Influence of Casting Temperature on the Near-Surface Structure and Wettability of Cast Silk Fibroin Films

The near-surface structure and the wettability of silk fibroin films cast from aqueous solutions on hydrophobic polystyrene substrates at various temperatures is investigated by Fourier transform infrared attenuated total reflection spectroscopy (FTIR-ATR) and measurement of contact angle. The FTIR data reveal that the near-surface region of the films is enriched in random coil conformations of the protein at the expense of a reduced fraction of α-helix and β-sheet conformations. The relative random coil/β-sheet content shows a marked dependence on the casting temperature, displaying a minimum at 50 °C. The minimum occurs concurrently with a maximum in the wettability of film surfaces by polar liquids. In the lower wettability region, the film surfaces of this hydrophilic protein are hydrophobic, whereas in the enhanced wettability range they are slightly hydrophilic. The experimental data indicate that during formation of fibron films, α-helix and β-sheet structures are rejected by the interface because of their non-surface-active character, whereas random coils are energetically favored because at the interface they convert into a surface-active conformation which effectively minimizes the interfacial free energy and renders the polymer surface hydrophobic. In the narrow range of casting temperatures centered at 50 °C, the effect of the interface is overweighed by the bulk thermodynamics favoring the β-sheet crystallization of fibroin. Though the interfacial conformation is not accessible by FTIR-ATR, its surface-active character in combination with the unique composition and amino acid sequence of fibroin allows one to conclude that the possible chain structure is one that separates the hydrophobic alanine and hydrophilic serine residues to opposite sides of the plane passing through the chain axis.