Structural changes of thin films from recombinant spider silk proteins upon post-treatment

Engineering of spider silk proteins offers the possibility to control their molecular sequence and thus their material properties. Spin coating was used to prepare films of engineered spider silk protein derived from the garden spider's (Araneus diadematus) dragline silk protein ADF-4. A conformational transition from α-helix to β-sheet-rich structures upon methanol treatment of the films was detected by external reflection IR spectroscopy. We present direct evidence for this structural transformation using grazing-incidence X-ray diffraction (GIXRD) and small-angle scattering (GISAXS). The protein film structure after the methanol treatment consists mainly of β-sheet polyalanine crystals dispersed in an amorphous protein matrix. The GIXRD intensity profiles show Bragg peaks from β-sheet polyalanine crystallites having an average size of 7.5 nm. The non-uniform and large crystal size distributions within the film were explained based on the protein composition. The effect of the chemical nature of the interface on the protein film structure was investigated as well.