1H-Nuclear magnetic resonance study on imidazole–imidazole interaction in L-histidyl-L-histidine and D-histidyl-L-histidine: Analysis including microscopic dissociation series

The 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 pK_a 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 pK_a 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.