Determination of chemical shift anisotropies of unresolved carbonyl sites by C-α detection under magic-angle spinning

We demonstrate that the static powder pattern line shape of chemical shift anisotropy (CSA) can be obtained for unresolved carbonyl sites of polypeptides under magic-angle spinning. The CSA interaction is first recoupled at the carbonyl site. The phase factors associated with the CSA recoupling are then transferred to the adjacent α carbon by an isotropic polarization transfer based on scalar spin-spin coupling. Because α carbons of polypeptides are usually better resolved, we can then obtain the CSA static powder pattern line shapes of the carbonyl sites after Fourier transformation in the second dimension. We validate our approach experimentally by measurements on [U-13C, 15N]-l-alanine, [U-13C, 15N]-l-valine and prion fibrils with uniform 13C and 15N labels on selected residues.