Abstract:

Objective Silk fibroin is a prominent textile material, and solution-based processing is central to its diverse functional applications. The properties of the resulting products are largely determined by the conformation of silk fibroin in solution. However, the inherent instability of silk fibroin in solution leads to time-dependent conformational changes, making it essential to clarify the underlying mechanism of these transitions.

Method Aqueous silk fibroin solutions with different concentrations were prepared by dissolving freeze dried powder in deionized water. Conformational transitions were monitored using fluorescence spectroscopy, while morphological evolution of aggregates during transition was characterized by atomic force microscopy (AFM). The solution viscosity was determined over a range of concentrations with a rheometer.

Results It was found that increasing the concentration of silk fibroin solution resulted in higher viscosity, a lower initial proportion of β-sheet structures, and a higher content of random coils. Beyond a threshold concentration (1 mg/mL), the proportions of β-sheet and random coil structures were stabilized. Spin-coated film morphology transitioned from fibrous to smooth with increasing concentration. At low concentrations, silk fibroin transitions from random coil to β-sheet over time, where β-sheet content increased initially and then plateaued followed by aggregation into fibers characterized by homogeneous nucleation and one-dimensional growth. In contrast, at high concentrations, a lag phase in conformational transition was observed, during which the structure initially remained unchanged. Subsequently, β-sheet content increased until an equilibrium was reached. Resultant β-sheet aggregates displayed three-dimensional network growth.

Conclusion This study demonstrates that solution concentration critically governs silk fibroin conformation and transition kinetics. At low concentrations, β-sheet formation is initially favored, proceeding via homogeneous nucleation and one-dimensional growth. High concentrations favor random coils, where reduced intermolecular distances promote interactions that require overcoming an initial kinetic barrier (manifested as a lag phase) followed by sigmoidal transition kinetics. The growth of β-sheet aggregates exhibits three-dimensional network characteristics. These findings provide insight into the molecular mechanisms of concentration-dependent conformational transitions and kinetics in silk fibroin solutions, offering a theoretical basis for designing high-performance silk-based materials by aqueous processing.

Key words: silk fibroin, solution concentration, conformational transition, β-sheet, random coil, kinetics, fluorescence spectroscopy