Crystalline regions of Bombyx mori silk fibroin may exhibit β-turn and β-helix conformations

Modeling of the (GSGAGA), consensus sequence of the crystalline region of Bombyx mori silk fibroin revealed two novel conformations. One was a series of four-residue p-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 a-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-l residue-l for an isolated strand of continuous;beta-turns, -11.7 kcal mol(-1) residue(-1) for a strand of p-turns on the edge of a sheet of parallel strands, -14.9 kcal mol-l residue(-1) for a strand of beta-turns embedded in a parallel array of B-turns, and -13.0 kcal mol-l residue(-1) for an isolated, right-handed P-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)NH2 had a circular dichroic spectrum characteristic of the P-helical conformation and formed long, curving fibrils that were measured on electron micrographs as having a 2 nm diameter.