Hydrogen bonding imidazoles

In enzymic mechanisms, we are not usually going to get imidazole-imidazole hydrogen bonding, but the ability of imidazole to hydrogen bond to water, to other small molecules, and to carboxylic acid side-chains facilitates the enzyme reaction by correctly positioning the reagents. We shall see examples of this in Section 13.4. 

Candour, R. D., Nabulsi, N. A. R., and Fronczek, F. R., Structural model of a short carboxyl-imidazole hydrogen bond with a nearly centrally located proton Implications for the Asp-His dyad in serine proteases, J. Am. Chem. Soc. 112, 7816-7817 (1990). 

The rNH/rND isotopic frequency ratio of imidazole hydrogen bonded NH and NH+ groups of some protonated nitrogen bases containing NH+..X- hydrogen bonds (Table 7) also decreases with decreasing... 

Nagata, N. Kugimiya. S. Kobuke. Y. Antenna functions of 5,15- i5(imidazol-4-yl)-10,20- i5(4-dodecyloxy-phenyl)-porphyrin supramolecular assembly through imidazole-imidazole hydrogen bonding. Chem. Commun. 2000, (15), 1389-1390. 

Imidazoles and pyrazoles with free NH groups form hydrogen-bonded dimers and oligomers (66AHC(6)347). 

Annular nitrogen atoms can form hydrogen bonds, and if the azole contains an NH group, association occurs. Imidazole (84) shows a cryoscopic molecular weight in benzene 20 times that expected. Its boiling point is 256 °C, which is higher than that of 1-methyl-imidazole (198 °C). 

Imidazole, 4-methyl-annular tautomerism, 5, 363 association, 5, 362 boiling point, 5, 362 bromination, 5, 398 deuteration, 5, 417 diazo coupling, 5, 403 hydrogen bonding, S, 350 hydroxymethylation, 5, 404 iodination, 5, 400 kinetics, 5, 401 mass spectra, 5, 358 melting point, 5, 362 methylation, 5, 364 sulfonation, 5, 397 synthesis, 5, 479-480, 482, 484, 489 Imidazole, 5-methyl-annular tautomerism, 5, 363 Imidazole, l-methyl-4-chloro-ethylation, 5, 386 Imidazole, l-methyl-5-chloro-ethylation, 5, 386 nitration, 5, 395... 

By using imidazole catalysis, it is possible to get a better understanding of the active forms that water takes in enzymatic processes Thus, at low concentrations m the presence of an enzyme, the water may not be fully hydrogen bonded and therefore more reactive [61] The rate of hydrolysis of p-nitrotrifluoroacetanilide in acetonitrile shows a strong dependence on water concentration at low levels in the presence of imidazole The imidazolium complex is the approximate transition state (equation 60)... 

In solution and in the solid state, imidazole and its N2-unsubstituted derivatives form large hydrogen-bonded associates 13 (Scheme 8) [76AHC(S1), p. 266 84CHEC-I(5)345,84JPC5882 96CHEC-II(3)77 97JST(415)187]. 

This mechanism does not require a decision as to the question of whether the association of imidazole occurs through hydrogen bonding or by ionization. - However, if the methylation with diazomethane is considered together with methylations with dimethyl sulfate, dimethyl sulfate and alkali, and methyl iodide and the silver derivative of the imidazole, then such a comparison is best done using the hydrogen-bonded association model. 

The nature of the interaction between the monomer and the template is more obvious in cases where specific ionic or hydrogen bonding is possible. For example, /f-vinyl imidazole has been polymerized along a PM A A template301 202 and acrylic acid has been polymerized on a Af-vinylpyrrolidone template.3 The daughter PAA had a similar degree of polymerization to the template and had a greater fraction of isotaclic triads than PAA formed in the absence of the template. 

However, when the X-ray crystal structure of the MoFe protein was examined, it was clear that homocitrate could not directly hydrogen bond to the histidine, since the carboxylate group and imidazole are stacked parallel to each other in the crystal. Nevertheless, as noted in the previous section, studies on model complexes have suggested that homocitrate can become monodentate during nitrogenase turnover, with the molybdenum carboxylate bond breaking to open up a vacant site at molybdenum suitable for binding N2. 

Fig. 3.15 Model for allosteric inhibition of a protein-DNA complex by a polyamide-inter-calator conjugate. (Top) The GCN4 homodimer (yellow) is displaced by the intercalating moiety (green) of the polyamide conjugate. Blue and red spheres represent pyrrole and imidazole amino acids, respectively. The blue diamond represents / -alanine. (Bottom, left) Hydrogen-bonding model of an eight-ring hairpin polyamide-intercalator conjugate...

P212121 Z — 8 Dx= 1.57 R = 0.085 for 1,743 intensities. The two independent molecules have similar conformations. The glycosyl dispositions are anti (90.1°, 91.2°), and the D-ribosyl groups are 3T4 (24.0°, 34.1° 15.6°, 35.5°). The exocyclic, C-4 -C-5 bond orientations are gauche+ (63.1°, 53.8°). The orientation of the methyl groups in both molecules is such that it is directed away from the imidazole moiety of the base, that is, the 0-6-C-7 bond is trans to the C-5-C-6 bond this arrangement constitutes an obstacle to formation of Watson-Crick hydrogen-bonds to the complementary base cytosine. In molecule A, 0-6 and C-7 are displaced from the purine plane by 79 and 87 pm, and, in molecule B, by 49 and 16 pm. The bases are stacked. 

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