Anion recognition in a model ion channel: effects of solvation on chloride binding to a porphyrin-metacyclophane

Semi-empirical methods (AM1, PM3 and PM5) are used to determine the enthalpies, entropies and free energies of a solvated Cl- ion. The Cl- ion is solvated with from 1 to 12 H2O or MeOH molecules and thermodynamics of the H2O or MeOH clusters are reported. The thermodynamic results of Cl- solvation are then compared with the thermodynamics obtained for the process of a solvated Cl- docking in a metallated endo-metacyclophane-porphyrin adduct. The metallated endo-metacyclophane-porphyrin adduct contains Zn(II), Fe(II) or Co(III). The metallated porphyrin is located at the base of the model ion channel and is solvated in the external axial position with either a H2O or MeOH molecule. The solvated Cl- ion is placed in the center of the metallated endo-metacyclophane-porphyrin adduct and allowed to migrate into a docked position above the metallo-porphyrin ring. Once a stable configuration is obtained, the thermodynamic properties are calculated using the PM3 and PM5 semi-empirical methods. The thermodynamics of the solvated Cl--metallated endo-metacyclophane-porphyrin complexes are compared with the thermodynamics of the Cl--solvent clusters. (C) 2004 Elsevier B.V. All rights reserved.