Circular dichroism, intrinsic fluorescence of protein and exogenous fluorescence probe of 8-anilino-1-naphtha- lenesulfonic acid hemimagnesium salt （ANS） was used to investigate the mechanism of conformational change of silk fibroin （SF） in aqueous alcohol including methanol and ethanol. The conformational transition of SF from random coil to β-sheet was found to be of a close relationship with the microstructure of the solvent. The alcohol-water mixture at low concentration had little effect on the solvation of the peptide unit, as the inherent water structure was conserved. At high alcohol concentration, the transition from the tetrahedral-like water structure to the chain-like alcohol structure in the mixtures induced a β-sheet conformation of SF, as a result of the formation of intramolecular hydrogen bond between the peptide units in order to eliminate the thermodynamic unfavorite from the contact to the solvent molecules. Meanwhile, the aggregating of hydrophobic side chains was decreased by the alcohol via the destruction of hydrogen bond network of water by alcohol and the binding of alcohol to hydrophobic group.
