Imidazolium-4-thiolat

The various fonns of betaines are very important for their charge control functions in diverse applications and include alkylbetaines, amidoalkylbetaines and heterocyclic betaines such as imidazolium betaines. Some surfactants can only be represented as resonance fonns having fonnal charge separation, although the actual atoms bearing the fonnal charge are not ftmctionally ionizable. Such species are mesoionic and an example of a trizaolium thiolate is illustrated in table C2.3.3. 

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. 

Fig. 3.9. The Cys-thiolate/His-imidazolium catalytic dyad and the oxyanion hole of papain,... 

The mesoionic 1,2,3-triazolium-4-olates 361a,b and l,2,3-triazolium-4-thiolate 361c did not react with benzocyclopropene (113), and the attempted reaction between l,3-diazolium-4-olate 362 and benzocyclobutadiene failed (114). In contrast to these negative results, Ichinari et al. (249) found that the isomeric l,3-diazolium-4-olates (l,3-imidazolium-4-olates) 363 and 364 react with DMAD to form pyridones and pyrroles, respectively. 

Figure 12-15 Schematic drawing of the active site of a cysteine protease of the papain family with a partial structure of an acyl-enzyme intermediate in green. The thiolate-imidazolium pair of Cys 25 His 159 lies deep in the substrate-binding cleft and bridges an interface between two major structural domains, just as the Ser His pair does in serine proteases (Fig. 12-10). This may facilitate small conformational changes during the catalytic cycle. Asn 175 provides a polarizable acceptor for positive charge, helping to stabilize the preformed ion pair, and allows easy transfer of an imidazolium proton to the product of substrate cleavage. The peptide NH of Cys 25 and the side chain of Gin 19 form an oxyanion hole.

Probably the aspect of primary importance for the catalytic activity of cysteine proteases is the high nucleophilicity of the active-site thiol group. It is now generally accepted that the active form of papain and of cysteine proteases in general consists of a thiolate-imidazolium ion-pair, built from Cys25 and Hisl59. 

Fig. 16. Schematic drawing showing the binding interaction in the S1-S2 intersubsite region of papain postulated to provide for the transition state, involving nucleophilic attack by the thiolate anion of Cys-25 assisted by association of the pyridyl nitrogen atom of the leaving group with the imidazolium cation of His-159. [Reprinted with permission from Ref. (99).]...

On the one hand, the reaction of imidazolium-2-dithiocarboxylate 111 with bromine gives the corresponding cationic thioacylsulfenyl bromide 112 [173]. On the other hand, treatment of 111 with an equimolar amount of iodine gives the charge transfer complex 113. Two iodine atoms and a thiolate sulfur atom are arranged in almost a linear fashion in 113. Further treatment of 113 with an excess amount of iodine causes oxidative coupling, affording the dicationic disulfide 114 (Scheme 27). 

Comparison of activation energy values for interaction of methyl thiolate with oxirane and epoxides (24, 25) shows decreasing of reaction rate for epoxyacid (Fig. 10.9) [47], while increasing of inhibition activity has been found experimentally for acid-substituted oxiranes [48]. According to [47] increased activity of epoxide (24) arises from interaction of carboxylate with imidazolium ion stabihzes the noncovalent enzyme inhibitor complex and thus improves Ki. 

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