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(83-10) QSAR Studies on the Tropospheric Degradation of Organic Compounds . Gramatica, Paola1, Papa, Ester*,1, Battaini, Francesca1, Pilutti, Pamela1, Pozzoli, Luca1, 1 QSAR Research Unit-Dep. Structural and Functional Biology- University of Insubria- Via J.H. Dunant 3, Varese, Italy ABSTRACT- Persistence is particularly relevant in assessing the environmental risk of organic chemicals as it describes the presence in the environment of a substance that can lead to adverse effects. The reactions of chemicals with OH, NO3 radicals and ozone are the principal degradation processes in the troposphere, thus an upper limit of the atmospheric persistence of chemicals is assessed by determining their rate constants. Our goal is to develop statistical models according to QSAR/QSPR approach, making it possible to rapidly predict the atmospheric degradability of organic chemicals from a simple description of the molecular structure. The molecular descriptors used are 1D-constitutional, 2D-topological and 3D-descriptors (i.e. WHIM and GETAWAY descriptors, quantum mechanical and others). The best descriptor subsets for each modelling were selected by the Genetic Algorithm - Variable Subset Selection strategy (GA-VSS). Model calculations were performed by Ordinary Least Squares regression (OLS). In order to make more reliable response predictions for new compounds, the models were validated by using the leave-one-out, leave-more-out procedures, external validation and scrambling of the responses; the reliability of the predictions was always checked by the leverage approach. All the obtained models were satisfactory at different levels (Q2 % =83-94%), mainly due to different experimental data dishomogeneity. A PCA model, based on the three principal degradation processes, has been proposed to evaluate the overall atmospheric persistence of chemicals; the obtained PC1 score is proposed as an ATmospheric Persistence INdex (ATPIN) and also modelled by theoretical molecular descriptors. This model can be used as an evaluative model for the screening and ranking of chemicals according to their atmospheric persistence in a straightforward manner just starting from their chemical structure. Key words: oxidation rate constants, molecular descriptors, atmospheric persistence index, QSAR |
