Light can transform the secondary structure of silk protein

Fibroin is the main component of silk and is expected to be used as a novel functional material in medicine and bioelectronics. The main secondary structures of this protein are of the random-coil and the β-sheet types. In this study, we carried out laser-induced transformation of the secondary structure, from the random-coil type to the β-sheettype, in solid fibroin films. We prepared two types of fibroin films with the random-coil structure. One is a fibroin film doped with a dye as a photosensitizer with a small amount (1 wt %), and the other is a neat fibroin film. The former was excited at 532 nm and the latter was excited at 266 nm. Irradiations were carried out with fluences much lower than each ablation threshold. The excitation of the dye at 532 nm did not affect the secondary structure of the random-coil type. By contrast, 266-nm laser irradiation, which excites tryptophan (an amino-acid residue) involved in fibroin, created the β-sheetdomain in the film. The structural transformation was revealed by infrared absorption spectroscopy and atomic force microscopy.