Structural activity relationship between Salmonella-mutagenicity and nitro-orientation of nitroazaphenanthrenes

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.

In silico prediction of the mutagenicity of nitroaromatic compounds using a novel two-QSAR approach

Certain drugs are nitroaromatic compounds, which are potentially toxic. As such, it is of practical importance to assess and predict their mutagenic potency in the process of drug discovery. A classical quantitative structure-activity relationship (QSAR) model was developed using the linear partial least square (PLS) scheme to understand the underline mutagenic mechanism and a non-classical QSAR model was derived using the machine learning-based hierarchical support vector regression (HSVR) to predict the mutagenicity of nitroaromatic compounds based on a series of mutagenicity data (TA98-S9). It was observed that HSVR performed better than PLS as manifested by the predictions of the samples in the training set, test set, and outlier set as well as various statistical validations. A mock test designated to mimic real challenges also confirmed the better performance of HSVR. Furthermore, HSVR exhibited superiority in predictivity, generalization capabilities, consistent performance, and robustness when compared with various published predictive models. PLS, conversely, revealed some mechanistically interpretable relationships between descriptors and mutagenicity. Thus, this two-QSAR approach using the predictive HSVR and interpretable PLS models in a synergistic fashion can be adopted to facilitate drug discovery and development by designing safer drug candidates with nitroaromatic moiety.

Electronic properties of environmental pollutants and their mutagenic activity: Nitro derivatives of azaphenanthrenes

The physico-chemical properties of nitroazaphenanthrene isomers: 4-nitro-9-azaphenanthrene (4-N-9-Aph), 5-nitro-9-azaphenanthrene (5-N-9-Aph), 6-nitro-4-azaphenanthrene (6-N-4-Aph), 8-nitro-1-azaphenanthrene (8-N-1-Aph), and 8-nitro-4-azaphenanthrene (8-N-4-Aph) have been investigated theoretically using Density Functional Theory (DFT) calculations. Equilibrium geometries, relative stability, ionization potentials, electron affinities, molecular electrostatic potentials, dipole moments, electric polarizabilities, and vibrational properties of these isomers are presented. Averaged O-N-C-C dihedral angle, dipole moment, polarizability, the summation of IR intensities (∑IIR) and the summation of Raman activities (∑ARaman) over all 3N-6 vibrational degrees of freedom are sensitive to the structure of isomers. A very good linear relationship between ∑ARaman values (R=1.00) and the Salmonella typhimurium strain TA98(-S9) mutagenic activity of the investigated nitroazaphenanthrene isomers (Tokiwa et al., 2003) reveals a very important role of inductive and dispersive forces on the mutagenic pathways of the investigated isomers.