Thermally Induced α-Helix to β-Sheet Transition in Regenerated Silk Fibers and Films

• Published in: Biomacromolecules Vol. 6; no. 6; pp. 3328 - 3333
• Main Authors: Drummy, Lawrence F, Phillips, David M, Stone, Morley O, Farmer, B. L, Naik, Rajesh R
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 01.11.2005
• Subjects: Animals; Applied sciences; Bombyx; Calorimetry, Differential Scanning; Circular Dichroism; Copper - chemistry; Crystallization; Exact sciences and technology; Hot Temperature; Insect Proteins - chemistry; Natural polymers; Physicochemistry of polymers; Protein Conformation; Protein Structure, Secondary; Protein Structure, Tertiary; Proteins; Silk - chemistry; Stress, Mechanical; Temperature; Time Factors; X-Ray Diffraction; Film; Insecta; Temperature effect; Bombyx mori; Experimental study; Solid state; Silk; Oriented polymer; Arthropoda; Natural fiber; Bombycidae; Lepidoptera; Helical structure; Invertebrata; Conformational transition
• ISSN: 1525-7797; 1526-4602
• DOI: 10.1021/bm0503524

The structure of thin films cast from regenerated solutions of Bombyx mori cocoon silk in hexafluoroisopropyl alcohol (HFIP) was studied by synchrotron X-ray diffraction during heating. A solid-state conformational transition from an α-helical structure to the well-known β-sheet silk II structure occurred at a temperature of approximately 140 °C. The transition appeared to be homogeneous, as both phases do not coexist within the resolution of the current study. Modulated differential scanning calorimetry (DSC) of the films showed an endothermic melting peak followed by an exothermic crystallization peak, both occurring near 140 °C. Oriented fibers were also produced that displayed this helical molecular conformation. Subsequent heating above the structural transition temperature produced oriented β-sheet fibers very similar in structure to B. mori cocoon fibers. Heat treatment of silk films at temperatures well below their degradation temperature offers a controllable route to materials with well-defined structures and mechanical behavior.