NMR Chemical Shift Powder Pattern Recoupling at High Spinning Speed and Theoretical Tensor Evaluation Applied to Silk Fibroin

The NMR pulse sequence RAI (recoupling of anisotropy information) has been improved to obtain
powder patterns at high MAS spinning speeds. The 2D iso-aniso experiment displays the static chemical
shift spectra on the indirect dimension and the MAS spectra on the direct dimension; hence overlapping
chemical shift tensor patterns can be well resolved. This efficient technique is applicable to compounds
containing 13C sp3 (Cα, Cβ) and sp2 (CO) sites with higher chemical shift (CS) anisotropy (CSA), and the
reliability of the method was tested here on the 13C chemical shift tensors of polycrystalline glycine, alanine,
and serine. Subsequently, the same experiment was applied to the native silk protein fibroin from Bombyx
mori, which consists mainly of these three amino acids. Molecular dynamics (MD) simulations of the silk
II crystal structure of Takahashi et al. (Takahashi et al. Int. J. Biol. Macromol. 1999, 24, 127−138) were
carried out to study the influence of motions on the chemical shift tensors. The 13C chemical shift tensors
were calculated using the bond polarization theory BPT on 200 structures created by an MD simulation.
Very good agreement of the theoretical chemical shift anisotropy values with the experimental NMR results
was obtained. The tensor orientations in the protein structure could thus be reliably derived.