Halide Anion Capture and Recognition by a Tetrahedral Tetrammonium Receptor in Water: A Molecular Dynamcis Investigation

摘要

We report molecular dynamics potential of mean force (PMF) simulations on the capture of halide anion X~- (F~-, Cl~-, Br~-) by a tetrahedral receptor L~(4+) built from four quaternary ammonium sites connected by six (CH_2)_n chains, leading to the formation of inclusion complexes X~- is contained in L~(4+). Simulations performed with a reaction field correction of the electrostatics and with PME-Ewald summation gave very similar energy profiles. In aqueous soluton, an energy barrier of 12-17 kcal mol~(-1) was found for the three anions, mainy due to their dehydration when they enter through the largest triangular face of L~(4+). 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 L~(4+) 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.