The principal values of the N-15 chemical shift tensors of crystalline histidine and histidine-containing peptides have been measured to document for the first time the systematic trends in values of imidazole CSA with changes in hydrogen bonds. NMR measurement of imidazole groups, both N-15 and H-1, is a key method for studying strong and pK(a)-matched hydrogen bonds and their roles in enzymes, but appropriate model compound data and calculations are largely lacking in the literature. On the basis of this database of experimental values for imidazole groups interacting with carboxylate hydrogen-bonding partners, and of ab initio calculations for similar structures, a correlation was found between the N-15 delta(22) tensor value and the hydrogen bond length for cationic species. As the hydrogen bond distance decreases, the delta(22) tensor value shifts downfield. No correlation was found between the N-15 CSA tensor elements of neutral imidazole and the corresponding hydrogen bond distance, probably because the range of hydrogen-bonding distances in our compounds is limited (similar to 0.05 Angstrom) and because this functionality is not involved in nearly pK(a)-matched hydrogen bonds. Ab initio N-15 Shielding calculations for an imidazolium acetate (cationic) model showed general agreement with the trends in the experimental results, although the breadths of the calculated CSA tensors are systematically larger than those determined experimentally, End the variation in the calculated CSA tensor Values is somewhat smaller than that obtained experimentally.