Halide anion capture and recognition by a tetrahedral tetraammonium receptor in water: A molecular dynamics investigation

We report molecular dynamics potential of mean force (PMF) simulations on the capture of halide anions X- (F-, Cl-, Br-) by a tetrahedral receptor L4+ built from four quaternary ammonium sites connected by six (CH2)(n) chains, leading to the formation of inclusion complexes X- C L4+. Simulations performed with a reaction field correction of the electrostatics and with PME-Ewald summation gave very similar energy profiles. In aqueous solution, an energy barrier of 12-17 kcal mol(-1) was found for the three anions, mainly due to their dehydration when they enter through the largest triangular face of L4+. In the inclusion complexes, the anion is anchored near the center of the cavity due to the electrostatic field of the four positively charged ammonium sites, shielded from the surrounding water molecules. It was predicted that L4+ is selective for Cl- over Br- which both form stable inclusion complexes, while the F- complex should dissociate. The comparison of PMFs in aqueous solution and in the gas phase and the energy component analysis demonstrates the importance of solvent on the nature of these complexes and on the complexation energy profiles. The Cl-/Br- selectivity obtained from the dissociation pathways in water was in good agreement with the results of free energy perturbation simulations based on the "alchemical route" of a thermodynamic cycle, and consistent with experimental observations.