DETERMINATION OF THE PEPTIDE TORSION ANGLE PHI BY N-15 CHEMICAL-SHIFTAND C-13(ALPHA)-H-1(ALPHA) 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 an d proteins under magic-angle spinning. The indirect dimension of the e xperiment is obtained by separate but synchronous evolution of the mag netization under the N-15 chemical shift and the C-H dipolar coupling. The resulting sum and difference spectrum of the two frequencies, wit h more than ten independent sidebands, depends strongly on the relativ e 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 f ull range of 360 degrees when the amide N-15 chemical shift tensor doe s not possess reflection symmetry with respect to the peptide plane, T hus it complements our previous HNCH experiment, in which two mirror-s ymmetric 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 Acade mic Press.