COMPLEX-FORMATION BETWEEN COPPER(II) AND HIPPURYL-L-HISTIDYL-L-LEUCINE IN AQUEOUS-SOLUTION - A MODEL FOR COORDINATION PROPERTIES OF INTERNAL CHAINS OF PROTEINS

Complex formation between copper(II) and the peptide hippuryl-L-histidyl-L-leucine (H(2)L(+)) was studied in aqueous solution at t = 25 degrees C and I = 0.1 mol.dm(-3) by means of potentiometric, visible spectrophotometric, and circular dichroism measurements. In the acidic pH range, potentiometric and spectrophotometric data are consistent with the formation of the monodentate CuL(+) species. At a half neutralization of proton bound to N(3)-imidazole nitrogen, before forming more stable deprotonated species, precipitation occurs; redissolution of precipitate is obtained when further base is added. Potentiometric data are satisfactorily explained by assuming the formation of the species CuLH degrees(-1), CuLH(-2)(-), and CuLH(-3)(2-). As regards CuLH(-2)(-) complex, the whole of the data are strongly consistent with three nitrogen donor groups in the equatorial plane. The comparison Between CD spectra calculalculul CuLH(-2)(-) and CuLH(-3)(2-) species suggests the participation of the L-leucyl residue to coordination only in the latter complex. Since the N-terminal amino group is not available for coordination, this tripeptide might be a reliable model for coordination properties of internal chains of proteins.