Equilibrium Dialysis Study of the Interaction between Poly(α-L-glutamic acid) and Transition Metal Ions in the Helix–Coil Transition Region

Abstract

By the combined equilibrium dialysis-colorimetric method, the fractions, fb, of bound Ni(II) and Cd(II) in the poly(α-L-glutamic acid)–Ni(II) and–Cd(II) solutions were determined at 25°C over a pH range of 7—4.2. Both Ni2+ and Cd2+ ions were dissociated by about 25% at pH 6.5 (fb≍75%), where poly(Glu) is in the random–coil form, and by nearly 90% at pH 4.5, where poly(Glu) is almost a complete helix. The ORD data of poly(Glu)–Ni(II) and–Cd(II) complexes were essentially the same as those of poly(Glu), indicating that these transition metal ions do not induce the helical structure in poly(Glu). This is a striking contrast with the case of Cu2+. From the equilibrium dialysis data, it was concluded that each Ni2+ or Cd2+ ion binds with two adjacent glutamyl residues in poly(Glu)–Ni(II) and –Cd(II) complexes. The stability constants of these complexes were estimated to be of the order of 103.