Imidazoles shifts

The Fe-N mode is at 222 in the R state and 207 cnY in the T state for the a subunits, but only shifted to 218 T state for the (3 subunits. This is consistent with the interpretation that the Fe-imidazole interations are weakened more in the T state of the a subunits than p subunits. Time-resolved resonance Raman studies have shown that the R T switch is complete on a 10 ps tuuescale [38]. Finally, UV excitation of the aromatic protein side chains yields... 

Tile data on H- and C-cliemical shifts, H-relaxation rates, and protonation for 2-arylimidazo[l,2- ]imidazoles (107) and their methyl derivatives 108 and 109 indicate that 107 exists predominantly as the 1/7 tautomer 107a in CDCI3 and as the IH tautomer 107b in DMSO-dg or CD3OD-D2O (91MRC1147). An X-ray crystallographic study has demonstrated the IH tautomer 107a in the lattice. 

Analogous to the synthesis of isocyanates, isothiocyanates are obtained in good yield by reacting AyV -thiocarbonyldiimidazole (ImCSIm) with primary aliphatic or aromatic amines in equimolar amount. In chloroform at room temperature the dissociation equilibrium of imidazole-A-thiocarboxamides is shifted completely to isocyanates. 

The equilibrium in this first step favors tlie shift to anhydride formation if the second mole of carboxylic acid in the second step with forms imidazole to form a salt that is insoluble in the solvent used (ether, tetrahydrofuran, benzene). 

For commonly encountered heterocycles, the chemical shifts of trifluo-romethyl substituents will depend somewhat upon where in the heterocycle they are located. Examples of trifluoromethyl derivatives for a number of common heterocycles, including pyridines, quinolines, pyrroles, indoles, thiophenes, benzothiophenes, furans, benzofurans, imidazoles, and uracils are given below. 

Examples providing fluorine chemical shift data for trifluoromethyl imidazoles and a benzimidazole bearing a trifluoromethyl group are given in Scheme 5.53. 

When the CF3 group is bound to a nitrogen of imidazole, there is not much difference in its chemical shift (Scheme 5.54). 

Equations (12.36) and (12.40) are the basis of the LRA method for calculating pi a shifts [59]. Indeed, to obtain the pKa shift due to the protein environment, we perform the same calculation for the protein and for a small molecule in solution, analogous to the side chain of interest. For a histidine side chain, for example, one would choose imidazole or methylimidazole in solution as a model compound. The pKa shift due to the protein environment will then have the form ... 

Electrophoretic band-shift The reaction mixtures contain 20 pi of 50 mM imidazole-HCl (pH 7.5) buffer, 50 mM NH4C1, 10 mM Mg acetate, and 1 pM each IF2 and f[35S]Met-tRNAmetf. After 10 min incubation at 37°,... 

Elution of the bound antibody-enzyme conjugate occurs by only a slight shift in pH to acidic conditions or through the inclusion of a metal-chelating agent like EDTA or imidazole in the binding buffer. Either method of elution is mild compared to most immunoaffinity separation techniques (discussed in the previous section). Thus, purification of the antibody-enzyme complex can be done without damage to the activity of either component. 

Similarly, heterocyclic amines such as imidazole were added to pyrimidines 57 with moderate to high regioselectivity to produce mainly 4,5-disubstituted pyrimidines 58 after a 1,3-hydride shift along with small amounts of the 2,5-disubstituted isomers 59 . 

A strictly dehned region of chemical shifts of C2, C4, and C5 atoms in A-oxides of 4A-imidazoles allows to dehne clearly the position of the A-oxide oxygen atom (102). Chemical shifts of the a-C nitrone group in a-N-, O-, and S-substituted nitrones are located in the region of 137 to 150 ppm (388, 413). On the basis of 13C NMR analysis of 3-imidazoline-3-oxide derivatives, the position of tautomeric equilibria in amino-, hydroxy-, and mercapto- nitrones has been estimated. It is shown that tautomeric equilibria in OH- and SH-derivatives are shifted toward the oxo and thioxo forms (approximately 95%), while amino derivatives remain as amino nitrones (413). In the compounds with an intracyclic amino group, an aminonitrone (A) - A-hydroxyaminoimino (B) tautomeric equilibrium was observed (Scheme 2.76), depending on both, the nature of the solvent and the character of the substituent in position 2 of the heterocycle (414). 

Determination of electrochemical oxidation potentials and electrochemical reduction of 13 p-phosphorylated acyclic nitrones shows that phosphorylated compounds have a clear anodic shift of potentials of both, oxidation (Ep 1.40 to 2.00 V versus SCE in CH3CN) and reduction (Ep—0.94 to —2.06 V). This is caused by a strong electron-acceptor influence of the diethoxyphosphoryl group (430). In contrast, a reversible one-electron oxidation of azulene nitrones (233) (Scheme 2.80) occurs 0.6 V below the Ep potential of PBN, that is at the value one observes the oxidation of AH -imidazole-1,3-dioxides (219) (428, 429). In other words, the corresponding RC (234) is 14 kcal more stable than the RC of PBN. Although the EPR spectrum of RC (234) was not recorded, RC (236) from dinitrone (235) turned out to be rather stable and gave an EPR spectrum (170). 

There are several reported spectra for this type of compounds, but they were used only for structural confirmation and there are no systematic studies. For example, chemical shifts were reported for the hexahydropyrrolo[l,2- ]imidazole 51 <1996TL1707>. 

The reaction of 2-aminobenzimidazoles 600 with 2,3-allenenitriles led to pyrimi-dobenzimidazoles 604 via initial attack of the unsaturated nitrogen atom in the imidazole ring followed by a 1,3-H shift to form 602. Intramolecular amination of the nitrile group in 602 and a subsequent 1,3-H shift afforded the final product 604... 

The lines in blue show shifts at the active site residues while the imidazole signal is not shifted. The effects on the NMR signals in black (medium effects) and magenta (strong effects) are caused by changes in the pH (as indicated by the shifted imidazole line) induced by the ligands. 

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