The thermodynamics of basic 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. Protonation 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 modied 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 and compacted
the macromolecule. Copper(II) ions were easily complexed by amino nitrogens in a stable five-member ring showing three complex species of different stoichiometry on the whole range of pH investigated (3-11). Thermodynamic 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.