The position of the nitro group affects the mutagenicity of nitroarenes

The ease with which a nitrated polyaromatic hydrocarbon (NO₂PAH) is activated by reductive metabolism is an important factor in determining mutagenicity. However, the mutagenicity of 3-nitrobenzo[a]pyrene (3-NO₂BaP) is stronger than that of 1-NO₂BaP despite similar reduction properties, and the more potent mutagenicity of 3,6-diNO₂BaP relative to that of 1,6-diNO₂BaP cannot be explained by relative reducibility. Here, we investigated structural factors leading to the mutagenicity of these compounds by synthesizing 1- and 3-NO₂BaP derivatives with C6-position substituents that affect reduction properties and testing the mutagenicity of the compounds and their derivatives against Salmonella typhimurium TA98 and TA98NR. The LUMO and LUMO+1 energies of 6-substituted 3-NO₂BaPs were found to correlate with mutagenicity, but such correlations were much weaker with 6-substituted 1-NO₂BaPs, indicating that the mutagenicity of 3-NO₂BaPs is influenced by the ease of reductive metabolic activation. In silico structural analyses demonstrated that the distances between the nitrogen of the N-acetoxyamino group in reductive metabolites and a DNA alkylation target were longer for 1-NO₂BaPs than for 3-NO₂BaPs. Therefore, the active metabolites of 6-substituted 3-NO₂BaPs intercalate with DNA at a distance where they can readily form adducts with guanine. Conversely, the unfavorable position of intercalated active metabolites of 1-NO₂BaPs relative to guanine leads to difficult adduct formation despite the facile formation of the active metabolite due to a low LUMO energy. Therefore, the chemical reducibility of the nitro group and, more importantly, the ease of adduct formation between an active metabolite and DNA are essential for the prediction of the mutagenicity of NO₂PAHs.