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 (h3JCalphaC') between the two carbons on each side of Calpha-Halpha...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 Calpha-Halpha...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 h3JCalphaC' correlations in the beta-sheet regions of the immunoglobulin binding domain of protein G. In total, six such correlations were detectable. These correspond to half of the Calpha-Halpha...O=C H-bonds of protein G with Halpha...O distances shorter than 2.5 A. The h3JCalphaC' 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.