Understanding Solid-State Structural Transitions and Stability of Silk Fibroin from a Bound Water Perspective

Area of Science:

• Biomaterials Science
• Protein Crystallography
• Physical Chemistry

Background:

• Silk fibroin's crystalline structure is crucial for its properties.
• Understanding water's role in silk fibroin's structural transitions is key for applications.
• Precise control over protein structure under non-physiological conditions is challenging.

Purpose of the Study:

• To investigate how water molecule states influence silk fibroin's crystalline structure.
• To establish a theoretical framework for controlling silk fibroin's structural transformations.
• To provide insights into the processing and stability of silk-based biomaterials.

Main Methods:

• Low-field Nuclear Magnetic Resonance (NMR) spectroscopy to analyze water states.
• Thermal analysis (Differential Scanning Calorimetry) to determine thermal transitions.
• X-ray Diffraction (XRD) to assess crystalline structure.
• Crystallization kinetics and molecular dynamics simulations.

Main Results:

• Weak bound water content directly correlates with silk fibroin's crystalline state (Silk I vs. Silk II).
• Increased weak bound water lowers glass transition and crystallization temperatures, enhancing chain mobility.
• Specific thresholds of weak bound water (e.g., >4% above 4°C) trigger Silk I formation, while lower levels or higher temperatures favor Silk II.
• Water molecules were confirmed to accelerate structural transformation by increasing chain segment mobility.

Conclusions:

• Water molecule states are critical regulators of silk fibroin's crystalline structure.
• Precise control of temperature, humidity, and resulting water states can predictably alter silk fibroin's structure.
• This study offers a scientific basis for optimizing the processing, storage, and stability of silk fibroin biomaterials.