Equilibrium and spectroscopic studies of the ternary system: L-histidine, copper(II), and the native sequence peptide representing the copper(II)-transport site of human serum albumin

Equilibrium and spectroscopic studies of Cu(II)-transfer of native sequence tripeptide, L-aspartyl-L-alanyl-L-histidine-N-methyl amide (AAHNMA), representing the Cu(II)-transport site of human serum albumin (HSA), and L-histidine (L-His) are reported. The equilibria in the ternary system, M–A–B (M = Cu(II), A = anionic form of AAHNMA, and B = anionic form of L-His) have been investigated by analytical potentiometry in I = 0.2 [(Na+,H+) (Cl−,OH−)] at 25 °C. The ternary system shows the presence of five mixed ligand complexes: MH2AB, MHAB, MAB, MH−1AB, and MH−2AB. The species distribution and their stability constants were evaluated by the mathematical analysis of the potentiometric data. The species were further confirmed by their individual spectra computed from the absorption measurements. At physiological pH, the equilibrium studies reveal the presence of 13% of MH−1AB (λmax = 530 nm.ε = 90 M−1 cm−1) and 3% MAB (λmax = 595 nm, ε = 97 M−1 cm−1). The combined results of equilibrium and spectroscopic studies indicate the mixed ligand complex CuH−1AB formed by deprotonation of peptide nitrogen as an important intermediate in the Cu(II)-transfer reaction. The stability constant of CuH−1AB is compared to those of other tripeptides which were designed to mimic the specific Cu(II)-transport site of human albumin.