Abstract
AbstractThe 1H‐nmr titration curves of chemical shifts versus pH were observed for the protons of D,L‐histidyl‐D,L‐histidine as representative of cases with two or more ionizable groups with similar pKa values. The titration curves of L‐histidyl‐L‐histidine and D‐histidyl‐L‐histidine were individually analyzed according to two mathematical models: one of a macroscopic dissociation series and one of a microscopic dissociation series. Most‐probable values and standard deviations were obtained for pKa values and intrinsic chemical shifts. An analysis including the microscopic dissociation series yielded an electrostatic interaction between twoimidazole rings of about 0.3 pH units for L‐histidyl‐L‐histidine and about 0.7 pH units for D‐histidyl‐L‐histidine. The difference of the magnitude of imidazole‐imidazole interactions between L‐histidyl‐L‐histidyne and D‐histidyl‐L‐histidine was interpreted in terms of the spatial arrangement of two imidazole rings in each molecule based on the solution conformation estimated from Gd(III)‐induced relaxation enhancements.
Published Version
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