Determination of the Peptide Torsion Angle f by N Chemical Shift and C-H Dipolar Tensor Correlation in Solid-State MAS NMR

We demonstrate a dipolar-chemical shift correlation technique for sign-sensitive determination of the torsion angle f 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 N 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 N chemical shift tensor and the C-H bond. This relative orientation reflects the C(O)i21-N-C a-C(O)i torsion angle. The technique can distinguish f angles over the full range of 360° when the amide N 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 H-N bond relative to the C-H bond around the N-C axis cannot be distinguished. © 1998 Academic Press