We demonstrate here a pulse sequence based on rotational-echo double resonance (REDOR) that can help distinguish resonances based on the \(\psi\) -torsion angle, which allows it to distinguish \(\alpha\) -helical and \(\beta\) -sheet regions in solid proteins under magic-angle-spinning. The method relies on conformation-dependent differences in distances between an amide \(^{1}\) H and amide \(^{15}\) N nuclei not covalently attached to it. Dephasing from this remote \(^{15}\) N nucleus is obtained in presence of the much stronger one-bond dipole-dipole coupling by using the \(\mathrm {\theta }\) -REDOR sequence. Experiments are demonstrated on perdeuterated (and 70% backexchanged) sample of uniformly \(^{13}\) C, \(^{15}\) N labeled model protein GB1 at the magic-angle spinning frequency of 41.67 kHz. This method will be useful in simplifying chemical-shift assignments in proteins where the structure is already known, and we anticipate a direct application in determining secondary structures without relying on \(^{13}\) C chemical-shifts.
