Structure of uniaxially aligned 13C labeled silk fibroin fibers with solid state 13C-NMR

Abstract Carbon-13 isotopic labeling of B. mori silk fibroin was achieved biosynthetically with [1- 13 C] glycine in order to determine the carbonyl bond orientation angle of glycine sites with the silk fibroin. Angular dependence of 13 C solid state NMR spectra of uniaxially oriented silk fibroin fiber block sample due to the carbonyl 13 C chemical shift anisotropy was simulated according to the chemical shift transformation with Euler angles, α F and β F , from principal axis system (PAS) to fiber axis system (FAS). The another Euler angles, α DCO and β DCO , for transformation from PAS to the molecular symmetry axis were determined from the [1- 13 C] glycine sequence model compounds for the silk fibroin. By the combination of these Euler angles, the carbonyl bond orientation angle with respect to FAS of the [1- 13 C] glycine sites of the silk fibroin was determined to be 90 ± 5°. This value is in agreement with the X-ray diffraction and our previous solid state NMR data of B. mori silk fibroin fiber (a typical β-pleated sheet) within experimental error.