EFFICIENT MULTISPIN HOMONUCLEAR DOUBLE-QUANTUM RECOUPLING FOR MAGIC-ANGLE-SPINNING NMR - C-13-C-13 CORRELATION SPECTROSCOPY OF U-C-13-ERYTHROMYCIN-A

We introduce a radio frequency (rf) pulse sequence for efficient homon uclear double-quantum dipolar recoupling under magic-angle spinning NM R. 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 suc h as this display a wide range of isotropic and anisotropic chemical s hifts and, therefore, require a broadband dipolar recoupling sequence that minimizes the errors arising from the interaction of chemical shi fts and rf inhomogeneity. The sequence should also preserve the theore tical 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. Ph ys. 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 permu ted elements (pi(phi)-2 pi(phi+180)-pi(phi), in analogy to the MLEV de coupling scheme) to remove error terms over a +/-10% range of rf ampli tude. The new sequence, which we refer to as CMR7 (combined MLEV refoc using and C7), yields for two-spin systems broadband double-quantum fi ltering efficiencies greater than 70%. For multispin systems, the impr oved polarization transfer efficiency results in greater cross-peak in tensities, facilitating assignment of U-C-13-labeled molecules in the solid state.