CuII Ion Coordination to an Unprotected Pentadecapeptide Containing Two His Residues: Competition Between the Terminal Amino and the Side-Chain Imidazole Nitrogen Donors

The complex-formation equilibria of the pentadecapeptide TLEGTKKGHKLHLDY, the 114-128 protein fragment of SPARC, with the Cull ion have been investigated, at I = 0.1 mol.dm(-3) (KNO3) and T = 298.2 K. Protonation and complex-formation constants have been determined potentiometrically, and formation enthalpies measured by direct solution calorimetry; the complex-formation model and species stoichiometry have been carefully checked by means of UV/Vis absorption, CD and EPR spectroscopy. The structure hypo-theses of the complex species are also based on detailed study of the H-1 and C-13 NMR spectra of the ligand in both the absence and presence of copper ions. The involvement in complex-formation of both the terminal amino and imidazole groups has been suggested and their specific behaviour at different pH values elucidated.