Self-Assembly of Natural Silk Fibroin under Osmotic Stress

Osmotic stress method was applied to investigate the supramolecular self-assembly behavior of natural silk fibroin within the gland. As with the experiments on re-generated silk fibroin, poly(ethylene glycol) 8K was used to apply an osmotic stress of 0.2-7.6 MPa to the gland of Bombyx mori silkworm, in vitro. Fibroin samples were extracted from the fibroin-dominant, water-soluble posterior region, and from three different parts in the middle region of the gland. Calcium chloride of 0.01 M was added to each stressing solution to balance the physiological salt content of the sample. Microscopic and thermodynamic details of this self-assembly process along the spinline have been assessed by wide angle X- ray diffraction, optical microscopy, etc. It is apparent that as osmotic stress increases, isotropic silk fibroin molecules in the posterior region are assembled together to form a water-soluble crystalline mesophase known as silk-I. Further increases in osmotic stress induce an anti- parallel beta-sheet structure known as silk-II.