Dielectric relaxation spectroscopy of hydrated and dehydrated silk fibroin cast from aqueous solution

The dynamics of silk protein in the presence and absence of water has been investigated by dielectric relaxation spectroscopy (DRS). The silk fibroin film cast from its water solution contains 4-7 wt % bound water molecules, which can be removed by dehydration at 165 °C. Temperature and frequency scans were performed on the hydrated and dehydrated samples over the temperature range from -100 to 280 °C, and frequency range from 20 to 1 MHz. Temperature scans of hydrated samples show three relaxation peaks, including β- and α-relaxations, related to bound water and to the glass transition. A new third peak, denoted as α', was seen in hydrated sample at around 60 °C, and its intensity increases with decreasing frequency. On the other hand, in the completely dehydrated sample, the β- and α'-relaxation peaks both disappeared, which reveals their origin from bound water molecules. The α' process is attributed to the removal of bound water, after which the glass transition of dehydrated silk appears at higher temperature as the α process. Real-time DRS has also been performed to monitor isothermal crystallization. Both the dielectric constant, ε', and conductivity, σ, decrease gradually as the crystallization proceeds. Analysis of dielectric modulus shows that both conductivity and the α-relaxation are observed at the beginning of crystallization. As the crystal grows, the α-relaxation starts gradually to diminish both in strength and in rate. Before crystallization, α-helices and random coils with dipole moments are the major components in silk fibroin. During crystallization, α-helices can be transformed into antiparallel β-sheets, which possess no dipole moment, causing the decreasing trend in the dielectric parameters as crystallization proceeds.