Drude polarizable force field for cation–π interactions of alkali and quaternary ammonium ions with aromatic amino acid side chains

Cation–π 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–π 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–π 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–π interactions involving small ions and proteins. © 2019 Wiley Periodicals, Inc. Cation–π interactions are important for protein stability and for molecular recognition in general, yet are not always adequately described by molecular mechanics models. The Drude polarizable force field is optimized for the interaction of Li+, Na+, K+, Rb+, Cs+, NH4+, (CH3)4+, and (C2H5)4N+ with aromatic compounds representing the side chains of Phe, Tyr, and Trp and used to evaluate the stability and geometry of the complexes in water.