Crystalline Regions of Bombyx mori Silk Fibroin May Exhibit β-Turn and β-Helix Conformations

Modeling of the (GSGAGA) n consensus sequence of the crystalline region of Bombyx mori silk fibroin revealed two novel conformations. One was a series of four-residue β-turns, consistent with published circular dichroic spectra and nuclear magnetic resonance data for the water-soluble silk I that is found in the abdominal glands of the silkworm larvae before spinning. The other conformation was a single-chain β-helix having 4.3 residues per turn, consistent with the circular dichroic spectra and water insolubility of silk II, the fibrous form of silk fibroin. Computer modeling of these structures provided a conformational energy of −9.9 kcal mol -1 residue -1 for an isolated strand of continuous β-turns, −11.7 kcal mol -1 residue -1 for a strand of β-turns on the edge of a sheet of parallel strands, −14.9 kcal mol -1 residue -1 for a strand of β-turns embedded in a parallel array of β-turns, and −13.0 kcal mol -1 residue -1 for an isolated, right-handed β-helix. These values are consistent with the transformation of silk I to silk II when an aqueous solution of silk fibroin is subjected to shear. The synthetic consensus peptide Ac(GSGAGA) 2 NH 2 had a circular dichroic spectrum characteristic of the β-helical conformation and formed long, curving fibrils that were measured on electron micrographs as having a 2 nm diameter.