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 a nd tertiary amino groups, peptide nitrogens were synthesised by a Michael-t ype 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 degreesC by potentiometry, calorimetry, viscometry and UV-Visi ble spectroscopy. Compared with the previously studied homologous PAAs, hav ing the rigid bis-diacylpiperazine ring inserted in the monomer unit, the b asic 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 repea ting units of the polymer. The presence of charges (positively ionised nitr ogens or negative carboxylate groups) produced almost unitary n values beca use of enhanced shielding due to the larger hydration shell surrounding the ionised groups. In contrast, neutralization of ampholytes to zwitterions r aised the n values owing to the release of water molecules, which increased the entropy contribution (DeltaS degrees) and compacted the macromolecule, Copper(II) ions were easily complexed by amino nitrogens in a stable five-m ember ring showing three complex species of different stoichiometry [CuL, C uH(-1)L and CuH(-2)L] (L means the repeating unit of the polymer) on the wh ole range of pH investigated (3-11). Tnermodynamic (log beta, -DeltaH degre es, DeltaS degrees) and UV-Visible data are in line with the proposed compl ex species that also involve deprotonation of one or two peptide nitrogens in different pH-ranges. Polyampholytes were able to bind copper(II) ions th rough peptide sites at low pHs (>3), while the basic PAAs needed to reach p H > 5. (C) 2000 Elsevier Science Ltd. All rights reserved.