Spectropolarimetric and spectrophotometric characterization of copper(II) complexes with oligopeptides in aqueous solution

The ligands investigated in this paper, L-alanyl-L-alanine methyl ester, L-alanyl-L-alanyl-L-alanyl-L-alanine and glycyl-l-leucyl-l-tyrosine, enable us to extend and improve a spectral map, developed in a previous work, concerning the intraligand and charge transfer to copper(IT) bands in ultraviolet circular dichroism spectroscopy, for copper(II) complexes. Visible spectrophotometry was also employed for L-alanyl-L-alanyl-L-alanyl-L-alanine and glycyl-l-leucyl-l-tyrosine. For each proton - copper(II) - ligand system the best chemical model was found by means of pi-I-metric measurements (t = 25 degrees C and ionic strength I = 0.1 mel dm(-3)). A stability comparison was made between CuLH-1 complex formed by L-alanyl-L-alanine methyl ester and L-alanyl-L-alanine in order to extimate the importance of the carboxylate group as regards copper(II) complex stability; we were also able to calculate a UV/CD spectrum for a monodentate copper(II) - amino group interaction (lambda(max) = 262 nm for charge transfer of NH2 --> copper(II) band in the Cut complex with the L-alanyl-L-alanine methyl ester). The UV/CD band located at 270 nm in CuLH-2 and CuLH-3 copper(II) - L-alanyl-L-alanyl-L-alanyl-L-alanine complexes allowed us to assume a carboxylate coordination for each species. By means of UV/CD spectra it was possible to exclude the involvement in copper(II) coordination of the phenolate oxygen of the L-tyrosine residue in copper(II) - glycyl-L-leucyl-L-tyrosine complexes.