Imidazole/imidazolium groups

Imidazolium groups of histidine are essential for the transfer of a hydrogen ion to the displaced fructosyl moiety [12], The reprotonation of the imidazole takes place by abstracting a proton from the C6 hydroxyl moiety and facilitating the formation of the glycosidic linkage. 

Figure 4 The enzyme ribonuclease A cleaves RNA using the imidazole group of histidine 12 as a base and the imidazolium group of histidine 119 as an acid in a simultaneous two-proton transfer process. Studies discussed in this article indicate that the process is not as simple as shown.

Buffer function of the imidazole/imidazolium functional groups of histidine residues in protein. 

The best catalyst was the AD isomer, the reverse of what we had seen in our ribonu-clease model studies. The pH/rate profile indicated that there was bifunctional catalysis by imidazole and imidazolium groups. This indicated that proton removal has a preferential direction of attack by base that is not directly along the C-H bond line, but instead pushes the electrons toward the carbonyl group (Figure 2.11). Such subtle information is available only because of the defined geometry in molecular complexes of cyclodextrin i w-imidazoles. 

In a similar approach, Stelzer and coworkers [114] prepared a range of phosphines bearing Al-methyl imidazolium groups placed close or remote from the phosphorus (Scheme 2.32). Aryl rings could be linked to the imidazol part via a Kosugi-Stille coupling. Additional abundant functional groups like in compound I may furthermore enhance the solubility in ILs [115]. 

An imidazole-substituted calix[4]arene 3 was designed as a phase-transfer catalyst for aromatic nucleophilic substitution reaction in water [21]. The stable conformation associating cavities of calixarene skeleton and the S5mergistic effect of imidazolium groups of calix[4]arene 3 expressed the excellent catalytic ability (see Fig. 27.6). Taking into accoimt the results represented in Table 27.4, calix[4]... 

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

An interesting synthesis of a nucleoside carbonic ester was conducted in excellent yield by reaction with 1 -methyl-3 - [2-(p-nitropheny l)ethoxycarbony 1]-imidazolium chloride (for an analogous introduction of the npeoc-protecting group, see also Section 3.10.1) [2393 as mentioned in Section 3.1.8, methylation of the imidazole unit increases significantly the reaction rate. 

Carbamates by Reaction of Imidazole- or Imidazolium-iV-carboxylates. Introduction of Amino Protecting Groups... 

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