Stretching-Induced Conformational Transition of the Crystalline and Noncrystalline Domains of ¹³C-Labeled Bombyx mori 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 13 C 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 13 C CP/MAS NMR using the conformation-dependent 13 C 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.

Characterization of water in hydrated Bombyx mori silk fibroin fiber and films by 2H NMR relaxation and 13C solid state NMR

“…The 13 C- 1 H cross polarization (CP) /MAS NMR method has been usually used for the structural analysis of SF [10,11, 38]. However, the presence of water causes an increase in chain mobility and consequential loss in CP signal of the amino acid residues in the SF that are hydrated by water [7, 8].…”

Engineering the Future of Silk Materials through Advanced Manufacturing

“…Generally, silk molecular structure consists of highly repetitive amino acid sequences staggered by nonrepetitive regions. Repetitive regions form large and stable, crystalline β‐sheet domains, while nonrepetitive sequences fold in semiamorphous structures, including helices and coils . Depending on the relative amount and distribution of crystalline and amorphous regions, different silks possess unique properties.…”

Glycerin-Induced Conformational Changes in Bombyx mori Silk Fibroin Film Monitored by 13C CP/MAS NMR and 1H DQMAS NMR

“…Here, we start from the definition of Silk I* form is different from the Silk I structure; the details have been reported elsewhere [ 12 , 34 ]. Briefly, Silk I is defined as the solid state structure of SF stored in the middle silk glands after drying without any external forces.…”

“…It is a soluble form that remains stable and non-viscous up to high concentrations without precipitating, this presumably being essential for the secretion of mature silk fibers [ 6 , 35 , 36 ]. According to solid state NMR spectra, the solid state Silk I contains random coil regions, together with regions having a well-defined ordered structure [ 13 , 14 , 33 , 34 ]. These ordered regions are defined as Silk I* [ 12 , 34 ].…”