Towards a unified Quantitative Structure-Activity Relationship describing toxicity of polar and nonpolar narcotic organic chemicals for the springtail Folsomia candida in different soils

The development of a unified Quantitative Structure-Activity Relationship (QSARs) describing the toxicity of polar and non-polar organic compounds in multiple soils was always hampered by two main factors. First, existing QSARs distinguish between narcosis I and II compounds, which exhibit different toxicities when compared with their logKow suggesting that polar compounds are more toxic. Second, to predict freely dissolved concentrations, the sorption to soil organic matter of compounds had to be estimated by using organic carbon-water partition coefficients (logKoc). To overcome these problems, in this study the compound interaction with the membrane, the liposome-water partition coefficient (logKlipw), and soil-specific logKoc were determined for two soils and a compound set of eight chlorobenzenes and four chloroanilines. Solid-phase microextration (SPME) was used to measure actual concentrations in the soil interstitial water at EC50 values for the effect on the reproduction of Folsomia candida. The QSAR corrected for the determined chemical descriptors still separated between soils, which was in agreement with the difference in the measured concentrations. A unified QSAR, reliably predicting toxicity of all compounds in both soils, was accomplished by the additional incorporation of the total water volume in the soil as an integral compartment of the soil matrix.