DOUBLE-QUANTUM FILTERING IN MAGIC-ANGLE-SPINNING NMR-SPECTROSCOPY - AN APPROACH TO SPECTRAL SIMPLIFICATION AND MOLECULAR-STRUCTURE DETERMINATION

We show how simple radio-frequency pulse sequences can be used to select resonances from pairs of magnetic dipole-coupled nuclear spins and to suppress resonances from isolated spins in magic-angle-spinning (MAS) NMR experiments, thereby simplifying the spectra and providing information about internuclear distances. Non-zero average dipole-dipole couplings are generated by means of DRAMA sequences (Tycko, R.; Dabbagh, G. Chem. Phys. Lett. 1990,173,461-465). Double-quantum filtering techniques are then used to select the resonances of coupled spin pairs. Two experimental demonstrations of double-quantum filtering in C-13 MAS spectra of mixtures of organic compounds are presented, one in which the NMR signal from labeled carbon sites in a doubly C-13-labeled compound ((CH3)2C(OH)SO3Na) is selected while natural abundance signals from an unlabeled compound (N-acetyl-L-valine) are suppressed, and one in which natural abundance signals from a singly C-13-labeled compound (methionine.HCl) are selected while signals from an unlabeled compound (N-acetyl-L-valine) are suppressed. The implications of these experiments, including potential applications to the simplification of MAS spectra of complex molecules, such as biopolymers, and to the determination of the structure of selected regions of complex molecules, are analyzed in detail.