Yf. Wei et al., Solid-state N-15 NMR chemical shift anisotropy of histidines: Experimentaland theoretical studies of hydrogen bonding, J AM CHEM S, 121(44), 1999, pp. 10389-10394
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.