The dissolution and characterization of Bombyx mori silk fibroin in calcium nitrate‐methanol solution and the regeneration of films

Abstract

Bombyx mori silk fibroin from the silkworm was found to be soluble in a calcium nitrate-methanol system. Fibroin dissolves in 75% w/v Ca(NO3)2/MeOH solution at a temperature of 67°C. The viscometric behavior of the fibroin-salt solution was analyzed and the fibroin's secondary structures were determined via 13C solution nmr. Fourier transform ir, solid state 13C-nmr, x-ray diffraction, differential scanning calorimetry, scanning electron microscopy (SEM), and polarizing microscopy were used to characterize regenerated films and fibers. A compositional phase diagram of fibroin in the salt solution was constructed. Viscosity data indicate that there is aggregation of fibroin chains within the salt solution. The extremely high value of intrinsic viscosity of 8.7 dL/g at 298 K may be due to aggregation. Aggregation may be caused by the complexing of calcium ions with the fibroin chains at their amide linkages. The energy required for viscous flow for the fibroin solution (ΔHvis = 9.03 kcal/mol) is greater than that of the solvent (ΔHvis = 7.01 kcal/mol). Chain entanglements may be hindering the free motion of chains thus increasing the energy required for the viscous flow. 13C-nmr shows that fibroin chains exist in two independent conformational environments. While most of the molecule is in a random coil conformation, there is evidence of some order within the chains of fibroin. In as-cast regenerated films, the fibroin chains are in a random coil/α-helix conformation with some β-sheet content. Crystallinity induced by immersion of thin films in methanol is evidenced via x-ray diffraction, which shows lattice spacings at 4.042 A. Thin films have a fibrillar morphology that is clearly shown under the SEM and the polarizing microscope. Fibers were hand pulled from the concentrated fibroin-salt solutions and coagulated with acetone and methanol. A microscopic analysis was done using the polarizer. © 1997 John Wiley & Sons, Inc. Biopoly 42: 61–74, 1997