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 com plexation behaviors of the ester derivatives of p-tert-butylcalix[6]arene t oward a variety of alkyl ammonium ions. Using the Finite Difference Thermod ynamic Integration (FDTI) method in Discover we have calculated the absolut e and relative Gibbs free energy of the different alkyl ammonium ions compl exed with the alkyl p-tert-butylcalix[6]aryl acetates. Semi-empirical AMI m ethod was used for calculating enthalpy of formation. CVFF and MM+ forcefie ld for molecular mechanics calculations were adapted to express the complex ation energies of the hosts. The local charges used to calculate electrosta tic energy term in MM+ were estimated using AM1 semi-empirical quantum mech anics methods. Molecular dynamics were performed to simulate the behavior o f 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 co nformation for host-guest complexes. Among the different orientations of al kyl ammonium cations complexed inside the cone-shape host, endo-cone comple x is calculated as the most stable conformer. Ethyl p-tert-butylcalix[6]ary l ester (2) showed better complexation efficiency toward alkylamines than m ethyl p-tert-butylcalix[6]aryl ester (1) and this calculation result agrees well with the reported experimental data.