Stability of Fused Rings in Metal Chelates. X. Structures and Stability Constants of the Copper (II) Complexes of Tripeptides Composed of Glycine and/or β-Alanine

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Abstract
On the basis of comparative studies of the equilibria involving copper(II) and triglycine (G·G·G), glycylglycyl-β-alanine (G·G·β-A), glycyl-β-alanylglycine (G·β-A·G), or β-alanylglycylglycine (β-A·G·G), the structures of the complexes formed at various pH values and the structure-stability relationship between the fusedring chelates have been discussed in reference to the equilibrium constants determined by potentiometric titration at 25°(μ=0.1 (KNO3)) and the properties of the copper(II) chelates isolated. For each tripeptide, the stability constant K1 for the 1: 1 complex and the constants for the deprotonation reactions Kc1 and Kc2 were calculated by the method of non-linear least-squares. For G·G·G, G·G·β–A, and G·β–A·G, the equilibrium indicating the formation of a protonated complex was detected at pH 3.5–4.5. The relatively small difference in the logK1 values (5.25–5.60) and the logKc1 values (5.23–5.36) and the larger difference in the −logKc2 values (5.54–6.73) have been interpreted in terms of the steric requirements around copper(II) in the fused-ring chelates. Relative stabilities of the fused-ring systems in the deprotonated chelates have been inferred from the K1Kc1Kc2 values to be in the order 5-6-5(G.β-A·G-Cu(II))\simeq6-5-5(β-A·G·G-Cu(II))>5-5-6(G·G·β-A-Cu(II))>5-5-5(G·G·G–Cu(II)).

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