Computer Simulations on Molecular Recognition of Alkylamines by Ester Derivatives of p-tert-Butylcalix[6]arene

In this study we have performed computer simulations to investigate the complexation behaviors of the ester derivatives of p-tert-butylcalix[6]arene toward a variety of alkyl ammonium ions. Using the Finite Difference Thermodynamic Integration (FDTI) method in Discover we have calculated the absolute and relative Gibbs free energy of the different alkyl ammonium ions complexed with the alkyl p-tert-butylcalix[6]aryl acetates. Semi-empirical AM1 method was used for calculating enthalpy of formation. CVFF and MM+ forcefield for molecular mechanics calculations were adapted to express the complexation energies of the hosts. The local charges used to calculate electrostatic energy term in MM+ were estimated using AM1 semi-empirical quan-tum mechanics methods. Molecular dynamics were performed to simulate the behavior of these complexes. Most stable conformation is found to be 1,2,3-alternate for uncomplexed alkyl p-tert-butylcalix[6]aryl ester host, and cone-type conformation for host-guest complexes. Among the different orientations of alkyl ammo-nium cations complexed inside the cone-shape host, endo-cone complex is calculated as the most stable con-former. Ethyl p-tert-butylcalix[6]aryl ester (2) showed better complexation efficiency toward alkylamines than methyl p-tert-butylcalix[6]aryl ester (1) and this calculation result agrees well with the reported experimental data