A QM/MM MOLECULAR DYNAMICS STUDY OF THE POTENTIAL OF MEAN FORCE FOR THE ASSOCIATION OF K+ WITH DIMETHYL ETHER IN AQUEOUS SOLUTION

摘要

We present a hybrid Quantum Mechanical/Molecular Mechanical (QM/MM) molecular dynamics study of the free energy profile for the association of K+ with dimethyl ether (DME) in H2O. The QM/MM method employs the semiempirical AM1 method to describe DME, the MM parametrization of Dang for K+, and the SPC/E model for H2O. The electrostatic and van der Waals parameters for the QM/MM coupling terms, which describe the interaction of K+/DME and H2O/DME, have been previously described.(1) We calculate a potential of mean force and fmd a weak solvent separated ion-dipole pair (SSIDP) at 5.4 Angstrom separation and a contact ion-dipole (CIDP) free energy minimum at 2.7 Angstrom separation of the K+ with the oxygen of DME. The latter distance agrees well with the gas-phase optimized K+/DME structure. There is a 3-kcal/mol barrier separating the CIDP and SSIDP which is centered at 3.8 Angstrom K+/DME separation. The estimated Delta A(bind) for the CIDP is 0.9 +/- 0.1 kcal/mol which predicts that K+/DME is not a strongly bound complex in aqueous solution. The SSIDP has only a 0.2-kcal/mol barrier separating it from completely uncomplexed K+/DME, and it also does not represent a stable bound structure. K+ and its first solvation shell waters exert an opposite effect an repolarizing the wave function of DME when the latter is inside the 3.8-Angstrom barrier. However, outside this barrier, both the solvent and the cation act to enhance the induced dipole moment of DME. This study demonstrates the nonadditive interactions of a solvated cation with a simple monodentate organic ligand. These results are useful for interpreting K+ complexation by multidentate ligands, such as the crown ethers. [References: 35]