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

Carbon-13 isotopic labeling of B. mori silk fibroin was achieved biosynthetically with [I-C-13] glycine in order to determine the carbonyl bond orientation angle of glycine sites with the silk fibroin. Angular dependence of C-13 solid State NMR spectra of uniaxially oriented silk fibroin fiber block sample due to the carbonyl C-13 chemical shift anisotropy was simulated according to the chemical shift transformation with Euler angles, alpha(F) and beta(F), from principal axis system (PAS) to fiber axis system (FAS). The another Euler angles, alpha(DCO) and beta(DCO), for transformation from PAS to the molecular symmetry axis were determined from the [1-C-13] 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-C-13] glycine sites of the silk fibroin was determined to be 90 +/- 5 degrees. 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 beta-pleated sheet) within experimental error. (C) 1998 Elsevier Science B.V.