Theoretical studies on the carcinogenicity of polycyclic aromatic hydrocarbons

The distinct molecular regions of a set of 28 polycyclic aromatic hydrocarbons (PAHs) showing varying degrees of carcinogenic activity (CA) have been analyzed on the basis of their calculated molecular electrostatic potential (MESP) at B3LYP/6-31+G(d,p) level of theory. The MESP, being a property directly related to electron density, clearly distinguishes the electron dense centers in the molecule into K, L, M, and newly defined N regions. Further, a quantitative structure activity relationship (QSAR) model of carcinogenicity is developed using the volume of MESP lobes at the named regions for a set of 17 carcinogenic molecules with experimentally known CA index. The QSAR equation suggested that all the geometrical regions are significant in determining the carcinogenic property of PAHs. The model clearly showed that K and M regions have activating carcinogenic effect whereas L and N regions have deactivating carcinogenic effect. The CA showed considerable enhancement when any three distinct regions are present in a PAH. On the other hand, all the PAH systems with only one type of region are inactive irrespective of whether the region is activating or deactivating. Similarly, molecules showing the presence of two types of regions are either inactive or weakly active. The essential features of both the "K, L region" and the "bay region" theories of carcinogenesis are well evident in the new QSAR model, as the former theory works on the basis of activating K and deactivating L regions whereas the latter theory is related with the activating M region.