INVESTIGATION OF MOLECULAR-STRUCTURE IN SOLIDS BY 2-DIMENSIONAL NMR EXCHANGE SPECTROSCOPY WITH MAGIC-ANGLE-SPINNING

An approach to the investigation of molecular structures in disordered solids, using two-dimensional (2D) nuclear magnetic resonance (NMR) e xchange spectroscopy with magic angle spinning (MAS)I is described. Th is approach permits the determination of the relative orientation of t wo isotopically labeled chemical groups within a molecule in an unorie nted sample, thus placing strong constraints on the molecular conforma tion. Structural information is contained in the amplitudes of crosspe aks in rotor-synchronized 2D MAS exchange spectra that connect spinnin g sideband Lines of the two labeled sites. The theory for calculating the amplitudes of spinning sideband crosspeaks in 2D MAS exchange spec tra, in the limit of complete magnetization exchange between the label ed sites, is presented in detail. A new technique that enhances the se nsitivity of 2D MAS exchange spectra to molecular structure, called or ientationally weighted 2D MAS exchange spectroscopy, is introduced. Sy mmetry principles that underlie the construction of pulse sequences fo r orientationally weighted 2D MAS exchange spectroscopy are explained. Experimental demonstrations of the utility of 2D MAS exchange spectro scopy in structural investigations of peptide and protein backbone con formations are carried out on a model C-13-labeled tripeptide, L-alany lglycylglycine. The dihedral angles phi and psi that characterize the peptide backbone conformation at Gly-2 are obtained accurately from th e orientationally weighted and unweighted 2D C-13 NMR exchange spectra . (C) 1996 American Institute of Physics.