High-resolution 13C NMR (75.46 MHz) spectra of silk fibroins in the solid state were recorded by the cross-polarization-magic angle spinning method with emphasis on revealing conformational features of the dimorphic structures, silk I and II, of Bombyx mori fibroin prepared under different conditions. It was found that the 13C chemical shifts of Gly, Ala, and Ser residues of silk II samples from B. mori fibroin by different preparations and of cocoon samples from several silkworms gave identical values with those of corresponding model polypeptides having the /3-sheet conformation. Thus, identification of the silk II type form is easily performed by examining their 13C chemical shift values. The 13C chemical shifts of samples having the silk I form are significantly displaced from those of the silk II form, and can be used for diagnostic purposes. As expected, the 13C chemical shifts of Ala and Gly residues of the silk I samples were identical with those of (Ala-Gly)"II. However, we found that none of the 13C chemical shifts predicted from the crankshaft model [Lotz, B.; Keith, H. D. J. Mol. Biol. 1971, 61, 201-215] in which Ala and Gly residues are close to the /S-sheet and -helix conformations, respectively, was in agreement with the 13C chemical shifts of (Ala-Gly)"II and silk I type form. Instead, we found that predicted 13C chemical shifts from the loose helix proposed by Konishi and Kurokawa [Konishi, T.; Kurokawa, M. Sen'i Gakkaishi 1968, 24, 550-554] on the basis of the calculated 13C contour map of chemical shifts for Ala residue are in good agreement with the displacement of the l3C chemical shifts.

Solid state high resolution 13C NMR spectra of the silk fibroins from Bombyx mori (B. mori) and Philosamia cynthia ricini (P. c. ricini) together with those of appropriate model peptides were recorded by cross polarization-magic angle spinning method. We found by examining 13C chemical shifts that anti-parallel β-sheet and α-helix forms were straightforwardly ascribed to the fibroins from B. mori and P. c. ricini, respectively.