformational humic structure due to peroxidase-promoted oxidative cross-coupling among humic molecules. Our findings show for the first time that the abiotic degradation of 2,4-D is catalyzed by dissolved humic substances at neutral pH.
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Dipartimento di Scienze Chimico Agrarie  Università di Napoli Federico II, Portici, Italy.
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Cozzolino
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Paci
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D015084:2,4-Dichlorophenoxyacetic Acid / Q000737:chemistry* / Q000378:metabolism; D002247:Carbon Isotopes / Q000378:metabolism; D006540:Herbicides / Q000737:chemistry* / Q000378:metabolism; D006735:Horseradish Peroxidase / Q000378:metabolism*; D006812:Humic Substances / Q000494:pharmacology*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*
D002247:Carbon Isotopes; D006540:Herbicides; D006812:Humic Substances; D015084:2,4-Dichlorophenoxyacetic Acid; D006735:Horseradish Peroxidase