Determination of Thiols

Thiols exhibit reducing properties. They are oxidized into disulfides by iodine. The couple disulfide/thiol obeys the half-redox equilibrium 

The standard potential somewhat depends on the structure R. The reaction can be observed in water and in ethanol, and the titrations can be direct or indirect. 

Electrochemical studies show that, from a mechanistic standpoint, the reaction is complex. From a theoretical point of view, the value of the oxidation state of the sulfur atoms in the thiol and in the disulfide is questionable, due to the similar electronegativity values of sulfur and carbon atoms. In the thiol, the sulfur may be considered at oxidation state —I or —II. In the disulfide, it may be in oxidation state 0 or -I. 

Their titrations are achieved in hydroalcoholic media the determination of 1,2-dimercaptopropanol  

11 Determination of Xanthogenates and Derivatives Determination ofHydrazoic Acid and of Azides 

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Thiols are easily oxidized to disulfides in solution, but this reaction occurs only very slowly at most electrode surfaces. However, use can be made of the unique reaction between thiols and mercury to detect these compounds at very favorable potentials. The thiol and mercury form a stable complex which is easily oxidized, in a formal sense it is mercury and not the thiol which is actually oxidized in these reactions. For the LCEC determination of thiols a Au/Hg amalgam electrode is used Using a series dual-electrode both thiols and disulfides can be determined in a single chromatographic experiment... 

Fahey, R. C., Newton, G. L., Dorian, R., and Kosower, E. M., Analysis of biological thiols quantitative determination of thiols at the picomole level based upon derivatization with monobromobimanes and separation by cation-ex-change chromatography, Anal. Biochem., Ill, 357, 1981. 

P.N. Deepa and S.S. Narayanan, Sol-gel coated Prussian blue modified electrode for electrocatalytic oxidation and amperometric determination of thiols. Bull. Electrochem. 17, 259-264 (2001). 

M.C. Liu, P. Li, Y.X. Cheng, Y.Z. Xian, C.L. Zhang, and L.T. Jin, Determination of thiol compounds in rat striatum microdialysate by HPLC with a nanosized CoHCF-modified electrode. Anal. Bioanal. Chem. 380, 742 (2004). 

Methods for determination of thiol drugs (i.e., captopril [21-25], penicillamine [26-28], hydrochlorothiazide [24, 25, 29, 30], and tiopronin [31, 32]) have been developed. These methods are based on CL from a cerium (IV) oxidation system sensitized by adequate fluorophores such as quinine and rhodamine B. By using HPLC-coupled CL-flow-injection analysis method, tiopronin and its metabolite 2-mercaptopropionic acid in human urine were sensitively determined with the detection limits of 0.8 and 1 pM, respectively [32],... 

Capillary electrophoresis has found use in the biotechnology industry for structural analysis of recombinant proteins. The high resolving power of CE for charged analytes makes it a powerful tool for the analysis of tryptic digests. Therefore, many of the techniques given here, such as the determination of thiols, carbohydrates, and amino acids, will be employed for this purpose. 

M Voldrich, D Dupont, J Dobias, J Phillippon. HPLC determination of thiol-containing antibrowning additives in fruit and vegetable products. Lebensm Wiss Technol 28 213-217, 1995. 

Fenton SS, Fahey RC (1986) Analysis of biological thiols determination of thiol components of disulfides and thioesters. Anal Biochem 154 34-42... 

Following its introduction in 1959 (Ellman), this sensitive spectro-photometric procedure for the determination of thiols has gained wide acceptance. The mixed disulfide product can be exploited as an intermediate in the preparation of other useful derivatives of the cysteinyl residue. 

R6. Rootwelt, K., Quantitative determination of thiols and disulfides in urine by means of Ellman s reagent and thiolated Sephadex and its application to cystinuria. Scand. J. Clin. Lab. Invest. 19, 325-330 (1967). 

Calibration curves were prepared with a standard mixture of thiols according to the labeling procedure and good correlations were observed between the CL intensity as peak height ratio and concentration of thiols up to 500 pmol/injection. The detection limits of standard thiols ranged from 0.23 to 0.30 pmol/injection at a signal-to-noise ratio (S/N) of 3. The repeatability of the proposed method was examined at the 50 pmol/injection level for thiols. The relative standard deviations (RSD) for within-day (n = 3) and between-day (n = 3) runs were < 4.6 and 4.9%, respectively. This method was successfully applied to the determination of thiols in human serum. A typical chromatogram of the extract from human serum determined by the proposed HPLC-CL method is shown in Fig. 3(B). 

Sulfur rank was then calculated from the results of the determinations of thiols formed and hydrogen sulfide liberated. 

Determination of Thiols. Again this depended on the method of Porter, Saville, and Watson (24). Thiol was added to an excess of silver nitrate dissolved in aqueous pyridine. The mixture was then diluted with water, and the pyridinium nitrate formed was titrated with alkali to a phenolphthalein end point. 

Among the large number of fluorescent sulphhydryl directed reagents known to protein chemists, some have found application as fluorigenic reagents for the sensitive determination of thiols. 

In the previous section we saw how isotope labelling has played an indispensable role in the elucidation of the motional processes and structure determinations of thiols. In fiiis section we turn to the dynamics of the rupture of the S—H bond. The chemical phenomenon of the... 

Radical-ion, for cysteine 490 for merc toacetate 490 for mercaptopropionate 490 Radiochemical methods, for determination of thiols 2, 300 Radiolysis of thiols, in oxygen-... 

D. Perrett and S.R. Rudge, The determination of thiols and related compounds using high-performance liquid chromatography, J. Pharm. Biomed. Anal., 1985, 3, 3-27. 

A. Wang, L. Zhang, S. Zhang and Y. Fang, Determination of thiols following their separation by CZE with amperometric detection at a carbon electrode, J. Pharm. Biomed. Anal., 2000, 23, 429-436. 

W.A. Kleinman and J.P. Richie, Determination of thiols and disulfides using high-performance liquid chromatography with electrochemical detection, J. Chromatogr. B, 1995, 672, 73-80. 

Argentometric methods were used, as mentioned above, in the determination of thiol compounds. The amperometric determination of mercaptansP > can be carried out as follows. 

An example of such a sample preparation is the determination of thiol-substances in serum proteins. In this example the low molecular weight substances were separated. 

Fig. 64. Determination of thiols (G) and ascorbic acid (A) in the tomato juice.

Lawrence, N., Davis, J., Jiang, L., Jones, T Davis, S., and Compton, R. (2000) Selective determination of thiols a novel electroanalytical approach. Analyst, 125, 661-663. 

Martinovic, A., S. Cerjan-Stefenovic, and N. Radic. 2008. Flow injection analysis with two parallel detectors Potentiometric and spectrophotometric determination of thiols and ascorbic acid in mixture. /. Chem. Metrol. 2 1-12. 

FIGURE 22.10 A map of the various routes that have been explored in the electroanalyti-cal determination of thiols. (Adapted from Harfield, J. C., Ch. Batchelor-McAuley, and R. G. Compton. 2012. Analyst 137 2285-2296.)... 

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