J couplings between pairs of protons separated by three or fewer covalent bonds can be measured. The value of a three-bond J coupling constant contains information about the intervening torsion angle. The other major source of structural information comes from through space dipole-dipole coupling between two protons called the NOE . Thus the NOE is a sensitive probe of short intramolecular distances. NOEs are categorized according to the location of the two protons involved in the interaction.
Stippled lines outline independent amino acid spin systems.
Knowledge of the amide and alpha proton chemical shifts are in principle all that is necessary although if this information is available it is likely that nearly complete assignments of sidechain protons are also available. While obtaining the sequential resonance assignments is a laborious task, the NMR method is perhaps the most powerful and certainly the most accurate method of secondary structure determination without a three-dimensional structure. The three-bond coupling constant between the intraresidual alpha and amide protons is the most useful for secondary structure determinations as it can be directly related to the backbone dihedral angle phi. Unfortunately, no there is no three-bond proton coupling that can be related to the angle psi. The solid curve is drawn for a L-amino acid in position i and the broken curve is drawn for Gly in position i.