Intrinsic fluorescence changes associated with the conformational state of silk fibroin in biomaterial matrices

Silk fibroin is emerging as an important biomaterial for tissue engineering applications. The ability to monitor non-invasively the structural conformation of silk matrices prior to and following cell seeding could provide important insights with regards to matrix remodeling and cell-matrix interactions that are critical for the functional development of silk-based engineered tissues. Thus, we examined the potential of intrinsic fluorescence as a tool for assessing the structural conformation of silk proteins. Specifically, we characterized the intrinsic fluorescence spectra of silk in solution, gel and scaffold configurations for excitation in the 250 to 335 nm range and emission from 265 to 600 nm. We have identified spectral components that are attributed to tyrosine, tryptophan and crosslinks based on their excitation-emission profiles. We have discovered significant spectral shifts in the emission profiles and relative contributions of these components among the silk solution, gel and scaffold samples that represent enhancements in the levels of crosslinking, hydrophobic and intermolecular interactions that are consistent with an increase in the levels of ss-sheet formation and stacking. This information can be easily utilized for the development of simple, non-invasive, ratiometric methods to assess and monitor the structural conformation of silk in engineered tissues.