Voltammetric evaluation on poly α-aspartic acid-zinc ion complex in the helix-coil transition pH region.

Helix-coil transitions of poly α-aspartic acid (PASP) were studied by dc polarography in the presence of Zn(2+) as a marker attached to the polymer. The diffusion current (i(d)) of Zn(2+) declined markedly in the pH range of 3.5-7.4 due to a formation of metal ion-PASP macromolecular complexes. The complex formation also reflects on an increase of the magnitude at ca. 222 nm of CD spectrum, suggesting that PASP forms the helix structure by coordination of Zn(2+) in the corresponding pH region. Helix content, determined by the decrease in i(d) of Zn(2+), corresponds favorably to that by CD measurements. In the lower acidic pH region, the coordination mode of Zn(2+) to PASP is different from that at neutral pH region. The decrease in i(d) of Zn(2+) is independent of the further formation of helix structure. Zn(2+) coordinates with sparsely dissociated carboxylate groups of the helical part of PASP, which bring about an aggregation of polypeptide strands. The diffusion current of the ion attached to the polymer, therefore, is a parameter sensitive to conformational changes of PASP from acidic through neutral pH region.