In continuing the investigation of designing the specific Cu(II)-transport site of human serum albumin, the peptide derivative glycylglycyl-L-histidine-N-methyl amide was designed to approximate more closely to the native protein. This peptide derivative was synthesized in good yield. The equilibria involved in the binary system, Cu(II)–glycylglycyl-L-histidine-N-methyl amide, have been studied, as well as those in the ternary system, L-histidine–Cu(II)–glycylglycyl-L-histidine-N-methyl amide. This peptide derivative was found to bind Cu(II) exclusively as a 1:1 complex in the pH range 4 to 11, having the same ligand atoms as those for the carboxyl-terminal free peptide and human albumin. However, it was found that glycylglycyl-L-histidine-N-methyl amide bound Cu(II) more strongly than did glycylglycyl-L-histidine, the stability constants being log β1–21 = −0.479 and −1.99 respectively. In the ternary system, only 10% of the mixed complex was detected at pH 7, in comparison to 80% found in the case of the carboxyl-terminal free peptide. This finding agrees well with the increased stability of this peptide binary complex. These observations are also consistent with the results obtained from the equilibrium dialysis experiments. The Cu(II) – peptide amide complex has a dissociation constant of 2.07 × 10−17, indicating a higher binding strength of this peptide derivative for Cu(II) over the native albumin by a factor of 3.
The determination of the stoichiometry of the mixed complex of vanadium with hydrogen peroxide and with 4-(2-Pyridilazo) Resorcinol