Characteristic 1H Chemical Shifts of Silk Fibroins Determined by 1H CRAMPS NMR

CRAMPS NMR of 1 H was used for the structural analysis of some natural silk fibroins such as Tussah Antheraea pernyi and Bombyx mori in the solid state. We were able to resolve all expected 1 H NMR resonances. When tied to the resolution of 13 C NMR via 2D 1 H− 13 C HETCOR experiments, overlapping proton resonance under CRAMPS are able to be further resolved. The simplified 1 H signals of these natural proteins could be successfully assigned on the basis of the conformation-dependent 1 H chemical shifts of model polypeptides. The 1 H chemical shift of the H α signals of Tussah A. pernyi fibroin adopting an α-helix conformation (4.0 ppm) agrees with that of α-helical poly( l -alanine) (3.9 ppm) to within ±0.1 ppm. A well-defined poly( l -alanylglycine), [Ala-Gly] 12, was used as a model polypeptide of B. mori silk fibroin. The 1 H CRAMPS spectra of B. mori fibroins adopting the silk I or silk II form were similar to those of [Ala-Gly] 12 adopting a corresponding conformation. The H α chemical shifts of the silk I fibroin were 3.9 ppm (singletlike) whereas those of the silk II fibroin exhibited peaks at 5.0 and 3.9 ppm. Further, we found that the 1 H chemical shift of side chains in silk I was downfield by 0.4 ppm compared with that in silk II. Thus, it is possible to assign the 1 H CRAMPS NMR spectra of natural proteins such as silk fibroins using model polypeptides of known structure as references.