Thermodynamic study of the interaction of N-Carboxymethyl chitosan with divalent metal ions

Abstract

Potentiometry and microcalorimetry indicate that calcium is not chelated by N-carboxymethyl chitosan and that the order of affinity of N-carboxymethyl chitosan for the other divalent metal ions studied is Cu > Cd ⪢ Pb, Ni > Co. For these metals the following binding constants were obtained, respectively: 7 × 10 5; 4 × 10 5; 3 × 10 3 and 1 × 10 3. The number of water molecules liberated in the binding process by the solvated metal ions and by carboxylate were 25, 14 and 16 for Cu(II), Cd(II) and Pb(II), respectively. The binding was enthalpy driven and the negative ΔH values were thus attributed to the interaction of metal ions with nitrogen atoms. Cu(II) and Pb(II) introduced new bands in the CD and UV spectra, whilst the other metal ions did not: for Cu(II) they were a positive band at 237 nm and a negative one at 277 nm in the CD spectrum, and an absorption band at 246 nm in the UV spectrum; for Pb(II) they were a positive band at 229 nm and a negative one at 244 nm in the CD spectrum, and an absorption band at 231 nm in the UV spectrum.