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 f rom aqueous solutions on hydrophobic polystyrene substrates at various temp eratures is investigated by Fourier transform infrared attenuated total ref lection 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 alpha -helix and beta -sheet conformations. The relative random coil/beta - sheet content shows a marked dependence on the casting temperature, display ing a minimum at 50 degreesC. The minimum occurs concurrently with a maximu m in the wettability of film surfaces by polar liquids. In the lower wettab ility 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, alpha -helix and beta -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 confor mation which effectively minimizes the interfacial free energy and renders the polymer surface hydrophobic. In the narrow range of casting temperature s centered at 50 degreesC, the effect of the interface is overweighed by th e bulk thermodynamics favoring the beta -sheet crystallization of fibroin. Though the interfacial conformation is not accessible by FTIR-ATR, its surf ace-active character in combination with the unique composition and amino a cid sequence of fibroin allows one to conclude that the possible chain stru cture is one that separates the hydrophobic alanine and hydrophilic serine residues to opposite sides of the plane passing through the chain axis.