Blocking of the Sulfhydryl Group

Of course, it is possible to decrease the stability of the S-benzyl group toward acids by substitution with electron-releasing groups. Thus, in the S-p-methoxybenzyl group the benzyl cation is sufficiently stabilized to allow cleavage of the S —C bond with liquid hydrogen fluoride under fairly mild conditions while the S-2,4,6-trimethylbenzyl group can be removed with trifluoromethanesulfonic acid in trifluoroacetic acid in the persence of w-cresol. 

Cleavage of the S-tert.butyl group with acids is too cumbersome and is better accomplished in two steps, by treatment with o-nitrobenzenesulfenyl chloride followed by reduction of the thus formed mixed disulfide with mer-captoethanol  

Acid sensitive blocking can be achieved by the application of the S-triphenyl-methyl (trityl) group. The S-trityl blocking is removed with HCl in organic solvents (e.g. chloroform) and also by treatment with iodine in methanol, a process which yields the disulfide  

The role of methanol is quite important ih this reaction it acts as the acceptor of the trityl group. In a similar fashion ethylmercaptane, used in considerable excess, facilitates the acidolytic cleavage of the S-trityl group and prevents realkylation of the regenerated sulfhydryl group  

Intermediate acid sensitivities can be found in the S-diphenylmethyl (benz-hydryl) group and its methoxy derivative  

---

Foster, 1968 Sogami and Foster, 1968). The observed heterogeneity was attributed to a sulfhydryl-disulfide interchange (Sogami et aL, 1969), since it was prevented by blocking of the sulfhydryl group. 

Conversion to the S-sulfenylsulfonate derivative The high reactivity of sulfhydryl groups imposes serious limitations on attempts to modify selectively other amino acid residues in proteins containing cysteinyl residues. The reversible blocking of free sulfhydryl groups in native proteins with tetrathionate offers one solution to this problem ... 

Lead inhibits plant growth, reduces photosynthesis, and reduces mitosis and water absorption. Inhibition of photosynthesis is attributed to the blocking of protein sulfhydryl groups and to changes in phosphate levels in living cells. For two species of roadside weeds, pollen germination was reduced by 90% and seed germination by 87% at lead levels of about 500.0 mg/kg (DW) in soil and about... 

The reason that Hb doesn t block the action of NO is due to a unique interaction between Cys93/3 of Hb and NO recently described by Li Jia, Celia and Joseph Bonaventura, andjohnathan Stamler at Duke University. Nitric oxide reacts with the sulfhydryl group of Cys93/3, forming an S-nitroso derivative ... 

Methyl methanethiosulfonate (MMTS) is a small reversible blocking agent for sulfhydryl groups (Thermo Fisher, Toronto Research). It reacts with free thiols to form a dithiomethane modification with release of sulfinic acid (Figure 1.122). The sulfinic acid component decomposes into volatile products, which don t affect the disulfide formed from the MMTS reaction Alkylthiosulfonates react rapidly with thiols under mild conditions at physiological pH. The MMTS compound is a liquid at 10.6 M concentration and is conveniently added to a reaction medium by pipette. Complete thiol modifications of available cysteine residues in proteins can... 

The current theory on the mode of action of copper compounds is that the copper ion, Cu, is the active component, which is released from the different salts on the leaf surfaces. The copper ions in the presence of CO2 from the air and the organic acids excreted from the plant and/or fungal spores interact together to produce the resultant activity. The copper ions and complex-bound copper are capable of penetrating the spores and lead to the inhibition of enzyme reactions. This can occur by the removal of other important metals from their compounds by chelation and also by blocking or interacting with the sulfhydryl groups of the spore enzymes. 

Pre-slaughter injection of the live animal has proved to be the most effective method of introducing proteolytic enzymes into meat so that they penetrate uniformly into the furthest interstices of the tissue (31). Oxidized papain (103) and proteases whereby the sulfhydryl group in the active site is reversibly blocked (104) have been injected into the jugular veins of animals approximately 30 min before slaughter to distribute the enzyme in the tissues. Subsequently, in the reducing environment of the meat, the protected enzymes are activated. However, the enzymes are apparently active only between 50°C and 82°C or while the meat is cooking. 

The study of succinylation of cheese whey protein concentrates gave a blocking of 83% of the amino groups, 51% of sulfhydryl, and 27% of hydroxyl groups [35],... 

---