Salt-catalyzed reaction between styrene oxide and silk fibroin

The addition reaction of styrene oxide (StO) with silk fibroin was studied in the presence of various salts in different solvents at 45–75°C. Some water was required to make StO react with silk padded with various salt solutions. The reaction rate increased with the salt concentration and reached a maximum value at a certain concentration of the salt. Padding with solutions of thiosulfate, cyanide, thiocyanate, bicarbonate, or carbonate resulted in high add-ons (to 65 mole/10⁵ g) and low solubilities in HCl and NaOH aqueous solutions. The weight gains increased with the epoxide concentration and reached a constant value at a certain concentration of StO solution in ethanol, while they decreased slightly with epoxide concentration over 10% of StO solution in n-hexane. Histidine, lysine, arginine, tyrosine, and aspartic and glutamic acids were found to react. The reaction rate decreased with increasing solubility parameter of the solvent used, reached a minimum value about at 10 or at the solubility parameter of the epoxide, and then increased with the parameter. The StO–silk reaction may depend on the distribution of StO between aqueous salt and an organic solvent phases, and on the swelling of silk fiber in different aqueous salt solutions or in various organic solvents. The mechanism for this epoxide-silk reaction and the reactivity difference between StO and phenyl glycidyl ether toward silk fibroin are discussed in the light of the observed phenomena.