Effects of PAH isomerizations on mutagenicity of combustion products

Most of the mutagenicity of mixtures of polycyclic aromatic hydrocarbons (PAH) mixtures found in combustion exhaust gases is contributed by a relatively small number of the many PAH present. Since PAH mutagenicity is structure and hence isomer sensitive, changes in the distribution of isomers can change the mutagenicity of the mixture. Whether isomerization reactions in combustion play a significant role in determining the distributions of PAH isomers and the mutagenicity of product mixtures is assessed here for the following pairs of isomers: 1. (1) fluoranthene-pyrene2. (2) fluoranthene-acephenanthrylene3. (3) cyclopenta[cd]pyrene-benzo[ghi]fluoranthene4. (4) benzo[k]fluoranthene-benzo[a]pyrene Concentration ratios of the isomer pairs were measured in ethylene combustion with naphthalene injection using a plug flow reactor at equivalence ratios of 1.2 and 2.2 and temperatures of 1520, 1620, and 1705 K, and compared with equilibrium ratios based on properties computed from molecular mechanics and semiempirical quantum mechanical programs [MM3; MNDO, AM1, and PM3 in both restricted- and unrestricted-Hartree-Fock forms]. Bacterial mutagenicity was measured by a forward mutation assay using Salmonella in the presence of rat liver supernatant, and found to vary significantly among the above compounds. The measured concentration ratios for isomer pairs (2) and (3) are near the equilibrium values and becoming more so as temperature increases, but the measured ratios for isomer pairs (1) and (4) are far from the equilibrium values at all the temperatures. From kinetics estimations, the characteristic isomerization time for isomer pairs (2) and (3) at 1705 K and perhaps at 1620 K is less than the experimental residence times, while the only isomerization mechanisms envisioned for isomer pairs (1) and (4) would not be kinetically viable at these temperatures. These results indicate that isomerizations affecting mutagenicity are significant under these conditions for isomer pairs (2) and (3), which involve only compounds containing both five- and six-membered rings, but not for isomer pairs (1) and (4), each of which includes a compound containing only six- membered rings.