Polarographic Investigation of Biopolymers. II. Cu (II)-poly (α,l-Glutamic Acid) in the Helix-Coil Transition Region

Abstract

Abstract The pH-induced helix-coil transition of the complex of Cu(II)-poly(α,l-glutamic acid) was studied with polarographic method. The diffusion current was found to change very sharply in a narrow range of pH and this change depended on the amount of Cu(II) ions. The half-wave potential of the complex was also dependent on pH and mixing ratio. The effect of Cu(II) ions on the secondary structure of the polymer was discussed from the viewpoint that the helix-coil transition of PGA is responsible for the change of the mode of binding interaction between Cu(II) ions and PGA sites. Based on the data of diffusion currents versus mixing ratios, the helical fractions of the complex at pH 3.4 and 4.0 were evaluated to be 0.37 and 0.11, respectively. From a value of the diffusion coefficient of the complex at pH 5.0, the molecular size in a random-coil form was calculated to be approximately 70 Å in diameter. The coordination number of the complex in the coiled form depended on the mixing ratio; the number was 3.4 in excess of polymer and 0.2–0.3 for the mixing ratio between 3.2 and 13. These values were discussed in terms of the possible mechanism of coordination. Finally, the activation energy of diffusion was obtained for complexes in helical and random-coil forms.