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 of the molecules as well as irradiation parameters. The energy difference between the ighest ccupied olecular rbital and the owest noccupied olecular rbital (HOMO-LUMO gap), which can be computed directly from structure, was found to be the molecular descriptor that best distinguishes phototoxic chemicals from non-phototoxic chemicals. Aromatic chemicals that are phototoxic in sunlight have HOMO-LUMO gap energies that fall in the range of 6.7 to 7.5 eV. This study showed that the effect of most substituents on the HOMO-LUMO gap was negligible, and that phototoxicity in an aromatic chemical is likely only if the parent 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 photoinduced toxicity.