COPPER(II) COMPLEXES IMMOBILIZED ON A POLYMERIC MATRIX - THERMODYNAMICS, SPECTROSCOPY, AND MOLECULAR MODELING

Polymer-immobilized copper(II) complexes were prepared by using deoxylactit-1-yl (1), 2-substituted pentanedioic acid (2), and 2-substituted propanoic acid (3) derivatives of chitosan (4) as polymeric ligands. The thermodynamics of formation, based on both equilibrium dialysis and microcalorimetric experiments, suggest that the functional groups in each monomeric residue are an effective site of binding for one metal ion. The enthalpies of complex formation (25 degrees C) are -(35.7 +/- 2.4) x 10(2) and -(46.0 +/- 7.5) x 10(2) J/mol for Cu(II)-1 at pH 5.6 and 8.0, respectively, and (13.6 +/- 2.0) x 10(2) J/mol for Cu(II)-2 at pH 4.4, while the entropies of formation are around 65 J . mol(-1) deg(-1) in the former two cases and 96 J . mol(-1). deg(-1) in the latter one. ESR results (100 K) indicate that all these compounds basically have a tetragonal symmetry, but visible CD spectra suggest that the order of increasing departure from this geometry is Cu-4 approximate to Cu-3 < Cu-2 less than or equal to Cu-1, the arrangement of ligands around the central metal ion being more symmetric in 3 and 4 for the lack of sterically constraining side chains. Molecular modeling of Cu(II)-1 and Cu(II)-2 active sites was performed by mimicking different pH conditions and using both a series of pairwise additive energy terms and a new distance-dependent dielectric function to tackle electrostatic effects that are believed to dominate metal-macromolecule interactions in solution. Hypothetical models of the metal complexes are presented and supported by experimental results, as far as the Limited data allow. Implications of steric effects on the computed structures are also discussed.