Recoupling of chemical shift anisotropies in solid-state NMR under high-speed magic-angle spinning and in uniformly C13-labeled systems

Abstract

We demonstrate the possibility of recoupling chemical shift anisotropy (CSA) interactions in solid-state nuclear magnetic resonance (NMR) under high-speed magic-angle spinning (MAS) while retaining a static CSA powder pattern line shape and simultaneously attenuating homonuclear dipole–dipole interactions. CSA recoupling is accomplished by a rotation-synchronized radio-frequency pulse sequence with symmetry properties that permit static CSA line shapes to be obtained. We suggest a specific recoupling sequence, which we call ROCSA, for which the scaling factors for CSA and homonuclear dipole–dipole interactions are 0.272 and approximately 0.05, respectively. This sequence is suitable for high-speed ${}^{\mathrm{13}}\mathrm{C}$ MAS NMR experiments on uniformly ${}^{\mathrm{13}}\mathrm{C}$ -labeled organic compounds, including biopolymers. We demonstrate the ROCSA sequence experimentally by measuring the ${}^{\mathrm{13}}\mathrm{C}$ CSA patterns of the uniformly labeled, polycrystalline compounds L-alanine and N-acetyl-D,L-valine at MAS frequencies of 11 and 20 kHz. We also present experimental data for amyloid fibrils formed by a 15-residue fragment of the β-amyloid peptide associated with Alzheimer’s disease, in which four amino acid residues are uniformly labeled, demonstrating the applicability to biochemical systems of high molecular weight and significant complexity. Analysis of the CSA patterns in the amyloid fibril sample demonstrates the utility of ROCSA measurements as probes of peptide and protein conformation in noncrystalline solids.