Additivity of cation-π interactions:: An ab initio computational study on π-cation-π sandwich complexes

The pi-cation-pi interaction between a cation or a cationic group and several aromatic residues, although rather prevalent in biological systems, has not been studied theoretically. The ab initio MP2 calculations were carried out on the systems composed of TMA with two aromatic rings, viz. benzene, pyrrole, or indole, to explore how a cation or a cationic group interacts simultaneously with two aromatic residues in proteins or nucleic acids. The calculated results on pi-TMA-pi complexes revealed additivities of both the geometries and the binding energies relative to cation-pi complexes. The preferred structure of such a complex can be constructed by superimposing the corresponding TMA-pi complexes via the cation. The binding energies of the pi-TMA-pi sandwiches are the sums of the two corresponding TMA-pi systems. The contribution of electron correlation to the overall binding energy is estimated to be at least 50%, with dispersion serving as the main component of the electron correlation interaction. Similar to geometrical and energetic additivity, the additivities in BSSE and DeltaZPE were also found. Therefore, our finding provides a convenient and effective way to construct pi-TMA-pi sandwiches and to estimate their binding energies. Morokuma decomposition analysis on the binding energy indicated that the electrostatic, charge transfer, and polarity interactions drive the binding of TMA with aromatics, whereas the exchange repulsion and high order coupling always obstruct the TMA approaching aromatics. Charge-transfer happens to some extent during the complexation of TMA with aromatics, and the transferred NPA atomic charges and charge-transfer energies are almost same in different complexes of TMA-pi or pi-TMA-pi. The interaction between the 2 aromatics in the sandwich pi-TMA-pi complexes is negligible because of their long interaction distances. All this information should be helpful in studying such interactions in biological systems.