Stretching-Induced Conformational Transition of the Crystalline and Noncrystalline Domains of ¹³C-Labeled <i>Bombyx mori</i> Silk Fibroin Monitored by Solid State NMR

Abstract

It is important to understand the conformational changes of both the crystalline and the noncrystalline domains of Bombyx mori silk fibroin as they are stretched from soluble Silk I to the mature Silk II fiber, to be able to design man-made strong and tough silk fibers. Here, the Ser, Tyr, and Ala Cβ of silk fibroin were labeled by 13C to obtain domain-specific structural information, because Ser residues are present predominantly in the crystalline domains, Tyr predominantly in the noncrystalline domains and Ala residues in both domains. The conformational change was monitored by 13C CP/MAS NMR using the conformation-dependent 13C NMR chemical shift. With increasing stretching ratio, initially there is a change from Silk I* (a repeated type II β-turn structure) to β-sheet in the longer AGSGAG sequences, followed by shorter AGSGAG sequences, while the noncrystalline domains change gradually from random coil to β-sheet during the whole conformational change.