1 H Nuclear magnetic resonance studies of histidine‐containing di‐ and tripeptides. Estimation of the effects of charged groups on the p K a value of the imidazole ring

The nmr titration curves of chemical shifts versus pH were observed for the protons of various histidine‐containing di‐ and tripeptides. With these results, the macroscopic p K a values and the chemical shifts intrinsic to each ionic species were determined by a computer curve‐fitting based on a simple acid dissociation sequence. The p K a value of the imidazole ring in N ‐acetyl‐ L ‐histidine methylamide was assumed to represent the intrinsic (or unperturbed) p K a of the imidazole rings of histidine having peptide linkages at both the CO and NH sides. The p K a values of the imidazole rings observed for most di‐ and tripeptides were reasonably reproduced by simple calculations using the intrinsic value and the perturbations due to the CO 2 − and NH 3 + groups located at various positions. Some other factors affecting the p K a value of the imidazole ring are also discussed.