Silver mercaptide

To remove the silver mercaptide formed from the facilitated protein thiolation reaction, add an excess of thiourea to convert all the silver into a soluble Ag(thiourea)2 complex and free the sulfhydryl modifications. 

Silver ion also catalyzes nucleophilic reactions of thiol esters, including reactions of acetylhomocysteine thiolactone (12) and diethylethylphosphonothiolate (52). In the first reaction, an insoluble complex of silver ion and the substrate was first produced at pH 7.5, which then reacted with the nucleophile, in this case an amino group of a protein. In the second reaction silver ion complexes of the substrate were also postulated, on the basis that silver ion complexes with sulfur are much more stable than those with oxygen (I). The complexes postulated were 1 1 and 2 1 silver ion-substrate complexes. These complexes were suggested to react with the nucleophiles, water and fluoride ion, giving as products phos-phonic acid and phosphonyl fluoride, respectively, and silver mercaptide. It is evident that the last reaction at least must involve only the direct interaction of a silver ion with the sulfur atom of the thiol ester without chelate formation. Therefore it appears the metal ion-catalyzed reactions of thiol esters are unique, in that they involve complex formation, but not chelate formation in their catalytic mechanism. 

Conte, P., Carotenuto, G., Piccolo, A., Perlo, R, and Nicolais, L., NMR-investigation of the mechanism of silver mercaptide thermolysis in amorphous polystyrene, J. Mater. Chem., 17, 201-205 (2007). 

T. Kuninori and J. Nishiyama, Measurement of biological thiols and disulfides by high performance liquid chromatography and electrochemical detection of silver mercaptide formation. Anal Biochem., 1991, 197, 19-24. 

Finally, it has been noted that the attempted detritylation of open-chain unsymmetrical disulphides containing S-trityl groups using silver nitrate in pyridine led to extensive decomposition and neither the silver mercaptide nor thiol (after treatment of the reaction mixture with hydrogen sulphide) could be isolated [94]. Thus there is some question as to whether peptides containing preformed cystine residues and S-trityl or S-benz-hydryl cysteine can be deblocked with heavy metals without causing cleavage of the disulphide bond. 

The hydrogen sulfide-free sample is dissolved in an alcoholic sodium acetate titration solvent and titrated potentiometiically with sQver nitrate solution, using as an indicator the potential between a glass reference electrode and a sUver/sUver-sulfide indicating electrode. Under these conditions, the mercaptan sulfur is precipitated as silver mercaptide and the end point of the titration is shown by a huge change in ceD potential. 

When mercaptans are present in excess, the end of the Ag2S precipitation occurs at about -550 to -350 mV, and is followed by the precipitation of the silver mercaptide to the +300 mV end point This situation is shown in the middle curve of Fig. 1. Since all of the Ag2S originates from an equivalent amount of mercaptan, the total titration to the mercaptide end point must be used to calculate the amount of mercaptan sulfiir. 

When elemental sulfur is present in excess, the end of the Ag2S precipitation is taken in the same region (+300 mV) as in the case of silver mercaptide, and is calculated as mercaptan sulfur. 

Fitch expanded on his work on gold(I) mercaptides in combination with silver(I) mercaptides to include the synthesis of novel gold-silver mercaptide precursor compounds [53], These were prepared by simply reacting the gold(I) mercaptide compounds with an eqnimolar amonnt of silver(I) mercaptide or silver(I) carboxylate at elevated temperatures in a suitable solvent (for example, toluene) (Equation 15.5). 

---