Partition of Pesticides of the Coumarin Family between Water and Amphiphilic Aggregates

Coumarin derivatives used as pesticides may accumulate in hydrophobic microstructures, such as biological membranes, aggregates of humic and fulvic acids, and micelles of pollutant surface active agents. In this work, we present a comparison of three photophysical methods for the determination of the membrane-water and micelle-water partition constants, K_p, of two pesticides (0,0-diethyl-0-(4- methylcoumarin-7-yl)thiophosphate (potasan) and 0,0-diethyl-0-(3-chloro-4-methylcoumarin-7-yl)- thiophosphate (coumaphos) and their parent compounds 4-methyl-7-methoxy coumarin and 3-chloro-4-methyl- 7-methoxy coumarin (MMC and CIMMC), respectively. Micelles of an anionic surfactant, sodium dodecyl sulfate (SDS); a nonionic surfactant, polyoxyethylene(10) lauryl ether (C₁₂E₁₀); and bilayers of a zwitterionic phospholipid 1,2-dimiristoyl-sn-glycerophosphocholine (DMPC) in aqueous suspensions buffered at pH 7.4 are used. The values of the partition coefficients obtained by the fluorescence anisotropy method proved to be more universal and reliable than those based upon the medium dependence of the fluorescence quantum yield and lifetime. The obtained partition constants are compared with the universal predictor, K_OW, and with the solubility in water. In this family of compounds, there is no correlation between the K_p and the hydrophobicity. The results are analyzed in terms of the molecular interactions and the solubilization site of the solutes in the aggregates, and the use of hydrophobicity in the analysis of bioconcentration of these compounds is discussed.