Thiolate group

Wang ZLetal 1998 Bundling and interdigitation of adsorbed thiolate groups in self-assembled nanoorystal superlattioes J. Phys. Chem. B 102 3068... 

FIGURE 24.17 The mechanism of the thiolase reaction. Attack by an enzyme cysteine thiolate group at the /3-carbonyl carbon produces a tetrahedral intermediate, which decomposes with departure of acetyl-CoA, leaving an enzyme thioester intermediate. Attack by the thiol group of a second CoA yields a new (shortened) acyl-CoA. 

Some interesting cases of isomerism in bridged complexes do arise. The thiocyanate bridged complex shown in Figure 3.41 is a good example of the ambidentate behaviour of the thiocyanate (confirmed by X-ray) while in the complexes [Pt(PR3)(SR)(SR )]2 the choice of isomer is determined by the order in which the thiolate groups are introduced (Figure 3.42). 

Estimated from Eq. (3) and parameters in Tables I and II. c These ions are reduced by thiols. Since Cu(II) forms less stable complexes with thiols than Cu(II), this will always occur in an environment of thiols alone. However, log Ki for Fe(III) with thiols is larger than for Fe(II), and so log p4 for Fe(III) with ME should be proportionately much larger them with Fe(II). Thus, coordination of four thiolate groups, as occurs in electron transfer proteins, will lead to a very stable Fe(III) complex, which is much less easily reduced to Fe(II) than when only a single thiolate is coordinated. 

The initial step of the adsorption of thiols on a Mo(100) surface is the formation of adsorbed thiolate groups. Phenyl thiolate is formed upon adsorption of thiophenol at 120 K on a clean Mo(110) surface.381 The thiolate intermediate subsequently undergoes competing C-S bond hydrogenolysis to form benzene, or C-S and C-H bond scission to form surface benzyne. The adsorption of thiophenol was also studied on a sulfur-covered Mo surface382 Phenyl disulfide is formed via S-H bond scission and S-S bond formation. The S-S linkage is oriented perpendicular and the phenyl ring parallel to the surface. 

The initial step of the adsorption of cyclic sulfides on a Mo(100) surface is also the formation of adsorbed thiolate groups.395-397 Adsorbed alkyl thiolates decompose to adsorbed sulfur, carbon, and hydrogen on the clean Mo surface, but once the surface is deactivated by adsorbed sulfur, alkanes and alkenes evolve from the surface. Tetrahydrothiophene (34) and trimethylene sulfide decompose on Mo(110) to alkanes and alkenes by way of a common intermediate, which is proposed to be a surface thiolate (33). The thiolate undergoes hydrogenation or dehydrogenation, depending on the surface hydrogen concentration (Scheme 4.115).398 399... 

The Mo(III) d3-d3 complexes [CpMo(dithiolene)]2 are characterized by a single Mo—Mo bond, further stabilized by interaction with the n system of the dithiolene ligands. Indeed, the analogous complexes where the two dithiolene are replaced by four thiolate groups were found to oxidize more easily and salts of the cationic d3-d2 [CpMo(SMe)4MoCp]+ were even isolated and structurally and magnetically characterized [50]. 

The important biochemical role played by this type of (non-heme) iron proteins has stimulated efforts to synthesise Fe4S4 complexes bound to thiolate groups. For instance, Figure 10 shows the voltammetric behaviour of two such complexes, [Fe4S4(SPh)4]2 and [Fe4S4(SBut)4]2-.la... 

The serine hydrolases, threonine hydrolases, and cysteine hydrolases, the attacking nucleophile of which is a serine or threonine OH group or a cysteine thiolate group, respectively, and which form an intermediate covalent complex (i. e., the acylated enzyme). Here, an activated H20 molecule enters the catalytic cycle in the second step, i.e., hydrolysis of the covalent intermediate to regenerate the enzyme. 

The mechanism of hydrolysis of cysteine peptidases, in particular cysteine endopeptidases (EC 3.4.22), shows similarities and differences with that of serine peptidases [2] [3a] [55 - 59]. Cysteine peptidases also form a covalent, ac-ylated intermediate, but here the attacking nucleophile is the SH group of a cysteine residue, or, rather, the deprotonated thiolate group. Like in serine hydrolases, the imidazole ring of a histidine residue activates the nucleophile, but there is a major difference, since here proton abstraction does not appear to be concerted with nucleophilic substitution but with formation of the stable thiolate-imidazolium ion pair. Presumably as a result of this specific activation of the nucleophile, a H-bond acceptor group like Glu or Asp as found in serine hydrolases is seldom present to complete a catalytic triad. For this reason, cysteine endopeptidases are considered to possess a catalytic dyad (i.e., Cys-S plus H-His+). The active site also contains an oxyanion hole where the terminal NH2 group of a glutamine residue plays a major role. 

Schematic diagram of the modification of protection from modification by application of a reversible ligand (agonist or antagonist, ). The reversible binding site specific ligand slows the rate at which the MTS reagent modifies the thiolate group of the introduced cysteine...

In the course of investigations using 4-chloro-7-nitrobenzofurazan as a reactivity probe for identifying the active sites of a number of enzymes such as papain, ficin, and bromelain, the intermediacy of Meisenheimer adducts derived from direct attack of thiolate groups located in the protein has been assumed on the basis of the spectral changes accompanying the process of replacement of the chloro group.232,233... 

Epoxy groups, such as that of E-64, a compound isolated from the culture medium of a species of Aspergillus, react irreversibly with the active site thiolate group of cysteine proteases.1/U/V Related epoxides may become useful medications against abnormal cathepsin levels. 

Another surprise was the discovery, from the X-ray structure,558-559 that a tyrosine side chain at the active site has been modified by addition of a thiolate group... 

The coordination chemistry of the thiolate group has been widely investigated where it was part of a polydentate ligand. Fewer studies involving monothiolates have been carried out to determine the coordination characteristics intrinsic to the RS- group.294,295... 

It is instructive, in this context, to compare the case of assembly of both nitrosylated iron sulfur tetranuclear clusters and the related clusters with pendant thiolate groups (13), with the laborious and stepwise initial synthesis (38) of cubane C8H8 this comparison is, of course, simply one between thermodynamically controlled processes, in the case of the iron sulfur systems, and kinetically controlled reactions for C8H8 synthesis. 

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