A spectroscopic study of aqueous solutions of Ac-WGHGHGHGPGHGHGH-NH2 (HGP) indicates that copper(II) binds to the peptide to form a 2:1 Cu2+/HGP complex with four nitrogen atoms in the copper coordination environment. Electron paramagnetic resonance (EPR) and UV-visible data suggest copper binding through the peptide backbone and imidazole nitrogen donors. Circular dichroism data show that HGP is unbound below pH 5.5 and is copper-saturated at pH 9 and above. The apo form of the peptide is unstructured in solution and is organized into a turn conformation in the presence of 2 mol equiv of Cu2+ at basic pH. EPR measurements for 2:1 Cu2+/HGP solutions in the g = 2 region and within the pH range 7-11 exhibit axial spectra. A molecular-mechanics-minimized model of the Cu2+/HGP complex gave a Cu center dot center dot center dot Cu separation of 8 angstrom.