Determination of the peptide torsion angle φ by 15N chemical shift and 13Cα-1Hα 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 N-15 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 C-alpha-H-alpha bond, This relative orientation reflects the C(O)(i-1)-N-C-alpha-C(O)(i) torsion angle, The technique can distinguish phi angles over the full range of 360 degrees when the amide N-15 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-N bond relative to the C-alpha-H-alpha bond around the N-C-alpha axis cannot be distinguished. (C) 1998 Academic Press.