Direct observation of Calpha-Halpha...O=C hydrogen bonds in proteins by interresidue h3JCalphaC' scalar couplings

The role of C−H···O hydrogen bonds in the stabilization of biomolecules is increasingly being recognized from the evidence of close C−H···O contacts in crystal structures. However, relatively little is known about their strength. Here, we report the observation of NMR scalar couplings (h3JCαC‘) between the two carbons on each side of Cα−Hα···O=C H-bonds in proteins. These couplings give direct evidence of the correlation of the electronic wave functions in the donor and acceptor groups of Cα−Hα···O=C H-bonds. A long-range H(NCO)CA experiment or a selective long-range H(NCA)CO experiment was used for the detection of h3JCαC‘ correlations in the β-sheet regions of the immunoglobulin binding domain of protein G. In total, six such correlations were detectable. These correspond to half of the Cα−Hα···O=C H-bonds of protein G with Hα···O distances shorter than 2.5 Å. The h3JCαC‘ couplings range from 0.2 to 0.3 Hz and are in good agreement with predicted average values based on DFT/FPT calculations. An anticorrelation is observed with the size of h3JNC‘ coupling constants across N−HN···O=C H-bonds, which share the same acceptor carbonyl oxygen.