Conformation-selective detection of residues in solid proteins under magic-angle-spinning

We demonstrate here a pulse sequence based on rotational-echo double resonance (REDOR) that can help distinguish resonances based on the \(ψ\) -torsion angle, which allows it to distinguish \(α\) -helical and \(β\) -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 \(θ \) -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.