Computational Study of the Alkylation Reaction of the Nitrogen Mustard Mechlorethamine Using NBO Model and the QTAIM Theory

Substances known as nitrogen mustards turn into aziridinium ion through the intramolecular cyclization SN1. This ion reacts with the DNA preferably at the N7 position of the guanine, and because of this, it is an important antineoplastic agent. Based on this, the objective of this study is to quantify the interaction between the nitrogen mustard mechlorethamine and the guanine, using the NBO analysis and the QTAIM theory. The results of the NBO analysis showed that when the triangular cycle C4-N1-C5 is formed, there is some resonance among these atoms. This analysis also showed that the electronic transition at the sigma antibondingorbital σ* N1-C4 presents higher perturbation energy of second order, indicating that this bond is broken at the nucleophilic attack of the N7 nitrogen of guanine. The analysis that refers to the electron density obtained by the QTAIM theory indicates that the guanine proximity enables an electron density polarization of the BCPs aziridinium ion of mechlorethamine making that the frontal part of the ion becomes electron deficient. Finally, the relative results to the Laplacian of the electron density obtained by the QTAIM theory showed that the guanine approximation increases the “hole” factor at the C4, proving that the nucleophilic attack based on the “lump-hole” concept causes the region of that atom is the site of alkylation reaction.