Studies on transition-metal–peptide complexes. Part 6. Influence of side-chain donor group on the equilibrium and thermodynamics of binary and ternary copper(II)–dipeptide complexes

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Abstract
Copper(II) binary and ternary complexes of dipeptides containing amide carbonyl, alcoholic hydroxy, and carboxy groups in their side-chain have been studied. The dipeptides (A ligands) were glycyl-L-serine (GlySer), glycyl-L-asparagine (GlyAsn), glycyl-L-aspartic acid (GlyAsp), and glycyl-L-glutamic acid (GlyGlu). In the case of copper(II)–dipeptide ternary complexes, L/D-α-alanine (α-Ala), β-alanine (β-Ala), L/D-aspartic acid (Asp), L/D-ornithine (Orn), and 2,2′-bipyridyl (bipy) were used as B ligands. pH-Titrimetry and calorimetry were applied to determine the stability constants and the enthalpy and entropy changes of the species formed in the systems studied. It was assumed that the carboxylate group in the GlyAsp side-chain co-ordinates in the axial position to copper(II). Above pH 10, deprotonation of the amide carbonyl in GlyAsn takes place, accompanied by its bonding in the square plane to copper(II). A [CuAB] type complex is formed in high concentration, mainly with bipy as B ligand. However, [CuABH–1] type deprotonated species are also formed in the case of copper(II)–dipeptide–amino-acid systems. Stereoselectivity in this complex was observed only with Asp and Orn. This effect may presumably be attributed to electrostatic interactions. Thermodynamic data support the assumption of bipy occupying two equatorial positions in [CuAB] species, and an equatorial and an axial position in [CuABH–1].

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