QSAR EVALUATION OF ALPHA-TERTHIENYL PHOTOTOXICITY

The concept that phototoxic chemicals can be identified in chemical ri sk assessments by computing the energy difference between highest occu pied and lowest unoccupied molecular orbitals (HOMO-LUMO gap) was eval uated using two new sets of phototoxicity data from the recent literat ure. The original model, developed from data with unsubstituted PAH to xicity to Daphnia magna, showed that phototoxicity was observed when t he HOMO-LUMO gap varied between 6.7 and 7.5 eV. All substituted alpha- terthienyls that were phototoxic to mosquito larvae and to brine shrim p had HOMO-LUMO gap energies within the 7.1 +/- 0.4 eV ''phototoxicity window''. The alpha-terthienyls within this range that did not exhibi t phototoxicity contained carboxyl or other polar substituents,which l ikely prevented bioaccumulation in the organisms. Polyamino and polyni tro derivatives of toluene in munitions wastes were reported to be pho totoxic to sea urchins even though the HOMO-LUMO gap energies exceeded 8.0 eV. Because the same toluenes were not phototoxic to D. magna nor to Escherichia coli, we suggest that the developmental effects observ ed in the sea urchins were caused by electrophilic species from metabo lic activation rather than the production of oxygen radicals from phot o induced excited states of the molecules.