Structural analysis of spider silk films

Due to their outstanding mechanical properties, spider silks have fascinated men for a long time. Silk is composed of proteins which can not only be processed into fibers, as found in nature, but also be cast to form films in vitro. Starting with a protein solution in hexafluoroisopropanol, we were able to cast films with different properties deviated from the two spider silk proteins employed. All as-cast films revealed an alpha-helixrich structure and were water soluble. However, the secondary structure of One-protein films (silk films cast from one spider silk protein) could be converted from an alpha-helical rich to a beta-sheet rich secondary structure by post-cast treatment with methanol or potassium phosphate. The structural conversion was accompanied by a higher stability as seen by water-insolubility. Depending on the employed proteins, silk films were stable in protein denaturants such as urea and guanidinium hydrochloride. Strikingly, Two-protein films (silk films cast from a mixture of both spider silk proteins) showed properties derived from both proteins, indicating that the process of film casting based on silk proteins is closely related to film casting of traditional chemical polymers. Our results reveal novel possibilities to generate protein films for applications that demand stable biocompatible polymer films.