Determination of the peptide torsion angle phi by 15N chemical shift and 13Calpha-1Halpha dipolar tensor correlation in solid-state MAS NMR.

We demonstrate a dipolar-chemical shift correlation technique for sign-sensitive determination of the torsion angle phi in solid peptides and proteins under magic-angle spinning. The indirect dimension of the experiment is obtained by separate but synchronous evolution of the magnetization under the 15N chemical shift and the C-H dipolar coupling. The resulting sum and difference spectrum of the two frequencies, with more than ten independent sidebands, depends strongly on the relative orientation of the 15N chemical shift tensor and the Calpha-Halpha bond. This relative orientation reflects the C(O)i-1-N-Calpha-C(O)i torsion angle. The technique can distinguish phi angles over the full range of 360 degrees when the amide 15N chemical shift tensor does not possess reflection symmetry with respect to the peptide plane. Thus it complements our previous HNCH experiment, in which two mirror-symmetric conformers of the HN-N bond relative to the Calpha-Halpha bond around the N-Calpha axis cannot be distinguished.