Semiempirical molecular orbital theory in carcinogenesis research

INSIGHT INTO MECHANISM OF FORMATION OF C8 ADDUCTS IN CARCINOGENIC REACTIONS OF ARYLNITRENIUM IONS WITH PURINE NUCLEOSIDES II: AB INITIO, DFT, AND ABEEM/MM-MD SIMULATION STUDIES OF CHEMICAL REACTIONS CONCERNING N9-SUBSTITUTED PURINE BASES WITH 4-BIPHENYLYLNITRENIUM ION

Theoretical insight into the mechanism of C8 adducts formation in a series of complicated carcinogenic reactions has been provided in a previous work. However, two important issues involved in this mechanism still need to be elucidated in detail. Hence, in this paper, we first present a new theoretical model to study the direct formation mechanism of C8 adduct. It is found that this model can well reflect the actual interactions in the real carcinogenic reactions. Thus, a better theoretical model to simulate other properties of these complicated reactions is found using ab initio and density functional theory (DFT) methods. Second, we simulate the formation process of C8 adduct in this new theoretical model using ABEEM/MM-MD method. According to the MD study, we approve that the higher aqueous-phase activation energy for transition state in this kind of reaction contributes to weaker interactions between central sites of reaction and water compared with those for reactants. This study once more supports the mechanism of formation of C8 adducts in the actual carcinogenic reactions where arylnitrenium ions directly attack at C8 positions of nucleoside bases in DNA.

Insight into mechanism of formation of c8 adducts in carcinogenic reactions of arylnitrenium ions with purine nucleosides

For the most important arylnitrenium ion-guanosine C8 adducts in the reactions involving arylamine-initiated carcinogenesis, a detailed mechanism of their formation still remains unclear. In this paper, we employ quantum chemistry methods to explore this issue. Our study indicates that formation of these C8 adducts proceeds directly by additions of arylnitrenium ions to C8 position of nucleoside bases in DNA. The good agreements of theoretical rate constants, pK(a) value, and NMR chemical shifts of C8 intermediate with experimental data support this theoretical finding. Excitingly, predictions of what adducts can be observed in reactions of arylnitrenium ions with guanine and hypoxanthine are in fair agreement with experimental observations. This study answers an important question, in carcinogenesis researches, of what is the mechanism for formation of C8 adducts.

Concerning the reaction between singlet nitrenium ions and water: a computational investigation on competitive reaction paths

The reaction between singlet nitrenium ions XNH(+) (X = F and Cl) and H(2)O has been investigated by high-level of theory ab initio calculations. The geometries of the involved intermediates, transition structures, and dissociation products have been optimized at the MP2(full)/6-31G(d) level of theory, and accurate total energies have been obtained using the Gaussian-3 (G3) procedure. The reaction commences by the exothermic formation of the F-NH-OH(2) (+) and Cl-NH-OH(2) (+) intermediates, which are in turn able to undergo two distinct low-energy reaction paths, namely, the isomerization to the N-protonated isomers of the hydroxylamines F-NH-OH or Cl-NH-OH, and the eventual extrusion of HF or HCl. The competitive or alternative occurrence of these two processes strictly depends on the nature of the substituent X. In the reaction between FNH(+) and H(2)O, the energy gained in the formation of the complex F-NH-OH(2) (+) from the association between FNH(+) and H(2)O, 52.1 kcal mol(-1), is by far larger than the activation barrier for the loss of HF from F-NH-OH(2) (+), computed as 24.9 kcal mol(-1). In addition, the F-NH-OH(2) (+) intermediate requires 33.0 kcal mol(-1) to overcome the barrier for the isomerization to F-NH(2)-OH(+). Therefore, the reaction between FNH(+) and H(2)O is expected to occur practically exclusively by HF elimination with formation of the HN-OH(+) ionic product. On the other hand, for the reaction between ClNH(+) and H(2)O, it is not possible to get a definitive conclusion on the competitive or alternative occurrence of the two reaction paths. In fact, the transition structure involved in the elimination of HCl from Cl-NH-OH(2) (+) is only 3.4 kcal mol(-1) lower in energy than the transition structure for the isomerization of Cl-NH-OH(2) (+) to Cl-NH(2)-OH(+). In addition, the absolute values of the energy barriers of these two processes, 24.2 and 27.6 kcal mol(-1), respectively, are comparable with the energy gained in the formation of the complex Cl-NH-OH(2) (+) from the association between ClNH(+) and H(2)O, 24.0 kcal mol(-).1 Therefore, the ClNH(+) cation is predicted to react with water significantly slower than FNH(+).

Quantum chemical characterization of the reactions of guanine with the phenylnitrenium ion

Density functional calculations at the B3LYP/6-311+G(2d,p)//pBP/DN level predict all cationic adducts combining guanine, at either its N2, O6, N7, or C8 positions, with phenylnitrenium ion, at either its N, 2, or 4 positions, to be lower in energy than the separated reactants. This relative stability of all adducts is preserved after addition of aqueous solvation free energies computed at the SM2 level, although some leveling of the adduct relative energies one to another is predicted. Cations having the lowest relative energies in solution correspond structurally to those adducts most commonly found when guanine reacts with larger, biologically relevant nitrenium ions in vitro and in vivo. One of these, the N-C8 adduct, is stabilized both by a rearomatized phenyl ring and by the operation of an anomeric effect not found in any of the others. On the basis of energetic analysis, direct conversion of an N-N7 cation to an N-C8 cation according to a previously proposed mechanism is unlikely; however, an alternative rearrangement converting a 2-N7 cation to an N-C8 cation via the intermediacy of a five-membered ring may be operative in nitrenium ions with aromatic frameworks better able than phenyl to stabilize endocyclic cationic charge.

Chemical and biological factors affecting mutagen potency

This review surveys the chemical and biological factors that are correlated with the mutagenic activity of the aromatic and heterocyclic amines. Particular attention is given to the predicted quantum chemical properties of the parent amines and their metabolites. A number of chemical properties have been found to correlate well with measured mutagenic potency, including log P, the energies of the frontier orbitals of the parent amines, and the thermodynamic stability of the nitrenium ion, possibly the ultimate DNA-binding species. These correlations are intriguing clues to the mutagenic activity of the aromatic amines; however, many factors still await final explanation, including the exact mechanisms of the metabolic enzymes, the identity(s) of the ultimate DNA-binding species, the reaction mechanism in the DNA-adduction, the role of sequence context in the covalent and non-covalent binding of the adducts, and the role of DNA repair.