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 polycyclic aromatic hydrocarbons (PAHs) are generally not acutely toxic in conventional laboratory tests, many are extremely toxic in the presence of sunlight. In an effort to develop a model for predicting which PAHs may exhibit photo-induced toxicity, Newsted and Giesy (1987) reported 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-induced PAH toxicity. Photo-induced toxicity is the result of competing processes such as stability and light absorbance which interact to produce a complex, multilinear relationship between toxicity and chemical structure. We sought a molecular descriptor which could be computed from structure rather than measured empirically. We found that a measure of the energy stabilization of the toxicant in the form of the HOMO-LUMO (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 shows, for example, why phenanthrene and tetracene are not toxic while anthracene is highly phototoxic. Those PAHs exhibiting photo-induced toxicity were consistently within HOMO-LUMO gap ''window'' of 7.2 +/- 0.4 eV.