Silk Fibroin/Montmorillonite Nanocomposites: Effect of pH on the Conformational Transition and Clay Dispersion

Abstract

By adjusting the solution pH value below the isoelectric point (pI) of silk fibroin (SF) protein, the SF was in the cation state and it could interact strongly with unmodified anionic montmorillonite (MMT) surface. In this way, novel SF-MMT nanocomposites with good clay dispersion were successfully obtained, which were confirmed by X-ray diffraction and transmission electron microscopy. Further 1H CRAMPS and 13C CP/MAS NMR experimental results revealed that beta-sheet content of SF was remarkably enhanced for nanocomposite prepared below the pI of SF (SF-MMTA) due to the strong interaction between MMT and SF. In SF-MMTA nanocomposite, clay layers acting as an efficient nucleator could efficiently enhance the beta-sheet crystallization. On the contrary, SF preserved the native random coil conformation in SF-MMTN nanocomposites due to the weak interaction between MMT and SF. A tentative model was suggested and used to explain the mechanism of clay dispersion and conformational transition of silk protein.