Nuclear magnetic resonance titration curves of histidine ring protons. I. Influence of neighboring charged groups.

Several imidazole derivatives bearing titratable charged groups adjacent to the imidazole ring have been studied by nuclear magnetic resonance. Curves relating the chemical shift values of the C-2 and C-4 ring proton resonance signals of these compounds to the solvent pH have been examined. These curves clearly demonstrate effects corresponding to the titration of the neighboring charged groups in addition to the effect of protonation of the imidazole ring in each case. The conclusions drawn from these data may be useful in interpreting similar titration curves of the histidine ring proton signals of proteins.