Reactivity of functional amino acid residues in silk fibroin of philosamia cynthia ricini towards monoepoxides.

Abstract

The heterogeneous addition reaction of the silk fibroin fibers of Philosamia cynthia ricini with various monoepoxides was investigated in ethanol and cyclohexane at 45-75°C after the fibers were padded with aqueous solutions of various salts as catalysts. The reactivity of the main functional amino acid residues was found to decrease in the order: histidine _??_ tyrosine _??_ arginine. The concentration of the aqueous salt solutions influenced the reactions of tyrosine and arginine, but did not affect largely those of histidine and lysine. The reaction rate of tyrosine was discussed in terms of the nucleophilicity of the anions of the catalyst. The reactions of tyrosine and histidine depended on both of the polar and steric factors of the substituents of the epoxides. Their effect was elucidated by the modified Taft equation. The reactivity difference of tyrosine, arginine, and histidine between Philosamia cynthia ricini and Bombyx mori silk fibroins towards the epoxide was partly explained with swellability and intermicellar distance of the fibroin fibers and the amino acid composition of the fibers.