Theoretical study concerning the reactivity of imine derivatives of polycyclic aromatic hydrocarbons

Ultimate Carcinogenic Metabolites from Aromatic and Heterocyclic Aromatic Amines: A Computational Study in Relation to Their Mutagenic Potency

The formation of nitrenium ions from their precursors was examined by density functional theory (DFT) calculations in order to analyze the role of these electrophilic intermediates on the mutagenic activity of the parent amines. The relative reactivities for N-O bond dissociation from the N-hydroxy, N-acetoxy and N-sulfate derivatives of aniline were evaluated. Furthermore, the N-acetoxy esters from a set of 17 aromatic and heteroaromatic amines of diverse structure were considered, and correlations were sought between the calculated properties and the reported mutagenic potencies. The mutagenic activity was found to increase when a more negative charge developed at the exocyclic nitrogen of the nitrenium ion (qN) and with nitrenium ion stability. Different functional correlations were observed for the amine derivatives grouped according to their classification as aromatic (Ar), imidazo-carbocyclic (Imi-C), and imidazo-heterocyclic (Imi-H). The formation of N-acetyl nitrenium ions from aromatic amides was also considered and found to be less favorable than nitrenium ion generation from the corresponding amines.

Oxidized metabolites from benzo[a]pyrene, benzo[e]pyrene, and aza-benzo[a]pyrenes. A computational study of their carbocations formed by epoxide ring opening reactions

A DFT study aimed at understanding structure-reactivity relationships and fluorine substitution effects on carbocation stability in benzo[a]pyrene (BaP), benzo[e]pyrene (BeP), and aza-benzo[a]pyrene (aza-BaP) derivatives are reported. The relative energies of the resulting carbocations are examined and compared, taking into account the available biological activity data on these compounds. O-Protonation of the epoxides and diol epoxides leads to carbocation formation by barrierless processes. Charge delocalization modes in the resulting carbocations were deduced via NPA-derived changes in charges, and fluorine substitution effects were analyzed on the basis of charge density at different carbocation positions. Thus, fluorine substitution at sites bearing negative charge generated inductive destabilization of the carbocation, whereas a fluorine atom at a ring position which presented significant positive charge density produced a less pronounced destabilization due to fluorine p-pi back-bonding. Protonation reactions were also studied for the azaBaPs. In selected cases, the covalent adducts generated via bond formation with the exocyclic nitrogen of cytosine were computed and relative energies and geometries of the resulting adducts were examined.

Carbocations from oxidized metabolites of benzo[a]anthracene: a computational study of their methylated and fluorinated derivatives and guanine adducts

Structure-reactivity relationships and substituent effects on carbocation stability in benzo[a]anthracene (BA) derivatives have been studied computationally at the B3LYP/6-31G and MP2/6-31G levels. Bay-region carbocations are formed by O-protonation of the 1,2-epoxides in barrierless processes. This process is energetically more favored as compared to carbocation generation via zwitterion formation/O-protonation, via single electron oxidation to generate a radical cation, or via benzylic hydroxylation. Relative carbocation stabilities were determined in the gas phase and in water as solvent (PCM method). Charge delocalization mode in the BA carbocation framework was deduced from NPA-derived changes in charges, and substitution by methyl or fluorine was studied at different positions selected on basis of the carbocation charge density. A bay-region methyl group produces structural distortion with consequent deviation from planarity of the aromatic system, which destabilizes the epoxide, favoring ring opening. Whereas fluorine substitution at sites bearing significant positive charge leads to carbocation stabilization by fluorine p-pi back-bonding, a fluorine atom at a ring position which presented negative charge density leads to inductive destabilization. Methylated derivatives are less sensitive to substituent effects as compared to the fluorinated analogues. Although the solvent decreases the exothermicity of the epoxide ring-opening reactions due to greater stabilization of the reactants, it provokes no changes in relative reactivities. Relative energies in the resulting bay-region carbocations are examined taking into account the available biological activity data on these compounds. In selected cases, quenching of bay-region carbocations was investigated by analyzing relative energies (in the gas phase and in water) and geometries of their guanine adducts formed via covalent bond formation with the exocyclic amino group and with the N-7.

A Computational Study of Carbocations from Oxidized Metabolites of Dibenzo[a,h]acridine and Their Fluorinated and Methylated Derivatives

In a model computational study aimed at understanding structure-reactivity relationships and substituent effects on carbocation stability in aza-polycyclic aromatic hydrocarbons, the epoxides, diol epoxides, and the dihydrodiols of dibenzo[a,h]acridine (DB[a,h]ACR) were studied by density functional theory at the B3LYP/6-31G level. Bay region carbocations were formed via the O-protonated epoxides in barrierless processes. Relative carbocation stabilities were determined in the gas phase and in water as solvent (polarized continuum model method). Charge delocalization modes in the resulting carbocations were deduced by gauge-independent atomic orbitals (GIAO) NMR (based on Delta delta13C values) and via the natural population analysis (NPA)-derived changes in charges. Although the solvent decreases the exothermicity of the epoxide ring-opening reactions due to greater stabilization of the reactants, relative reactivity trends remain the same. Whereas fluorine substitution at ring positions bearing significant positive charge leads to carbocation stabilization by fluorine p-pi back-bonding, fluorine substitution at a ring position that presented negative charge density in the unsubstituted compound leads to inductive destabilization. Methylated derivatives exhibit less sensitivity to substituent effects as compared to the fluorinated analogues. A bay region methyl group produces structural distortion, and this deviation from planarity destabilizes the epoxide, favoring ring opening. Relative energies, changes in NPA charges, and GIAO NMR data in the resulting "benzylic" carbocations are examined collectively and discussed, taking into account the available biological activity data on these compounds.

Theoretical study of aza-polycyclic aromatic hydrocarbons (aza-PAHs), modelling carbocations from oxidized metabolites and their covalent adducts with representative nucleophiles

Protonation of the epoxides, diol epoxides, and dihydrodiols of benzo[h]quinoline (BhQ), benzo[f]quinoline (BfQ), phenanthrene (Phe), benzo[c]phenanthridine (BcPhen), and chrysene (Chry) were studied by DFT at the B3LYP/6-31G* level, and selected cases were calculated with the 6-31+G* diffuse-function augmented basis set for comparison purposes. Bay-region carbocations were formed from O-protonated epoxides via a barrierless processes. Relative carbocation stabilities were determined in the gas phase and with water as solvent (PCM method). The presence of a heteroatom changes the regioselectivity of epoxide ring opening, in some cases favoring non-bay-region carbocations. The epoxide ring opening mode is also greatly influenced by N-protonation. The dications resulting from initial N-protonation followed by epoxide protonation were also studied by DFT. Charge delocalization modes in the resulting mono- and dications were derived by GIAO-NMR (based on Delta delta13C values) and via the NPA-derived changes in charges. Relative aromaticity in different rings in the arenium ions was gauged by NICS. In representative cases, the covalent adducts (syn and anti) formed by reaction of the benzylic carbocations derived from diol epoxides and dihydrodiols with methoxide and methanethiolate anions were studied. Relative energies (in the gas phase and with water as solvent) and geometries of the adducts formed by quenching of the carbocations derived from BhQ and Phe-epoxides with guanine via the exocyclic amino group and via the N-7 were also investigated computationally. Although aqueous phase calculations change the energy for the addition reactions because of greater stabilization of the reactants, relative reactivity trends remain the same. The data are discussed, taking into account the available experimental results concerning the biological activity of these compounds.