Rotational resonance in uniformly 13C-labeled solids: effects on high-resolution magic-angle spinning NMR spectra and applications in structural studies of biomolecular systems

Abstract

We describe investigations of the effects of rotational resonance ( R 2) on solid state 13C NMR spectra of uniformly 13C-labeled samples obtained under magic-angle spinning (MAS), and of the utility of R 2 measurements as structural probes of peptides and proteins with multiple uniformly labeled residues. We report results for uniformly 13C-labeled l-alanine and l-valine in polycrystalline form, and for amyloid fibrils formed by the 15-residue peptide Aβ 11–25 with uniform labeling of a four-residue segment. The MAS NMR spectra reveal a novel J-decoupling effect at R 2 conditions that may be useful in spectral assignments for systems with sharp 13C MAS NMR lines. Pronounced dependences of the apparent isotropic 13C NMR chemical shifts on MAS frequency near R 2 conditions are also observed. We demonstrate the feasibility of quantitative 13C– 13C distance determinations in l-valine, and qualitative determinations of inter-residue 13C– 13C contacts in Aβ 11–25 fibrils. Finally, we demonstrate a “relayed” R 2 technique that may be useful in structural measurements on systems with poorly resolved 13C MAS NMR lines.