The quantitative reactions of thiols and disulphides with silver nitrate

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Abstract
Potentiometric titration showed that reduced glutathione (GSH) reacts with one equivalent of AgNO3 to form a Ag mercaptide and with a 2d equivalent to form a soluble complex. This reaction was used in the estimation of GSH. An excess of AgNO3 was added to the GSH, and the free Ag ion remaining was estimated by back-titration with KBr. This titration was also followed potentiometrically; the end points were reproducible to 10-8 g. eq. The method can be used for other thiols. Thiols may be estimated in the presence of disulfides. The soln. must first be acidified to pH 2 with dilute HNO3 in order to prevent the hydrolytic decomposition of the disulfides. The reaction between thiols and AgNO3 is not affected by this reduction of pH. Cystine and oxidized glutathione (G2S2) were estimated by reaction with Na2SO3, followed by estimation of the thiol formed by the method described. The conditions for the reaction with Na2SO3 were reinvestigated. The reaction kinetics of the hydrolytic decomposition of G2S2 and cystine in the presence of an excess of AgNO3 were studied. Acetate catalyzed the reaction of cystine and, to a much lesser extent, that of G2S2. Similarly, barbitone inhibited the cystine reaction completely, but that of G2S2 only slightly. In the presence of GSH and G2S2, cystine was shown to behave in a manner resembling G2S2. A possible explanation for the greater reactivity of the S in GSH and G2S2 as compared with that in cysteine in cystine in given. This involves a hydrogen bond between the S in GSH and G2S2 and the NH3 group of the glutamyl radical.