pH dependence of 15N NMR shifts and coupling constants in aqueous imidazole and 1-methylimidazole. Comments on estimation of tautomeric equilibrium constants for aqueous histidine

15N, 1H and 13C NMR spectra for [15N2]imidazole and [15N2]-1-methylimidazole in aqueous solution as functions of pH provide shift and coupling-constant information useful in characterizing the protonated and unprotonated forms of these compounds and as background for determining N binding to other species, such as metal ions. When combined with similar data for the imidazole-ring atoms in histidine, these data give more reliable estimates of tautomeric equilibrium constants for the amphionic and anionic forms of histidine than possible from the histidine data alone