The interfacial behavior of Bombyx mori silk fibroin.

摘要

A new crystal structure has been observed for Bombyx mori silk fibroin at air-water interfaces. This structure, silk III, incorporates a left-handed three-fold polyglycine II conformation and an approximately hexagonal lattice. Detailed crystallographic studies using electron diffraction data have been used to characterize the silk III crystal structure. There is a hexapeptide repetitive sequence found in the crystallizable portions of silk fibroin. When this sequence, Gly-Ala-Gly-Ala-Gly-Ser, is in a threefold helical conformation, a row of alternating glycine and serine residues parallel to the helical axis results. One third of the helix thus becomes slightly hydrophilic, whereas the other two thirds consist of glycine and hydrophobic alanine residues. The data indicate that the helices are arranged so that the serine residues pack preferentially in serine-rich sheets in the (110) planes of the crystal. The result is a monoclinic crystal structure, where the basal plane angle {dollar}gamma{dollar} is 116{dollar}spcirc{dollar} rather than the 120{dollar}spcirc{dollar} expected for perfect hexagonal packing, due to the distortion in nearest neighbor interhelical packing distances that results when the serine residues have a preferred packing. The separation of hydrophobic and hydrophilic residues in a threefold helical conformation of fibroin suggests that the air-water interface may stabilize the threefold conformation because this conformation allows the fibroin to behave as a surfactant at the interface. The sheet-like arrangement of serine residues deduced for the monoclinic silk III crystallites also supports a role for surfactancy in stabilizing the silk III structure at the air-water interface. If the crystallizable portions of fibroin are behaving as a surfactant, then the three-fold helical silk III structure should be oriented with the axes of the three-fold fibroin helices in the plane of the interface. This orientation is observed in uncompressed fibroin films which were picked up onto TEM grids. In LB films compressed to 16.7 mN/meter on the trough prior to being picked up onto TEM grids a uniaxial orientation is observed for silk III, with the helical axes perpendicular to the plane of the sample film. A surface compression of 34 mN/meter results in films containing silk II crystallites with the same uniaxial orientation, placing the helical axes perpendicular to the plane of the film. In addition to air-water interface experiments several experiments were carried out using aqueous-organic interfaces. A hydrated crystal structure incorporating a left-handed 6/2 helix which is still roughly three-fold, is observed at the water-hexane and water-chloroform interfaces. Large lamellar crystallites possessing a hexagonal habit were observed at both of these interfaces. A banded cholesteric mesophase which in some regions crystallizes into the same silk III hydrate structure as the lamellae was also observed and characterized.