MECHANISTIC QUANTITATIVE STRUCTURE-ACTIVITY RELATIONSHIP MODEL FOR THE PHOTOINDUCED TOXICITY OF POLYCYCLIC AROMATIC-HYDROCARBONS .1. PHYSICAL MODEL-BASED ON CHEMICAL-KINETICS IN A 2-COMPARTMENT SYSTEM

A quantitative structure-activity relationship model for the photoindu ced toxicity of 16 polycyclic aromatic hydrocarbons (PAHs) to duckweed (Lemna gibba) in simulated solar radiation (SSR) was developed. Lemna gibba was chosen for this study because toxicity could be considered in two compartments: water column and leaf tissue. Modeling of photoin duced toxicity was described by photochemical reactions between PAHs a nd a hypothetical group of endogenous biomolecules (G) required for no rmal growth, with damage to G by PAHs and/or photomodified PAHs in SSR resulting in impaired growth. The reaction scheme includes photomodif ication of PAHs. uptake of PAHs into leaves, triplet-state formation o f intact PAHs, photosensitization reactions that damage G, and reactio ns between photomodified PAHs and G. The assumptions used were: the PA H photomodification rate is slower than uptake of chemicals into leave s, the PAH concentration in aqueous solution is nearly constant during a toxicity test, the fluence rate of actinic radiation is lower withi n leaves than in the aqueous phase, and the toxicity of intact PAHs in the dark is negligible. A series of differential equations describing the reaction kinetics of intact and photomodified PAHs with G was der ived. The resulting equation for PAH toxicity was a function of treatm ent period, initial PAH concentration, relative absorbance of SSR by e ach PAH, quantum yield for formation of triplet-state PAH, and rate of PAH photomodification. Data for growth in the presence of intact and photomodified PAHs were used to empirically solve for a photosensitiza tion constant (PSC) and a photomodification constant (PMC) for each of the 16 PAHs tested. For 9 PAHs the PMC dominates and for 7 PAHs the P SC dominates.