Orientation Distribution\nof Crystalline β‑Sheet\nDomains in Bombyx mori Silk Fiber Studied\nwith Vibrational Sum Frequency Generation Spectroscopy

Silk fibers have good biocompatibility and mechanical\nproperties,\nwhich make them attractive in biomaterial applications as well as\ntextile industries. It is believed that the superior mechanical property\nis associated with the crystalline β-sheet structure in the\nfiber; but a deeper understanding of the structure–property\nrelationship is still needed for full exploitation of its physical\nproperties. Especially, accurate information on hydrogen-bonding interactions\nwithin β-sheet domains at the nanoscale and their spatial distributions\nat the mesoscale are critically needed. In this study, we demonstrate\nthe selective detection of crystalline β-sheet domains in Bombyx mori silk fiber using sum frequency generation\n(SFG) spectroscopy and its use to determine the angular distribution\nof the β-sheet crystallites with respect to the fiber axis.\nNumerical simulations of the SFG signal of the amide-I band were carried\nout using tensors based on the B2 symmetry of the D<sub>2</sub> point\ngroup and compared with experimental data. This comparison found that\nthe crystalline β-sheet domains are aligned along the fiber\naxis with a standard deviation of ∼27° and parallel to\nthe fiber surface with a standard deviation of ∼5°. It\nwas also found that the amide bands in the SFG spectra cannot be fully\nexplained with the assumption that the crystalline β-sheet vibrations\ncan be described with the D<sub>2</sub> point group. Being able to\nmonitor the amide group vibrations sensitive to both interchain hydrogen\nbonding and crystallite orientations, SFG analysis has a potential\nto unveil the structure–mechanical property relationship that\nmay not be readily assessable with other characterization techniques.