| Abstract
| - The π−cation−π interaction between a cation or a cationic group and several aromatic residues, althoughrather prevalent in biological systems, has not been studied theoretically. The ab initio MP2 calculationswere 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 proteinsor nucleic acids. The calculated results on π−TMA−π complexes revealed additivities of both the geometriesand the binding energies relative to cation−π complexes. The preferred structure of such a complex can beconstructed by superimposing the corresponding TMA−π complexes via the cation. The binding energies ofthe π−TMA−π sandwiches are the sums of the two corresponding TMA−π systems. The contribution ofelectron correlation to the overall binding energy is estimated to be at least 50%, with dispersion serving asthe main component of the electron correlation interaction. Similar to geometrical and energetic additivity,the additivities in BSSE and ΔZPE were also found. Therefore, our finding provides a convenient and effectiveway to construct π−TMA−π sandwiches and to estimate their binding energies. Morokuma decompositionanalysis on the binding energy indicated that the electrostatic, charge transfer, and polarity interactions drivethe binding of TMA with aromatics, whereas the exchange repulsion and high order coupling always obstructthe TMA approaching aromatics. Charge-transfer happens to some extent during the complexation of TMAwith aromatics, and the transferred NPA atomic charges and charge-transfer energies are almost same indifferent complexes of TMA−π or π−TMA−π. The interaction between the 2 aromatics in the sandwichπ−TMA−π complexes is negligible because of their long interaction distances. All this information shouldbe helpful in studying such interactions in biological systems.
|
