Solid-state N-15 NMR chemical shift anisotropy of histidines: Experimentaland theoretical studies of hydrogen bonding

The principal values of the N-15 chemical shift tensors of crystalline hist idine and histidine-containing peptides have been measured to document for the first time the systematic trends in values of imidazole CSA with change s 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 calculation s are largely lacking in the literature. On the basis of this database of e xperimental values for imidazole groups interacting with carboxylate hydrog en-bonding partners, and of ab initio calculations for similar structures, a correlation was found between the N-15 delta(22) tensor value and the hyd rogen bond length for cationic species. As the hydrogen bond distance decre ases, the delta(22) tensor value shifts downfield. No correlation was found between the N-15 CSA tensor elements of neutral imidazole and the correspo nding hydrogen bond distance, probably because the range of hydrogen-bondin g distances in our compounds is limited (similar to 0.05 Angstrom) and beca use this functionality is not involved in nearly pK(a)-matched hydrogen bon ds. Ab initio N-15 Shielding calculations for an imidazolium acetate (catio nic) model showed general agreement with the trends in the experimental res ults, although the breadths of the calculated CSA tensors are systematicall y larger than those determined experimentally, End the variation in the cal culated CSA tensor Values is somewhat smaller than that obtained experiment ally.