NMR STUDY ON THE PROTONATION OF IMIDAZOLE RING OFN-ACETYL-L-HISTIDINE METHYLAMIDE, A MODEL FOR HISTIDINE RESIDUES EXPOSED TO AQUEOUS SOLVENT

Abstract The pH titration curves of proton NMR chemical shifts were observed for N-acetyl-l-histidine methylamide and l-histidine in H2O and D2O. Macroscopic pKa and intrinsic chemical shifts for each ionic species were obtained. The enthalpy and entropy changes on protonation of the imidazole ring were determined. These data are useful for the discussions of microenvironments of histidine residues in proteins.