Stopped-flow spectroscopic studies on the ligand-exchange reaction of Cu-(glycine-peptide) complexes, Cu(H-iL), with cysteine.: Cu(II) transport and characterization of the intermediate ternary complexes Cu(H-1L)(Cys-);: L = glycylglycine (i=1), triglycine (i=2), tetraglycine (i=2 or 3), and pentaglycine (i=2 or 3)

Cu(II) complexes of glycine-peptides, abbreviated as Cu(H-iL): L = glycylglycine, triglycine, tetraglycine, and pentaglycine (i = 1-3), react with cysteine to form ternary complexes, Cu(H-1L)(Cys(-)), as first intermediates. The spectral parameters of the ternary complexes, which were similar irrespective of the peptides, were as follows: lambda(max) = 332 +/- 1 nm (epsilon = 4250 +/- 50 M-1 cm(-1)), g(parallel to) = 2.170 +/- 0.005, g(perpendicular to) = 2.00 +/- 0.05, and A(parallel to) = (2.05 +/- 0.01) x 10(-4) cm(-1), indicating that the ternary complexes have identical coordination structures: the Cu(II) coordinates with the peptides via the nitrogens from the terminal amino and the neighboring deprotonated-amide group, and with cysteine via the amino nitrogen and the thiolate sulfur. Based on the absorbance-time curves, the concentrations of each Cu(II) and Cu(I) species during the reaction were calculated. The species distribution Curve clearly visualized the pathway of the Cu(II) transport from Cu(H-iL) to Cu(Cys(-))(2) via Cu(H-1L)(Cys(-)). The rate of Cu(H-1L)(Cys(-)) formation, which was evaluated from the initial increase in the species distribution curve, depended on the coordination modes of the Cu(H-iL) complexes. Both the Cu(H-1L), involving N,N-,O-,O* donors, and the Cu(H-2L), involving N,N-,N-,O-* donors, rapidly formed Cu(H-1L)(Cys(-)) complexes, where the donors asterisked represent the fourth i ligand in the Cu(H-iL) complexes. The second-order rate constant, k(1+), was on the order of 10(6) M-1 s(-1), or bigger. The Cu(H-3L), involving N,N-,N-,N-* donors, reacted relatively slowly; k(1+) was on the order of 10(4) M-1 s(-1). Those results indicate that the affinity of the Cu(II) for the fourth donor in Cu(H-iL) determines the rate of metal-transport.