1 Department of Chemistry, Faculty of Science, Assiut University, Assiut, 71516, Egypt.
2 Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada.
3 Department of Chemistry and Center for Computational and Integrative Biology (CCIB), Rutgers University, Camden, New Jersey, 08102.
4 Centre for Research in Molecular Modeling (CERMM), Concordia University, Montréal, Québec, H4B 1R6, Canada.

Cation-p interactions play important roles in molecular recognition and in the stability and function of proteins. However, accurate description of the structure and energetics of cation-p interactions presents a challenge to both additive and polarizable force fields, which are rarely designed to account for the complexation of charged groups with aromatic moieties. We calibrate the Drude polarizable force field for complexes of alkali metal ions (Li⁺ , Na⁺ , K⁺ , Rb⁺ , Cs⁺ ), ammonium (NH₄⁺ ), tetramethylammonium (TMA⁺ ), and tetraethylammonium (TEA⁺ ) with aromatic amino acid side chain model compounds (benzene, toluene, 4-methylphenol, 3-methylindole) using high-level ab initio quantum chemical properties of these complexes. Molecular dynamics simulations reveal that cation-p complexes of the hard and tightly coordinated Li⁺ and Na⁺ ions are not stable in water but that larger ions form stable complexes, with binding free energies ranging between -0.8 and -2.9 kcal/mol. Like in gas phase, all complexes at equilibrium adopt an "en-face" complexation mode in water. The optimized Drude polarizable model provides an accurate description of the cation-p interactions involving small ions and proteins. © 2019 Wiley Periodicals, Inc.