Mechanisms of abiotic degradation and soil-water interactions of pesticides and other hydrophobic organic compounds part 9 -: Complexation of diazinon, an organophosphorus pesticide, with α-, β-, and γ-cyclodextrin -: NMR and computational studies

Complexation of the organophosphorus pesticide, diazinon, with alpha-, beta- and gamma- cyclodextrin has been investigated through NMR and computational methodologies. Binding constants (K-b) determined by H-1 and P-31 NMR follow the order gamma-CD > alpha-CD = beta-CD, in contrast with reported K-b data for other pesticides and thus indicative of steric encumbrance by the isopropyl group in diazinon being an important factor influencing binding constants. The interaction of diazinon with the CDs has also been investigated through computational studies via molecular dynamics - molecular mechanics (MD-MM2) and density functional theory (DFT), B3LYP/6-31G*. It is shown that the most favorable orientation in binding corresponds to the hydrophobic heterocyclic residue of diazinon being pulled deepest into the CD cavity, in agreement with the experimentally determined order of binding constants. Moreover, the computations show that it is only with gamma-CD that the heterocyclic residue of diazinon and the phosphoryl residue are both largely encrypted in the CD cavity, marking a clear differentiation with alpha-CD and beta-CD where the phosphoryl residue is located largely outside the cavity. Thus, the computational results are in essential agreement with the experimental binding constants where gamma-CD stands out with the highest K-b value. Our work could point to the potential usefulness of computational studies to be undertaken in tandem with experimental work in environmental situations such as soil remediation.