Association complexes between Fe (III) or Cu (II) ions and chitosan denvatives: a thermodynamic and spectroscopic investigation

Association complexes between iron(III) or copper(II) ions and deoxylactit-1-yl (1), 2-substituted pentanedioic acid (2), or 2-substituted propanoic acid (3) derivatives of chitosan were prepared and characterized by thermodynamic and spectroscopic measurements. Complex solutions did not show any precipitate or even opalescence, owing to the hydrolysis of free metal ions, within a wide range of [Me(n+)]/[P] molar ratio, even al a pH as high as 10.5 (Me(n+) = Fe3+ or Cu2+). Both equilibrium dialysis and Job plot experiments suggest that the functional groups in each monomeric residue are an effective site of binding for one metal ion. Reduction potentials, as obtained by cyclic voltammetric measurements, indicate that (i) coordination of the aforementioned polymeric ligands to Cu2+ ions stabilizes the oxidized species, and (ii) iron complexes have an oxidation power definitely higher than that of the corresponding copper compounds. Electron paramagnetic resonances (100 or 6 K) and Mossbauer (r.t.) spectra suggest that the order of increasing distortion from idealized geometry is Me(n+)-chitosan almost-equal-to Me(n+) -(3) < Me(n+) -(2) less-than-or-equal-to Me(n+) -(1). These results are discussed briefly in the light of a few general considerations concerning the structural features of association complexes between macromolecules and transition metal ions.