Silk I structure in Bombyx mori silk foams

Single crystals of Bombyx mori silk fibroin in the metastable silk I polymo rph have been produced using a new foaming technique. Foams of silk protein are generated by bubbling pure nitrogen gas through an aqueous solution of regenerated silk fibroin. The foamed material is collected, dried, and the n sonicated to yield individual crystals which were examined using transmis sion electron microscopy and electron diffraction. It is found that slightl y acidic conditions in the solution from which the foam was generated favor the formation of silk II while neutral to slightly basic solutions favor s ilk I formation. More dilute solutions favor the formation of silk II while more concentrated solutions (about 7 wt.% or greater) favor the formation of silk I. X-ray powder diffraction patterns from the dried silk I foams di splayed features highly indicative of silk I. We also report the first sing le crystal electron diffraction patterns of silk I. These patterns indicate a large unit cell, possibly 22.66 x 5.70 x 20.52 Angstrom, with six chains of six residues, Gly-Ala-Gly-Ala-Gly-Ser. Although we have not fully chara cterized this complex structure it appears that the chain is nearly fully e xtended and thus our data is consistent with models possessing general feat ures similar to those proposed by Fossey SA, Nemethy G, Gibson KD, Scheraga HA. (Biopolymers 1991;31:1529-1541). (C) 1999 Elsevier Science B.V. All ri ghts reserved.