Efficient multispin homonuclear double-quantum recoupling for magic-angle spinning NMR:: 13C-13C correlation spectroscopy of U-13C-erythromycin A

We introduce a radio frequency (rf) pulse sequence for efficient homonuclear double-quantum dipolar recoupling under magic-angle spinning NMR. The sequence is optimized for two-dimensional double-quantum C-13-C-13 chemical shift correlation spectroscopy in multiple spin systems, such as the U-C-13-labeled antibiotic erythromycin A. Spin systems such as this display a wide range of isotropic and anisotropic chemical shifts and, therefore, require a broadband dipolar recoupling sequence that minimizes the errors arising from the interaction of chemical shifts and rf inhomogeneity. The sequence should also preserve the theoretical efficiency over the powder average (similar to 73%) provided by the C7 experiment of Levitt and co-workers (Lee, Y. K.; Kurur, N. D.; Helmle, M.; Johannessen, O. G.; Nielsen, N. C.; Levitt, M. H. Chem. Phys. Lett. 1995, 242, 304-309). We satisfy these criteria by combining the standard C7 (2 pi(phi)-2 pi(phi+180)) elements with ct-pulse permuted elements (pi(phi)-2 pi(phi+180)-pi(phi), in analogy to the MLEV decoupling scheme) to remove error terms over a +/-10% range of rf amplitude. The new sequence, which we refer to as CMR7 (combined MLEV refocusing and C7), yields for two-spin systems broadband double-quantum filtering efficiencies greater than 70%. For multispin systems, the improved polarization transfer efficiency results in greater cross-peak intensities, facilitating assignment of U-C-13-labeled molecules in the solid state.