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

In this study, the crystalline structure of silk fibroin was regulated by controlling water molecule states under precise temperature and humidity. Low-field NMR revealed that strong bound water remained stable at 5-6%, while weak bound water varied significantly with environmental conditions. Thermal analysis showed that glass transition and crystallization temperatures decreased as weak bound water increased. X-ray diffraction indicated that weak bound water critically influences crystallization: at 4-60 °C, exceeding 4% weak bound water induced transformation to Silk I; below this threshold, the amorphous state remained. Above 70 °C, weak bound water dropped below 4%, leading to Silk II formation. Crystallization kinetics and molecular dynamics simulations confirmed that water molecules enhance chain segment mobility, accelerating structural transformation. This work provides a theoretical basis for regulating the processing, storage, and structural stability of protein-based biomaterials under nonphysiological conditions.