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Spectroscopic and equilibrium dialysis studies of poly(α‐L‐glutamic acid)–Cu(II) complexes in the pH range 4–7

The formation of complex between the Cu2+ ion and poly(α‐L‐glutamic acid) [poly(Glu)] in 150 mM NaCl solutions was studied by uv–visible absorption and equilibrium dialysis methods at the mixing ratios of Glu residues to Cu2+, R, of 32, 16, and 8 and in the pH range 4–7. The results showed that more than 90% of Cu2+ ions bind to the poly(Glu) at pH > 4.9, but the bound Cu(II) begins to dissociate with a decrease in pH. The absorption spectra of bound Cu(II) varied with pH and R in a complicated manner. Three different component spectra were disclosed from the analysis of the pH dependence of the bound spectra. We concluded that poly(Glu)–Cu(II) complexes fall into three classes in the pH range 4–7, with the proportions of these complexes varying with both pH and R. The three complexes predominate either in the helix or extended‐coil region, in the helix–coil transition region, or in the helix‐aggregate region. The stability constant and binding mode of each Cu(II)–Glu complex were estimated from the dialysis data. With these results, the possible structure of each complex is discussed.