Silk I structure in Bombyx mori silk foams

Area of Science:

• Biomaterials Science
• Protein Crystallography
• Materials Chemistry

Background:

• Bombyx mori silk fibroin is a key biomaterial with diverse applications.
• Understanding silk fibroin polymorphs, particularly the metastable silk I, is crucial for controlling its properties.
• Existing methods for obtaining silk I crystals are limited, hindering detailed structural investigation.

Purpose of the Study:

• To develop a new method for producing single crystals of Bombyx mori silk fibroin in the silk I polymorph.
• To characterize the structural features of silk I crystals obtained via this method.
• To investigate the influence of solution conditions on silk I and silk II formation.

Main Methods:

• A novel foaming technique involving bubbling nitrogen gas through regenerated silk fibroin solutions.
• Collection, drying, and sonication of foamed material to isolate single crystals.
• Analysis using transmission electron microscopy (TEM), electron diffraction, and X-ray powder diffraction (XRD).

Main Results:

• Successfully produced single crystals of silk I polymorph using the foaming technique.
• Identified optimal conditions for silk I formation: neutral to slightly basic pH and concentrated solutions (≥7 wt.%).
• Obtained the first single crystal electron diffraction patterns of silk I, suggesting a large unit cell and a nearly fully extended chain structure (Gly-Ala-Gly-Ala-Gly-Ser).

Conclusions:

• The new foaming technique provides a viable method for generating silk I single crystals.
• Solution pH and concentration are critical factors controlling silk I versus silk II polymorph formation.
• The preliminary structural data of silk I is consistent with existing theoretical models, offering insights into its molecular arrangement.