RHODAMINE 6G BINDS TO SILK FIBROIN AND INHIBITS ITS SELF-AGGREGATION

Amyloid structures are universal structures, widely diffuse in nature. Silk, capable of forming some of the strongest tensile materials on earth represents an important example of formation of functional amyloid fibrils, a process reminiscent of the oligomerization of peptides involved in neurodegenerative diseases. The stability of silk fibroin solutions in different conditions and its transition from alpha-helix/random coil to beta-sheet structures, at the basis of gelation processes and fibril formation, have been here investigated and monitored employing different biophysical approaches. Silk fibroin aggregation state as a function of concentration, pH and aging has been characterized employing NMR ordered diffusion spectroscopy. The change of silk fibroin diffusion coefficient over time, which reflects the progress of oligomerization, has been monitored for silk fibroin alone and in the presence of a polycondensed aromatic dye, namely rhodamine 6G. NMR, UV and DLS measurements indicated that rhodamine specifically binds to silk fibroin with a micromolar KD. The reported data reveal, for the first time, that RHD is capable of inhibiting fibroin self-association, thus controlling beta-conformational transition at the basis of fibril formation. The described approach could be extended to other amyloid forming proteins, allowing a better control of the oligomerization process. The development of nanoparticles as inhibitors of amyloid protein aggregation has recently gained attention. In this line, further investigations are presented on the capability of 5 nm anionic citrate-coated gold nanoparticles to modulate, at low substoichiometric ratios, silk fibroin beta-sheet driven oligomerisation.