Four faces of the interaction between ions and aromatic rings

Non-covalent interactions between ions and aromatic rings play an important role in the stabilization of macromolecular complexes; of particular interest are peptides and proteins containing aromatic side chains (Phe, Trp, and Tyr) interacting with negatively (Asp and Glu) and positively (Arg and Lys) charged amino acid residues. The structures of the ion-aromatic-ring complexes are the result of an interaction between the large quadrupole moment of the ring and the charge of the ion. Four attractive interaction types are proposed to be distinguished based on the position of the ion with respect to the plane of the ring: perpendicular cation-π (CP_⊥ ), co-planar cation-π (CP_∥ ), perpendicular anion-π (AP_⊥ ), and co-planar anion-π (AP_∥ ). To understand more than the basic features of these four interaction types, a systematic, high-level quantum chemical study is performed, using the X^-  + C₆ H₆ , M⁺  + C₆ H₆ , X^-  + C₆ F₆ , and M⁺  + C₆ F₆ model systems with X^-  = H^- , F^- , Cl^- , HCOO^- , CH₃ COO^- and M⁺  = H⁺ , Li⁺ , Na⁺ , NH4+, CH₃ NH3+, whereby C₆ H₆ and C₆ F₆ represent an electron-rich and an electron-deficient π system, respectively. Benchmark-quality interaction energies with small uncertainties, obtained via the so-called focal-point analysis (FPA) technique, are reported for the four interaction types. The computations reveal that the interactions lead to significant stabilization, and that the interaction energy order, given in kcal mol^-1 in parentheses, is CP_⊥ (23-37) > AP_⊥ (14-21) > CP_∥ (9-22) > AP_∥ (6-16). A natural bond orbital analysis performed leads to a deeper qualitative understanding of the four interaction types. To facilitate the future quantum chemical characterization of ion-aromatic-ring interactions in large biomolecules, the performance of three density functional theory methods, B3LYP, BHandHLYP, and M06-2X, is tested against the FPA benchmarks, with the result that the M06-2X functional performs best. © 2017 Wiley Periodicals, Inc.