The thermodynamics of basic and amphoteric poly(amido-amine)s containing peptide nitrogens as potential binding sites for metal ions. Part 1

Five basic and amphoteric poly(amido-amine)s containing, besides carboxyl and tertiary amino groups, peptide nitrogens were synthesised by a Michael-type polyaddition reaction of a primary (N-methylamine) or a bis-secondary (N,N′-dimethyl-ethylenediamine) amine to flexible bis-acrylamides. ation and copper(II)-complex formation studies were performed in 0.1 M NaCl at 25°C by potentiometry, calorimetry, viscometry and UV–Visible spectroscopy. Compared with the previously studied homologous PAAs, having the rigid bis-diacylpiperazine ring inserted in the monomer unit, the basic polymers showed lower basicity constants and increased polyelectrolyte behaviour. In fact, the n parameter of the modified Henderson–Hasselbalch equation was always higher than 1, suggesting reduced shielding among repeating units of the polymer. The presence of charges (positively ionised nitrogens or negative carboxylate groups) produced almost unitary n values because of enhanced shielding due to the larger hydration shell surrounding the ionised groups. In contrast, neutralization of ampholytes to zwitterions raised the n values owing to the release of water molecules, which increased the entropy contribution (ΔS°) and compacted the macromolecule. (II) ions were easily complexed by amino nitrogens in a stable five-member ring showing three complex species of different stoichiometry [CuL, CuH(−1)L and CuH(−2)L] (L means the repeating unit of the polymer) on the whole range of pH investigated (3–11). Thermodynamic (log β, −ΔH°, ΔS°) and UV–Visible data are in line with the proposed complex species that also involve deprotonation of one or two peptide nitrogens in different pH-ranges. Polyampholytes were able to bind copper(II) ions through peptide sites at low pHs (>3), while the basic PAAs needed to reach pH>5.