Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/14/2003
Publication Date: 7/15/2003
Citation: Shalini Jayasundera, Walter F. Schmidt, Cathleen J. Hapeman, and Alba Torrents Examination of molecular interaction sites of acetanilides with organic matter surrogates using nuclear magnetic resonance techniques Submitted to Journal of Agricultural and Food Chemistry Accepted by Journal: 3/14/03 Published: 51(13):3829 - 3835. Interpretive Summary: The degree and extent of the availability of a pollutant in the aqueous phase versus bound to soil particles can influence its toxicity, resistance to degradation, and subsurface transport. The availability of sorbed organic pollutants is primarily a function of the interactions formed between the chemical and the soil matrix during sorption. Pollutants that form strong physical and chemical bonds would be less available than weakly held chemicals for biodegradation and remediation. In addition, the specific interaction sites of a pollutant and its orientation on the surface can influence its biodegradation properties. Experiments were conduct using Nuclear Magnetic Resonance (NMR) spectroscopy to measure the type and extent of interactions. The results of this study shows that the organic pollutants with polar functional groups can form specific interactions of varying degrees of strength in different organic matter fractions. Certain parameters that have routinely been used in models to predict the extent of sorption do not accurately describe specific interactions and thus the environmental fate of the pollutant. This information is also needed for input in the establishing guidelines for total maximum daily loads (TMDLs) for pollutants.
Technical Abstract: The dynamics of acetamide pesticide interactions with organic matter (OM) surrogates were examined using Nuclear Magnetic Resonance (NMR) Spectroscopy. The 13C and 1H spin-lattice relaxation times (T1) were measured to investigate and identify the preferential binding sites and interaction mechanisms within the substrate. T1 relaxation times were determined as a function of the colloidal and dissolved OM concentration. The decrease in T1, at specific sites of acetamide molecules, with OM concentration was a function of the properties of the OM. The non- aromatic sites were involved in forming specific interactions initially with the polar OM such as cellulose, chitin, and collagen while more non- specific interactions were formed with lignin. The initial rate of decrease in T1 of the individual sites involved in binding was higher in the more polar organic matter such as cellulose and chitin than in the less polar lignin. The NMR data suggests that acetamide pesticides can form weak hydrophobic interactions with some OM and more specific, stronger interactions with others. Changes in relaxation were correlated to calculated and experimentally determined binding coefficients (Koc). This study shows that relaxation measurements can identify the ligand sites which have a high affinity and are most actively involved in interactions, but that this dynamic information does not always predict the extent of binding of metolachlor and acetanilide to different organic matter surrogates.