Thiols thioacylation

The cleavage mechanism of the caspases is shown schematically in Fig. 15.5. They use a typical protease mechanism with a catalytic diad for cleavage of the peptide bond. The nucleophilic thiol of an essential Cys residue forms a covalent thioacyl bond to the substrate during the catalysis. The imidazole ring of an essential histidine is also involved in catalysis and this facilitates hydrolysis of the amide bond in the sense of an acid/base catalysis. 

The sulfinate is the immediate product in the absence of a strong proton donor under sufficiently acidic conditions reduction to mercaptans may take place. The thiosul-fonate may be formed on thioacylation of the mercaptan or dismutation of the sulfinic acid the thiosulfonate is reducible under the conditions employed, so the final product is the thiol. 

Thioamide synthesis from dithiocarboxylates and amines under basic conditions gives thiols as rather unpleasant parallel products. This problem is avoided by using S-thioacylated thioglycolic acids (17), as... 

Thioacylation of Thiols with Thioacyl Halides, Thioacyl Anhydrides and Thioketenes... 

Thioacyl halides are not easily accessible. Aliphatic derivatives, in particular, can only be isolated in rare cases. Thioacylations of alcohols are therefore less important as synthetic methods than acylation of thiols with acyl hdides. Nevertheless, a variety of 0-alkyl and 0-aryl arenecarbothioates (2) have been obtained according to equation (4i).6.i4,25,2s... 

Thioacyl halides react with thiols in the presence of base and high yields of alkyl or aryl dithiocarbox-ylates (3) are obtained according to equation (57). ... 

Thiols can also be thioacylated, in the same way as alcohols (Section 2.5.3.2), by reaction with bis-thioacyl thioanhydrides (equation 58). The preparation of aromatic as well as aliphatic thioanhydrides is possible according to novel procedures described by Kato et a/., e.g. from the corresponding dithiocarboxylic acids and DCC or 2-chloro-N-methylpyridinium iodide.Even sterically hindered di-thioesters are obtained (equation 58). ... 

The highly activated sodium salts of carboxymethyl carbodithioates, which can be easily obtained by alkylation of sodium carbodithioates with bromoacetic acid, - have especially found application for the thioacylation of thiols. - Since the products are insoluble in water, the equilibrium reaction (equation 63) is shifted to the product side. Also vinyl arenecarbodithioates are reactive enough to be in-terconverted with potassium alkanethiolates. ... 

In general, thiocarbonyl halides (59) function as thioacylation reagents with a variety of nucleophiles to yield the appropriate thio derivatives (60) (Scheme 31). For example, (59) on condensation with thiols, amines, potassium cyanide or potassium thiocyanide yields the corresponding thio compounds (60). Thiolocarboxylic acids (50) characteristically acylate alcohols and amines with desulfuration (Scheme 32). Dithiocarboxylic acid esters (54b) react with organolithium or Grignard reagents to give the dithioketals (61) after treatment with an alkyl halide (Scheme 33). 

Since the NMR resonances of the carbonyl of thioesters are shifted (A 20 to 30 ppm), NMR spectroscopy should allow the direct monitoring of the formation and decay of a thioacyl intermediate. Using = O] N-benzoylimidazole (5 = 168.7 ppm), we were able to observe directly a thioacyl intermediate at 8 = 195.9 ppm in the presence of papain under the cryoenzymological conditions of -t C in 25 percent aqueous dimethyl sulfoxide. Moreover, the thioacyl species is clearly a productive intermediate since the decrease in its signal intensity was accompanied by an increase in the product resonances and by release of free enzyme (half-life, 96 minutes) determined by titration of its thiol group. The line width of the resonance at 195.9 ppm was 25 Hz [5,6]. 

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