In addition to the developments described below, we have recently demonstrated highly precise internuclear distance measurements with the CMR7 pulse sequence. Homonuclear couplings can be measured to a precision of better than 5 Hz with this approach, implying internuclear distance measurements of better than 0. 1-0.2 A precision out to 4-5 'A. We have developed a pulse sequence for efficient double-quantum dipolar recoupling in multiple spin systems under magic-angle-spinning NMR, based on the C7 sequence of Levitt and co-workers. For two-spin systems, the C7 sequence offers higher overall polarization transfer and double-quantum filtration (DQF) efficiency (-73%) than the MELODRAMA sequence (-52%), because the dependence of the recoupled interaction on rotor phase is eliminated. Experimentally, however, DQF efficiency with the C7 sequence depends significantly on the errors that arise from cross terms between chemical shifts and radiofrequency (rf) field inhomogeneity. Many applications require the excitation of DQ coherence in multi-spin systems, which display a wide range of isotropic and anisotropic chemical shifts. Also, the experimental reliability of the sequence is crucial for successful implementation under conditions of low sensitivity and temperature. To meet these requirements, we have constructed a pulse sequenc e that recouples dipolar interactions independent of chemical shifts, by combining Cn elements of various symmetries. The error terms inherent to the C7 sequence are removed by composite MLEV-type rotations; therefore, we refer to the new sequence as CMR7 (Combined MLEV-Refocusing with C7). We have demonstrated the utility of this approach with double-quantum filtration of U-""""""""Clabeled amino acids and """"""""C-""""""""C chemical shift correlation spectroscopy of the U-""""""""C-labeled antibiotic, erythromycin A. With 73% polarization transfer, the ratio of crosspeak to diagonal intensity is expected to be almost 3: 1, and in two-spin cases such as U- """"""""C, """"""""N-Gly, we have observed better than 2: 1 relative intensities in 2D spectra. However, the full theoretical DQF efficiency is usually not realized in multi-spin systems and similar behavior is observed in correlation spectra with respect to crosspeak intensities. Nevertheless, in many cases crosspeak intensities exceed the diagonal peaks, and in favorable instances the ratio of intensities is greater than 2: 1, even in the multi-spin limit. An illustrative example of these effects, and the improvement in resolution observed upon extending to a second 13C chemical shift dimension, is provided by the U-""""""""C-labeled macrolide antibiotic erythromycin A (EA). EA inhibits protein synthesis by binding to a bacterial ribosome. Actual structural information on the erythromycin-ribosome complex has been inaccessible due to the paucity of available crystal s and poor resolution of the solution NMR spectra due to the slow reorientational motion of the ribosome. As a result, the erythromycin-ribosome complex exhibits a solid-state NMR spectrum even in solution. Therefore, in order to facilitate structural studies of EA in both its free and complexed forms, it is necessary to make unambiguous chemical shifts assignments in the solid state. We have accomplished the chemical shift assignments using the CMR7 method and are pursuing further structural studies based upon the CMR7 method.
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