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

Abstract

Abstract 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.