Voltammetric Study of Poly Aspartic Acid-Cd2+ and/or Zn2+ Complex in the Helix-coil Transition pH Region

Helix-coil transition of poly aspartic acid (PASP) was investigated by dc polarography in the presence of Cd2+ and/or Zn2+ as a detecting marker which attached to the polymer. The diffusion currents (id) of Cd2+ and Zn2+ decreased sharply in the pH range of 3.5-7.0 due to the formation of the metal ions-PASP macromolecular complexes. The complex formation revealed on an increase in the magnitude at 222 nm of CD spectrum of the polymer, indicating that PASP forms helix structure by coordination of both ions in the corresponding pH region. The helix content of PASP, estimated by polarography is in good agreement with that by CD in neutral pH region. The degree of helicity induced by either of these two ions also can be determined independently by polarography. The size of the complex was calculated by Ilkovic’s equation and depends on strongly mixing ratio of PASP residue to cation marker. On the other hand in the lower acidic pH region, the coordination mode of these ions to PASP is different from that at neutral pH region. The decreases in id of the ions are independent of the further formation of helix structure. The metal ions coordinate with sparsely dissociated carboxylate groups of helical part of PASP, which bring about an aggregation of polypeptide strands. The diffusion current of the metal ion attached to the polymer, therefore, is a parameter sensitive to a conformational change of PASP from acidic through neutral pH region.