Measuring 13C- 13C connectivity in spinning solids by selective excitation

Abstract

In 1983, Ernst and co-workers (I) used DANTE: pulse trains (2) in CPMAS NMR experiments to irradiate selectively a single spinning sideband, as well as to saturate an entire sideband family (I). They also described the selective inversion of a sideband family by DANTE irradiation which was synchronous with the magic-angle spinning (that is, irradiation sidebands were matched to spinning sidebands). More recently, VanderHart has used DANTE pulse trains in natural-abundance CPMAS 13C NMR experiments on semicrystalline polymers to study crystalline polymorphy by measuring 13C-13C spin exchange (3). These experiments were not complicated by spinning sidebands of significant intensity. In this note, we report how DANTE pulse trains in CPMAS experiments can be used to measure specific 13C-l ‘C connectivity in complex, multiple 13C-labeled solids such as intact bacterial cells, even though the 13C NMR spectra are complicated by sizable spinning sidebands arising both from 13C-l 3C dipolar coupling and from chemical-shift anisotropy. We show that entire spinning sideband families can be inverted by asynchronous DANTE pulse trains whose irradiation sidebands are outside the spectrum of interest. The DANTE pulse trains we use typically consist of 65 0.7 ~LS pulses generated by a 20 kHz RF field. The spacing between pulses is 75 ps. The resulting irradiation sidebands are always outside the carbon spectrum being examined. A 3 ms interval immediately preceding the DANTE pulse train is used to switch the 13C carrier frequency to the selected irradiation frequency, and to permit the decay of residual transverse magnetization arising from pulse imperfections during the 38 kHz matched spin-lock cross-polarization and z-restoration parts of the experiment (Fig. 1). Polarization imbalances created by selective DANTE irradiation are equilibrated during the delay period, T. The CPMAS-delay and CPMAS-DANTE sequences (Fig. 1, top and bottom) differ only in the DANTE pulse train of the latter. The difference of spectra obtained with and without the DANTE pulse train (which we sometimes call the DANTE difference spectrum) therefore reveals directly the selectively irradiated resonance. If this resonance arises from one of the carbons of a covalently bonded pair of 13C-labeled nuclei, the other carbon of the pair can be identified by magnetization transfer during, say, a T = 10 ms period immediately following DANTE irradiation. This time is too short to allow spin diffusion between nonbonded carbons (2). Differences of spectra, obtained