Abstract
AbstractThe 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 pKa 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 pKa value of the imidazole ring in N‐acetyl‐L‐histidine methylamide was assumed to represent the intrinsic (or unperturbed) pKa of the imidazole rings of histidine having peptide linkages at both the CO and NH sides. The pKa 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 CO2− and NH3+ groups located at various positions. Some other factors affecting the pKa value of the imidazole ring are also discussed.
Published Version
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