Combination of RISM and cheminformatics for efficient predictions of hydration free energy of polyfragment molecules : application to a set of organic pollutants

Here, we discuss a new method for predicting the hydration free energy (HFE) of organic pollutants and illustrate the efficiency of the method on a set of 220 chlorinated aromatic hydrocarbons. The new model is computationally inexpensive, with one HFE calculation taking less than a minute on a PC. The method is based on a combination of a molecular integral equations theory, one-dimensional reference interaction site model (1D RISM), with the cheminformatics approach. We correct HFEs obtained by the ID RISM with a set of empirical corrections. The corrections are associated with the partial molar volume and structural descriptors of the molecules. We show that the introduced corrections can significantly improve the quality of the ID RISM FIFE predictions obtained by the partial wave free energy expression [Ten-no, S. J. Chem. Phys. 2001, 115, 3724] and the Kovalenko-Hirata closure [Kovalenko, A.; Hirata, F. J. Chem. Phys. 1999, 110, 10095]. We also show that the quality of the model can be further improved by the reparametrization using QM-derived partial charges instead of the originally used OPLS-AA partial charges. The final model gives good results for polychlorinated benzenes (the mean and standard deviation of the error are 0.02 and 0.36 kcal/mol, correspondingly). At the same time, the model gives somewhat worse results for polychlorobiphenyls (PCBs) with a systematic bias of -0.72 kcal/mol but a small standard deviation equal to 0.55 kcal/mol. We note that the error remains the same for the whole set of PCBs, whereas errors of HFEs predicted with continuum solvation models (data were taken from Phillips, K L. et al. Environ. Sci. Technol. 2008, 42, 8412) increase significantly for higher chlorinated PCB congeners. In conclusion, we discuss potential future applications of the model and several avenues for its further improvement