A QSAR ANALYSIS OF SUBSTITUENT EFFECTS ON THE PHOTOINDUCED ACUTE TOXICITY OF PAHS

Photoinduced toxicity of polycyclic aromatic hydrocarbons (PAHs) is a result of competing effects including stability and light absorbance o f the molecules as well as irradiation parameters. The energy differen ce between the Highest Occupied Molecular Orbital and the Lowest Unocc upied Molecular Orbital (HOMO-LUMO gap), which can be computed directl y from structure, was found to be the molecular descriptor that best d istinguishes phototoxic chemicals from non-phototoxic chemicals. Aroma tic chemicals that are phototoxic in sunlight have HOMO-LUMO gap energ ies that fall in the range of 6.7 to 7.5 eV. This study showed that th e effect of most substituents on the HOMO-LUMO gap was negligible, and that phototoxicity in an aromatic chemical is likely only if the pare nt aromatic structure is phototoxic. Exceptions included substituents that add to delocalization (nitro and alkenyl) which could shift some chemicals with a HOMO-LUMO gap just above 7.5 eV into the domain of ph otoinduced toxicity.