Two decapeptides containing three His and two Pro–Gly β-turn inducer units (C-Asn, cyclic) and three His and a single Pro–Gly unit (O-Asn, open) have been synthesized. A detailed potentiometric study showed that while O-Asn binds up to 3 equiv. of Cu2+ ions, C-Asn only coordinates two before precipitation occurred. Nonetheless, at a 1 : 1 Cu2+/peptide ratio both peptides form a major [CuHL]3+ species and spectroscopic studies (UV-Vis, CD and EPR) revealed a very similar copper(II) complex where the metal ion is coordinated solely by the imidazole rings of the His residues adopting a square planar or square pyramidal geometry. The corrected stability constants of the protonated species (log KCuH(O-Asn) = 8.17 and log KCuH(C-Asn) = 9.11) indicate that the cyclic peptide binds Cu2+ with higher affinity and this value represents the highest value reported so far for this type of coordination. Additionally, the calculated value of the effective stability constant, Keff, showed that C-Asn has a higher affinity for Cu2+ at all pH values not only at a 1 : 1 ratio but even at a 2 : 1 ratio. The replacement of the asparagine residue by an aspartic amino acid increases the Cu2+ affinity of the aspartic counterparts, C-Asp and O-Asp, which at a 1 : 1 Cu2+/peptide ratio also form a major species, [CuHL]2+ in these cases, with Cu2+ coordinated to the three histidine residues and one aspartic residue. These data show how cyclization and coordination to the aspartic residue increase the binding strength and preclude the coordination of the amidenitrogen up to higher pH values, stabilizing therefore, the species where Cu2+ is solely coordinated by the side chain functionalities.
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