COUPLING AMPLIFICATION IN 2D MAS NMR AND ITS APPLICATION TO TORSION ANGLE DETERMINATION IN PEPTIDES

A technique for amplifying the apparent magnitudes of C-13-H-1 and N-1 5-H-1 dipolar interactions in magic-angle spinning experiments is desc ribed, By inserting rotor-synchronized 180 degrees pulses in the evolu tion period of a 2D dipolar-chemical shift experiment, heteronuclear d ipolar couplings are doubled or quadrupled relative to the spinning sp eed, The increased number of dipolar sidebands is desirable for retain ing structural information in the indirectly detected dipolar dimensio n while resolving inequivalent sites in the isotropic chemical shift d imension at relatively high spinning speeds, This coupling amplificati on method is incorporated into an experiment that determines the pepti de torsion angle phi through the relative orientation of the C-alpha-H -alpha and N-H-N bonds. It is shown both experimentally and theoretica lly that the angular resolution of the measurement is enhanced signifi cantly by the selective doubling of the N-H-N coupling. (C) 1997 Acade mic Press.