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

Tetsuo Asakura,Misaki Hirayama,Yugo Tasei,Akihiro Aoki,Hiroki Arai,Masanori Endo

doi:10.3390/ijms17091517

Abstract

In order to improve the stiff and brittle characteristics of pure Bombyx mori (B. mori) silk fibroin (SF) film in the dry state, glycerin (Glyc) has been used as a plasticizer. However, there have been very limited studies on the structural characterization of the Glyc-blended SF film. In this study, 13C Cross Polarization/Magic Angle Spinning nuclear magnetic resonance (CP/MAS NMR) was used to monitor the conformational changes in the films by changing the Glyc concentration. The presence of only 5 wt % Glyc in the film induced a significant conformational change in SF where Silk I* (repeated type II β-turn and no α-helix) newly appeared. Upon further increase in Glyc concentration, the percentage of Silk I* increased linearly up to 9 wt % Glyc and then tended to be almost constant (30%). This value (30%) was the same as the fraction of Ala residue within the Silk I* form out of all Ala residues of SF present in B. mori mature silkworm. The 1H DQMAS NMR spectra of Glyc-blended SF films confirmed the appearance of Silk I* in the Glyc-blended SF film. A structural model of Glyc-SF complex including the Silk I* form was proposed with the guidance of the Molecular Dynamics (MD) simulation using 1H–1H distance constraints obtained from the 1H Double-Quantum Magic Angle Spinning (DQMAS) NMR spectra.

Highlights

Silk fibroin (SF) from Bombyx mori (B. mori) is a well-known and highly prized material for textiles
The 13C chemical shifts of random coil, Silk II and Silk I of Glyc-blended silk fibroin (SF) films are summarized in Table 1 together with 1H chemical shift data [32]
Without Glyc, the conformation of regenerated SF film was roughly random coil according to the Ala Cβ chemical shift of 16.5 ppm, there was a significant amount of β-sheet structure as mentioned below

Summary

Introduction

Silk fibroin (SF) from Bombyx mori (B. mori) is a well-known and highly prized material for textiles. SF has been used as a promising biomaterial because of the combination of high strength and toughness together with excellent biocompatibility [1,2,3,4,5]. SF film tends to become stiff and brittle in the dry state over time, exhibiting high tensile strength but low elongation [6]. Alcohols such as methanol have been widely used for the treatment of water-soluble SF, methanol induces further stiffness and reduces the biodegradability of SF [1,3,7]. These shortcomings hinder extensive use of SF in biomaterials

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Conclusion