Theoretical Studies of Chemical Reactivity of Metabolically Activated Forms of Aromatic Amines toward DNA

The metabolism of aromatic and heteroaromatic amines (ArNH2) results in nitrenium ions (ArNH+) that modify nucleobases of DNA, primarily deoxyguanosine (dG), by forming dG-C8 adducts. The activated amine nitrogen in ArNH+ reacts with the C8 of dG, which gives rise to mutations in DNA. For the most mutagenic ArNH2, including the majority of known genotoxic carcinogens, the stability of ArNH+ is of intermediate magnitude. To understand the origin of this observation as well as the specificity of reactions of ArNH+ with guanines in DNA, we investigated the chemical reactivity of the metabolically activated forms of ArNH2, that is, ArNHOH and ArNHOAc, toward 9-methylguanine by DFT calculations. The chemical reactivity of these forms is determined by the rate constants of two consecutive reactions leading to cationic guanine intermediates. The formation of ArNH+ accelerates with resonance stabilization of ArNH+, whereas the formed ArNH+ reacts with guanine derivatives with the constant diffusion-limited rate until the reaction slows down when ArNH+ is about 20 kcal/mol more stable than PhNH+. At this point, ArNHOH and ArNHOAc show maximum reactivity. The lowest activation energy of the reaction of ArNH+ with 9-methylguanine corresponds to the charge-transfer π-stacked transition state (π-TS) that leads to the direct formation of the C8 intermediate. The predicted activation barriers of this reaction match the observed absolute rate constants for a number of ArNH+. We demonstrate that the mutagenic potency of ArNH2 correlates with the rate of formation and the chemical reactivity of the metabolically activated forms toward the C8 atom of dG. On the basis of geometric consideration of the π-TS complex made of genotoxic compounds with long aromatic systems, we propose that precovalent intercalation in DNA is not an essential step in the genotoxicity pathway of ArNH2. The mechanism-based reasoning suggests rational design strategies to avoid genotoxicity of ArNH2 primarily by preventing N-hydroxylation of ArNH2.