Applications of frontier molecular orbital energies in QSAR studies

Comparative toxic and genotoxic effects of chloroacetanilides, formamidines and their degradation products on Vibrio fischeri and Chironomus riparius

Toxic and genotoxic effects of alachlor, metolachlor, amitraz, chlordimeform, their respective environmentally stable degradation products 2,6-diethylaniline, 2-ethyl-4-methylaniline, 2,4-dimethylaniline, and two other related compounds, 3,4-dichloroaniline and aniline were compared. Acute toxicity tests with Chironomus riparius (96 h) and Vibrio fischeri (Microtox) and genotoxicity tests with a dark mutant of V. fischeri (Mutato) were carried out. Our results demonstrate that toxicity and genotoxicity of the pesticides are retained upon degradation to their alkyl-aniline metabolites. In the case of the herbicides alachlor and metolachlor, the toxicity to V. fischeri was enhanced upon degradation. Narcosis alone explains toxicity of the compounds to the midge, but not so for the bacteria suggesting a disparity in the selectivity of the test systems. All compounds showed direct genotoxicity in the Vibrio test. but amitraz and its metabolite were genotoxic at concentrations 10(3)-10(5) lower than all the other compounds. The observations indicate that stable aniline degradation products of the pesticides may contribute considerably to environmental risks of pesticides application and that genotoxic effects may arise upon degradation of pesticides.

QSAR treatment of electronic substituent effects using frontier orbital theory and topological parameters

A methodology for the estimation of Hammett substituent constants from computational-based descriptors utilizing quantitative structure activity/property relationships (QSAR/QSPR) formalism is presented. Electronic descriptors derived from quantum chemical calculations and molecular topology were used to generate computational-based analogues of empirical Hammett substituent constants from statistical analysis. Global quantum chemical reaction indices were drawn from frontier orbital theory and density functional theory and formulated from AM1-based calculations. A localized index based on the electrotopological state index was used to encode information on individual group properties. From a training set consisting of 150 meta and para-substituted benzoic acids, statistical analysis of computational-based descriptors as a function of empirical substituent constants yielded a five-parameter QSAR/QSPR model which generates computational-based constants exhibiting a strong correlation with empirical values (r2 = 0.958). Both internal (PRESS) and external (independent testing set of benzoic acids) validation procedures suggest that the electronic effects QSAR/QSPR model derived in this work from computational-based parameters is a statistically viable paradigm. Both predicted and empirical constants were used in Hammett-type validation analyses as functions of chemical, biological, and spectroscopic data for thirty structurally diverse meta and parasubstituted aromatic testing sets. Statistical measures of ensuing correlations were examined and compared, and the empirical and predicted results were of similar quality. Validation results reveal that a large number of computational-based substituent constants can be accurately estimated from semiempirical AM1 frontier orbital energies and electronic structure information obtained directly from substituted benzoic acids without the aid of empirical parametrization.