Direct detection of N-H center dot center dot center dot O=C hydrogen bonds in biomolecules by NMR spectroscopy

A nuclear magnetic resonance (NMR) experiment is described for the direct detection of N-H center dot center dot center dot O=C hydrogen bonds (H-bonds) in N-15 and C-13 isotope-labeled biomolecules. This quantitative 'long-range' HNCO-COSY (correlation spectroscopy) experiment detects and quantifies electron-mediated scalar couplings across the H-bond (H-bond scalar couplings), which connect the magnetically active N-15 and C-13 nuclei on both sides of the H-bond. Detectable H-bonds comprise the canonical backbone H-bonds in proteins as well as other H-bonds in proteins and nucleic acids with N-H donors and O=C (carbonylic or carboxylic) acceptors. Unlike other NMR observables, which provide only indirect evidence of the presence of H-bonds, the H-bond scalar couplings identify all partners of the H-bond, the donor, the donor proton and the acceptor, in a single experiment. The size of the scalar couplings can be related to H-bond geometries. The time required to detect the N-H center dot center dot center dot O=C H-bonds in small proteins (<= approximate to 10 kDa) is typically on the order of 1 d at millimolar concentrations, whereas H-bond detection for larger proteins (<= approximate to 30 kDa) may be possible within several days depending on concentration, isotope composition, magnetic field strength and molecular weight. The proteins ubiquitin (8.6 kDa), dimeric RANTES (2 x 3 8.5 kDa) and MAP30 (30 kDa) are used as examples to illustrate this procedure.