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_(-1)L)(Cys~-);L=Glycylglycine (i=1),Triglycine

摘要

Cu(II)complexes of glycine-peptides,abbreviated as Cu(H_(-i)L):L=glycylglycine,triglycine,tetraglycine and pentaglycine (i=1-3),react with cysteine to form ternary complexes,Cu_(H_(-1)L)(Cys~-),as first intermediates.The spectral parameters of the ternary complexes,which were simlar irrespective of the peptides,were as follows:lambda_(max)=332+-1 nm (omega=4250+-50 M~(-1)cm~(-1)),g_(||)=2.170+-0.005,g_(perpendicular)=2.00+-0.05,and A_(||)=(2.05+-0.01)X10~(-4)cm~(-1),indicating that the ternary complexes have identical coordiantion structures:the Cu(II)coordinates with the peptides via the nitrogens from the terminal amino adn the neighboring deprotonated-amide group,and with cysteine via the amino nitrogen adn 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_(-i)L)to Cu(Cys~-)_2 via Cu_(H_(-1)L)(Cys~-).The rate of Cu_(H_(-1)L)(Cys~-)formation,whcih was evaluated from the intial increase in the species distribution curve,depended on the coordination modes of the Cu(H_(-i)L)complexes.Both the Cu_(H_(-1)L),involving N,N~-,O~-,O donors,and the Cu_(H_(-2)L),involving N,N~-,N~-,O~- donors,rapidly formed Cu_(H_(-1)L)(Cys~-)complexes,where the donors asterisked represent the fourth ligand in the Cu(H_(-i)L)complexes.The second-order rate constant,k_(1+),was on the order of 10~6 M~(-1)s~(-1),or bigger.The Cu(H_(-3)L),involving,N,N~-,N~-,N~- donors,reacted realtively slowly;k_(1+)ws 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_(-i)L)determines the rate of metal-transport.