| Abstract
| - We report results of simulations of association between p-sulfonatocalix[4]arene and inorganic (rare-earthmetal cations) and organic cations (a series of quaternary ammonium cations) in aqueous solution. Our maingoals were to obtain structural features for these complexes in aqueous solution and to study the role of wateron the cation binding by the p-sulfonatocalix[4]arene. The MD calculations show that the organic and inorganiccations bind in very different modes. The lanthanide cations are located outside the cavity of the calixareneforming an outer-sphere complex, while for the organic cations, the quaternary ammonium cation is includedinto the cavity of the calixarene. In fact, the Me4N+ cation penetrates deeply into the cavity. As concerns theEt4N+ cation, one of the alkyl chain is close to the center of mass of the calixarene, whereas two other alkylchains are located near the border of the cavity. In the case of Pr4N+ cation, only one propyl chain is insidethe cavity, while the others are outside the cavity of the calixarene. Additional simulations have been carriedout using two different free energy perturbation formalisms to calculate differences in Gibbs free energies ofcomplexation of lanthanide complexes. These simulations are consistent with the thermodynamic propertiesof association obtained recently by microcalorimetry, and the calculated differences in Gibbs free energies ofcomplexation are in excellent agreement with the experimental ones.
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