Hydrogen bonding monitored by deuterium isotope effects on carbonyl 13C chemical shift in BPTI: intra-residue hydrogen bonds in antiparallel beta-sheet.

Deuterium isotope effects on carbonyl 13C magnetic shielding were measured for the backbone carbonyl groups in BPTI (basic pancreatic trypsin inhibitor), and interpreted as a measure of hydrogen bond energies. The effects originate from peptide amide proton deuterium substitution and were observed on carbonyl carbons separated by two or three covalent bonds from the amide H/D. Two-bond isotope effects depend on the energy of the hydrogen bond donated by NH/D. Calibration of the effect with model compound data leads to hydrogen bond enthalpies less than 4.7 kcal/mol. Isotope effects over three bonds from the amide H/D to the carbonyl carbon of the same amino acid residue are observed for seven carbonyl resonances in BPTI. The three-bond isotope effects are highly related to the various backbone conformations. The largest effects are observed for residues with an approximate syn- periplanar conformation of the H-N-C alpha-C = O atoms, as realized for many residues in the BPTI antiparallel beta-sheet. The residues that show measurable three-bond effects have unusually short distances between H and O. The size of this effect decreases rapidly with increased O..H distance in the open five-membered ring. This observation suggests appreciable interactions in these rings.