NMR ^h3J_NC' couplings provide comprehensive geometrical constraints for protein H-bonds in solution

Protein backbone H-bonds (>N-H⋯O-C<) show relationships between NMR ^h3J_NC' couplings measured in solution and H-bond geometry parameters seen in X-ray crystal structures. Assuming that the solution and crystal structure of protein backbone is the same, the ^h3J_NC' couplings can be calibrated to provide good estimates of both angular and radial H-bond parameters in a solution. The crucial premise of equality between the solution and the crystal structure of protein backbone we validated on the level of the NH-bonds orientation, by comparing the orientations inferred from X-ray crystal structures with the solution ones determined from NMR residual dipolar couplings.