QSARS FOR PHOTOINDUCED TOXICITY .1. ACUTE LETHALITY OF POLYCYCLIC AROMATIC-HYDROCARBONS TO DAPHNIA-MAGNA

Research with a variety of aquatic species has shown that while polycy clic aromatic hydrocarbons (PAHs) are generally not acutely toxic in c onventional laboratory tests, many are extremely toxic in the presence of sunlight. In an effort to develop a model for predicting which PAH s may exhibit photo-induced toxicity, Newsted and Giesy (1987) reporte d a parabolic relationship between the toxicity and the energy of the triplet state of a variety of PAHs. We have reexamined these data and propose a more mechanistic explanation for the prediction of photo-ind uced PAH toxicity. Photo-induced toxicity is the result of competing p rocesses such as stability and light absorbance which interact to prod uce a complex, multilinear relationship between toxicity and chemical structure. We sought a molecular descriptor which could be computed fr om structure rather than measured empirically. We found that a measure of the energy stabilization of the toxicant in the form of the HOMO-L UMO (Highest Occupied Molecular Orbital - Lowest Unoccupied Molecular Orbital) gap provided a useful index to explain the persistence, light absorption, and photo-induced toxicity of PAHs. The model clearly sho ws, for example, why phenanthrene and tetracene are not toxic while an thracene is highly phototoxic. Those PAHs exhibiting photo-induced tox icity were consistently within HOMO-LUMO gap ''window'' of 7.2 +/- 0.4 eV.