Silver peptide complexes

Recently the structures of silver(I) complexes with cyclo-(glycyl-L-histidyl), cyclo-(L-methionyl-L-histidyl) and cyclo-(L-histidyl-L-histidyl) were examined using 7H and 13CNMR spectroscopy. The NMR measurements suggested that the silver ion was bound to the sulfur atom of the thioether and to the nitrogen atom of imidazole groups in these cyclic peptides, but not to their amide groups.417... 

Li H, Michael Siu KW, Guevremont R, Le Blanc JCY (1997) Complexes of silver with peptides and proteins as produced in electrospray mass spectrometry. J Am Soc Mass Spectrom 8 781-792... 

Another approach to the study of proteins has been the reaction of silver ions with simpler model compounds, for example di- and tri-peptides.416 The reaction with glycylglycine was first studied in 1951 and the formation constants for 1 1 and 1 2 complexes were determined.403 Thermodynamic parameters for these reactions have since been measured (log 0i = 2.90, logp2 = 5.65, AHfo = -56.9 0.8kjmol"1, ASj32 = -80.5 3 JK"1 mol"1).416... 

Use nondenaturing polyacrylamide gel electrophoresis (PAGE) to separate and visualize LMW-Cd2+-PC2 and HMW-Cd2+-PC2 complexes contained in the crude extract. A detailed protocol for doing this is given by Abrahamson el al.27. The peptides should be visualized by silver staining but referenced to the 109Cd result since proteins other than PCs may be present. 

Stable structures such as the naturally occurring ferric porphyrin complexes or porphyrins substituted with copper, cobalt, silver or vanadyl probe the active site of heme-containing enzyme [227]. Complexes of copper not associated with heme are also common. They are frequently formed at an amino terminus because the amino group provides a good primary amine donor atom. Two or three amino acid residues beginning at the N-terminus are often flexible until a more rigid portion of the polypeptide, such as the a helix, is encountered. A peptide nitrogen is available to... 

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... 

Amino groups involved in peptide bonding and N-terminal amino groups are in themselves insufficient for visualization with silver stain. If they were capable of independently reducing silver ions, all peptides, proteins, and amino acids would stain positively. However, the amino groups involved in peptide bonding and N-terminal atoms may be of some importance for the intensity of the stain, as these atoms have been observed to form 13 different complexes with copper between pH 1.5 to pH 11.0 (47,). Bound copper may be reduced under the conditions some stain protocols and then be displaced by silver. Alternatively, silver may also interact directly, but weakly, with these groups. 

Sihca impregnated with saturated and unsaturated hydrocarbons (squalene, paraffin oil) silicone and plant oils complexing agents (silver ions, boric acid, and borates carbohydrates unsaturated and aromatic compounds) chelating compounds [ethylene diamine tetra-acetic add (EDTA), digitonin] transition metal salt synthetic peptides 18-crown-6 and ammonium sulfate silanized sdica gel impregnated with anionic and cationic surfactants Cross-hnked, polymeric dextran gels (Sephadex)... 

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