KINETIC AND THERMODYNAMIC STUDIES OF THE COPPER(II) AND NICKEL(II) COMPLEXES OF GLYCYLGLYCYL-L-HISTIDINE

The protonation constants of the tripeptide glycylglycyl-L-histidine (L-) have been determined at 25-degrees-C and I = 0.1 mol dm-3 as log K 8.06, 6.82, and 2.80. Complexation with copper(II) can be represented by the series of equilibria Cu2+ + L- + H+ half arrow right over half arrow left [CuLH]2+; log beta111 = 12.40(1) Cu2+ + L- half arrow right over half arrow left [CuL]+; log beta110 = 7.64(1) Cu2+ + L- half arrow right over half arrow left [CuLH-1] + H+; log beta11-1 = 2.55(3) Cu2+ + L- half arrow right over half arrow left [CuLH-2]- + 2H+; log beta11-2 = - 1.55(1) in the case of nickel(II) only the species [NiLH]2+, [NiL]+, and [NiLH-2]- are of importance with log beta111 = 11.33(2); log beta110 = 4.74(6), and log beta11-2 = -6.93(1). The tripeptide acts as a quadridentate ligand to give complexes with copper and nickel with an amino group, two deprotonated amide groups and an imidazole pyridine nitrogen (Im-N3) as donors. At 1:1 ligand-to-metal ratios the purple copper(II) complex [CuLH-2]- is essentially 100% abundant above pH 7 and the planar yellow [NiLH-2]- above pH 8. The displacement of the tripeptide ligand from the nickel(II) complex by L-histidine has been studied kinetically over the pH range 7-8. There is a small solvolytic reaction and a reaction which is first-order in the hydrogen ion concentration. Under the experimental conditions employed, the reaction is essentially independent of the L-His concentration and displacement occurs by a proton-assisted nucleophilic pathway with rate-determining cleavage of the first nickel(II)-N(peptide) bond.