On the Nature of Fluorescence Modification Induced by Deformation in Regenerated Silk Fibroin

The mechanical deformation of regenerated silk fibroin (RSF) films induces notable changes in their fluorescence properties, yet the underlying molecular mechanisms remain poorly understood. In this work, we investigate the interplay between mechanical, structural, and optical response of RSF films processed with glycerol (gly) and with polyethylene glycol (PEG) as a plasticizer. PEG-containing films exhibited enhanced ductility, a significant increase of the absorption coefficient in the UV spectral range and narrower fluorescence spectra, with reduced intensity at longer wavelengths, suggesting fewer cross-links. These differences in the mechanical and optical properties of the investigated polymers may be due to the higher amorphous phase content of the samples with PEG, as it is revealed by wide-angle X-ray scattering (WAXS). Ab initio calculations of model systems confirmed that fluorescence, primarily due to tryptophan residues, is highly sensitive to local conformation and hydrogen-bonding interactions. To elucidate the effect of deformation on silk fibroin fluorescence, WAXS patterns, using synchrotron radiation, and fluorescence spectra were acquired while simultaneous tensile measurements were performed. Notably, strain-induced backbone alignment favors the formation of electronically coupled cross-links, which give rise to the observed changes in fluorescence. These findings provide further insights into the structure-property relationships governing RSF fluorescence, with potential implications for the design of deformation-responsive biomaterials.