Copper peptide displacement from

Substitution Kinetics of Copper(II)—Peptide Complexes. Three main reaction pathways have been found for the displacement of copper from peptide complexes—(1) proton transfer to the peptide group, (2) nucleo-... 

A further competition experiment was carried out using copper bound patellamide C and EDTA. Two equivalents of EDTA were added to the copper bound peptide and the spectrum measured and compared with that of copper/patellamide C but there was no measurable difference. A further two equivalents of EDTA were added but no change to the spectrum was observed. This shows that the EDTA could not displace the copper from patellamide C despite the formation constant for copper-EDTA being 6 x 10, many orders of magnitude greater than that measured for patellamide C. 

The first step in this new stain protocol employs copper acetate, a metal salt that is both a good fixative ( ) and a silver stain enhancer. The mechanism of copper s stain enhancement, in this and other silver stains, may be similar to its action in the biuret reaction (15.), in which a characteristic color shift, from violet to pink, is achieved by titrating peptides in the presence of copper ions. Copper complexes formed with the N-peptide atoms of the peptide bonds are primarily responsible for this reaction. There are also some number of secondary sites which may interact with copper. Any elemental copper formed may displace positive silver ions from solution as copper has a greater tendency to donate electrons than silver, indicated by its position in the electromotive series of the elements. Following the treatment with copper acetate, the membrane is sequentially soaked in a solution containing chloride and citrate ions and then in a solution containing silver nitrate. The membrane is then irradiated with light while it is in the silver nitrate... 

Displacement of triclycine (G3) from [CuH-aGg]- by macrocyclic tetra-amine ligands (Table 2), is much slower than the corresponding process with trien. The proposed mechanism involves initial binding at the carboxyl site followed by proton transfer to the adjacent peptide link, thus weakening the copper(ii)-peptide bond. The rate with [CuH 2G3] is 4 x 10 higher than the rate with [Cu(edta)] , which has no proton acceptor site and requires nucleophilic displacement by the macrocycle. 

Where the buffer has metal-complexing ability, problems may arise, particularly if heavy metal ions are present. Competition between metal ions and protons for attachment to buffer species can lead to a lowering of pH. Thus, the pH of phosphate and citrate buffers is markedly reduced by the addition of calcium ions (Davies and Hoyle, 1953, 1955). This effect is also to be expected with heavy metal ions and amines or amino acids. In addition, copper ions can displace protons from peptide groups. 

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