The Visible-UV Absorption and Circular Dichroism Studies of Poly(α-l-glutamic acid)–Copper(II) Complexes with Emphasis on the Extrinsic Cotton Effect

The visible-UV absorption and circular dichroism, CD, of poly(α-l-glutamic acid)–Cu(II) solutions have been measured at 25 °C in the pH range 7 to 3.5 and at the mixing ratio, R, (Glu residues-to-Cu2+) 32 to 4. Complex formation between poly (Glu) and Cu2+ was evident from the three absorption bands of bound Cu(II) around 700, 370, and 250 nm. The molar absorption coefficients, ε, varied with pH and R in a complicated but systematic manner. The pH dependence of ε, however, was not in parallel with the helix-coil transition of poly(Glu)–Cu(II) complexes. The extrinsic CD bands of bound Cu(II) have been detected in the three absorption band regions, and found to depend on both pH and R. The optical dissymmetry factor indicated that the 700 and 250 nm absorption bands of bound Cu(II) are associated with multiple CD bands of opposite signs. The intrinsic CD showed that the Cu(II)- and pH-induced helices of the poly(Glu)–Cu(II) complex are alike regarding the polymer backbone.