A nitrogen-15 nuclear magnetic resonance study of the acid-base and tautomeric equilibriums of 4-substituted imidazoles and its relevance to the catalytic mechanism of .alpha.-lytic protease

Abstract

the electrode does not contain bulky substituents; (v) anthraquinones are adsorbed parallel to the surface and are not influenced by changes in concentration; (vi) the biphenyl moiety, initially adsorbed with both rings parallel to the surface, undergoes reorientation first to a structure in which both rings are bonded to the surface edgewise and then to a 1 2 structure in which one ring is attached and the other is pendant; (vii) preferential adsorption of organic functional groups on Pt follows the order thiol > N heteroaromatic > diphenol ring = quinonoid ring > benzene ring > alkene > amine > hydroxyl > ketone; (viii) in general, adsorbed molecules reorient to structures occupying smaller surface area.

Tautomeric states of the active‐site histidines of phosphorylated and unphosphorylated IIIGlc, a signal‐transducing protein from escherichia coli, using two‐dimensional heteronuclear NMR techniques

“…bExpected "N chemical shifts were derived from solution NMR studies of model compounds (Witanowski et al, 1972;Blomberg et al, 1977;Bachovchin & Roberts, 1978;Roberts et al, 1982;Bachovchin, 1986) and are consistent with solid-state NMR studies of imidazole and histidine (Harbison et al, 1981;Munowitz et al, 1982).…”

“…More recently, the presence of hydrogen bonds has been inferred from perturbations of "N chemical shifts from their canonical (non-hydrogenbonded) values. In a detailed study of "N chemical shifts of nitrogens in imidazole-based model compounds (Schuster & Roberts, 1979;Roberts et al, 1982), it was shown that type-a and type-a+ nitrogens acting as hydrogenbond donors show systematic downfield shifts (up to 10 ppm) relative to expected values (Table 2), whereas type-0 nitrogens acting as hydrogen-bond acceptors show systematic upfield shifts (up to 10 ppm). These data have been used to show that His-57 N"-H and His-57 NE2 of a-lytic protease are both involved in hydrogen bonds (Bachovchin, 1986).…”

“…"N NMR spectroscopy has been used extensively to study the imidazole ring of histidine (Blomberg et al, 1977;Bachovchin & Roberts, 1978;Harbison et al, 1981;Munowitz et al, 1982;Roberts et al, 1982;Bachovchin, 1986). To simplify the discussion, the terms type-a, type-a+, and type-@ have been introduced to describe the protonation state of a particular nitrogen (Table 2) (Witanowski et al, 1972;Bachovchin, 1986).…”

“…5B). Comparison of the data with the previous "N assignments (Table 4) shows that the proton at 13.65 ppm (Witanowski et al, 1972;Blomberg et al, 1977;Bachovchin & Roberts, 1978;Roberts et al, 1982;van Dijk et al, 1990). Expected chemical shifts were recalculated from l M HN03 by subtracting reported values from 377.5 ppm (Witanowski et al, 1986).…”

Advances in development of new tools for the study of phosphohistidine

“…70 Hultquist's phosphoryl transfer experiments can be used as a plausible explanation for the accumulation of τ-pHis with the decrease in π-and bis-pHis over time (Scheme 3). Phosphoryl transfer reactions (1)(2)(3)(4)(5)(6)(7)(8) suggest that the phosphoryl group of pHis can be donated to the imidazole nitrogen of His or α-N-acetyl-His.…”