Imidazole chemical exchange

Although the kinetics of imidazole exchange between NP2 and its low-spin imidazole adduct are very slow and no chemical exchange cross peaks in the NOESY/EXSY spectrum of a mixture of the high-spin and low-spin forms of NP2 are observed, the V-methylimidazole complex... 

G.R. Goward, M.F.H. Schuster, D. Sebastiani, I. Schnell, H. W. Spiess, High-resolution soHd-state NMR studies of imidazole-based proton conductors structure motife and chemical exchange from H NMR, J. Phys. Chem. B 106 (2002) 9322-9334. 

C NMR studies, especially in the solid state (83H(20)1713), are of value in studies of tautomerism (83H(20)1713, 86ZC378). Solid state studies on imidazole (and pyrazole) show there are three distinct signals for the annular carbon atoms (imidazole C-2, 136.3 C-4, 126.8 C-5, 115.3 ppm). Proton exchange does not occur in the solid, hence the compounds resemble their crystal structures. Comparison with the corresponding chemical shifts for 1-methylimidazole (137.6, 129.3, 119.7 ppm) implies that the tautomerism has been frozen in the solid state (81CC1207). Solid-state examination of 2,2 -bis-lH-imidazole also reveals frozen tautomerism. 

Crystalline 2-methylimidazole exhibits different 13C (CPMAS) chemical shifts for C-4 and C-5 (125.0, 115.7 ppm). The average (120.3 ppm) is close to that reported for imidazole in deuterated DMSO (121.2 ppm). These results imply that solid state chemical shifts can be used instead of N-methyl models in tautomerism studies (87H(26)333). For imidazole the solid state l3C shifts are 137.6 (C-2), 129.3 (C-4), and 119.7 (C-5) (81JA6011). No proton exchange occurs in the solid, and the data support a structure resembling the crystal structure. Cooling imidazole solutions has not yet allowed the detection of individual tautomers, but by symmetry the compound exists in equal tautomeric forms, as does pyrazole (81CC1207). 

Deuterium (D20) exchange causes the chemical shifts at 7.18 and 9.0-10.0 to disappear. These resonances are assigned to three exchangeable hydrogens attached to nitrogen on the imidazole ring and in the side-chain. 

The structure and enzyme kinetics of bovine erythrocyte superoxide dismutase are reviewed. The protein has a novel imidazolate-bridged copper(II)-zinc(II) catalytic center in each of two identical subunits. Since a C /Cu1 redox couple is responsible for the dismutase activity of the enzyme, the role of zinc is of interest. Both 220-MHz NMR measurements of the exchangeable histidine protons and chemical modifications using diethylpyrocarbonate demonstrate that zinc alone can fold the protein chain in the region of the active site into a conformation resembling that of the native enzyme. Other possible roles for zinc are discussed. Synthetic, magnetic, and structural studies of soluble, imidazolate-bridged copper complexes of relevance to the 4 Cu(II) form of the enzyme have been made. 

It is not possible to separate the isomers of imidazole in which the 4- or 5-positions are substituted (and the imino nitrogen is unsubstituted), although, as mentioned previously (Section III,G), they are reported to enter into chemical reactions in one of the tautomeric forms.226,305 Exchange of the imino hydrogen atom across the... 

Noda et al. [ 168] reported the details of Bronsted acid-based ionic liquids consisting of a monoprotonic acid and an organic base, in particular solid bis(trifluorometha-nesulfonyl)amide (HTFSI) and solid imidazole (Im) mixed at various molar ratios to form liquid fractions. Studies of the conductivity, H NMR chemical shift, selfdiffusion coefficient, and electrochemical polarization results indicated that, for the Im excess compositions, the proton conductivity increased with an increasing Im molar fraction, with rapid proton-exchange reactions taking place between the protonated Im cation and Im. Proton conduction was found to occur via a combination of Grotthuss- and vehicle-type mechanisms. Recently, Nakamoto [169] reported the... 

Figure 10,5 (a) logCk J versus pK plots at different temperatures for the hydrogen exchange reaction at the C-2 position of four different Imidazoles Including 1, histamine 2, N -acetyl-L-hlstldlne methyla-mide (Ac-Hls-NHMe) 3, N -acetyl-DL-hlstldine (Ac-Hls-OH) 4, 1 H-lmldazole-5-propanolc acid (IPA). (b) The relationship between the y-Intercepts of the plots In (a) and temperature. The correlation observed for the y-intercepts at a pK of 7.0 Is shown in (b). Reproduced with permission from Ref [28]. 2012, American Chemical Society... 

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