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-histid yl-L-leucine (H(2)L(+)) was studied in aqueous solution at t = 25 degr ees C and I = 0.1 mol.dm(-3) by means of potentiometric, visible spect rophotometric, and circular dichroism measurements. In the acidic pH r ange, potentiometric and spectrophotometric data are consistent with t he formation of the monodentate CuL(+) species. At a half neutralizati on of proton bound to N(3)-imidazole nitrogen, before forming more sta ble deprotonated species, precipitation occurs; redissolution of preci pitate is obtained when further base is added. Potentiometric data are satisfactorily explained by assuming the formation of the species CuL H degrees(-1), CuLH(-2)(-), and CuLH(-3)(2-). As regards CuLH(-2)(-) c omplex, the whole of the data are strongly consistent with three nitro gen donor groups in the equatorial plane. The comparison Between CD sp ectra calculalculul CuLH(-2)(-) and CuLH(-3)(2-) species suggests the participation of the L-leucyl residue to coordination only in the latt er complex. Since the N-terminal amino group is not available for coor dination, this tripeptide might be a reliable model for coordination p roperties of internal chains of proteins.