A preliminary investigation of the additivity of pi-pi or pi+-pi stacking and T-shaped interactions between natural or damaged DNA nucleobases and histidine.

Previous computational studies have examined pi-pi and pi(+)-pi stacking and T-shaped interactions in nucleobase-amino acid dimers, yet it is important to investigate how additional amino acids affect these interactions since simultaneous contacts often appear in nature. Therefore, this paper investigates the geometries and binding strengths of amino acid-nucleobase-amino acid trimers, which are compared to the corresponding nucleobase-amino acid dimer interactions. We concentrate on systems containing the natural nucleobase adenine or its (cationic) damaged counterpart, 3-methyladenine, and the aromatic amino acid histidine, in both the neutral and protonated forms. This choice of molecules provides information about pi-pi and pi(+)-pi stacking and T-shaped interactions in asymmetric, biologically relevant systems. We determined that both stacked and T-shaped interactions, as well as both pi-pi and pi(+)-pi interactions, exhibit geometric additivity. To investigate the energetic additivity in our trimers, the synergy (E(syn)) and the additivity (E(add)) energy were examined. E(add) reveals that it is important to consider the interaction between the two amino acids when examining the additivity of nucleobase-amino acid interactions. Additionally, E(syn) and E(add) indicate that pi(+)-pi interactions are quite different from pi-pi interactions. The magnitude of E(add) is generally less than 2 kJ mol(-1), which suggests that these interactions are additive. However, the interaction energy analysis does not provide information about the individual interactions in the trimers. Therefore, the quantum theory of atoms in molecules (QTAIM) was implemented. We find inconsistent conclusions from our QTAIM analysis and interaction energy evaluation. However, the magnitudes of the differences between the dimer and trimer critical point properties are extremely small and therefore may not be able to yield conclusive descriptions of differences (if any) between the dimer and trimer interactions. We hypothesize that, due to the limited number of investigations of this type, it is currently unclear how QTAIM can improve our understanding of pi-pi and pi(+)-pi dimers and trimers. Therefore, future work must systematically alter the pi-pi or pi(+)-pi system to definitively determine how the geometry, symmetry, and system size alter the QTAIM analysis, which can then be used to understand biologically relevant complexes.