Abstract
Silk fibroin from Bombyx mori caterpillar is an outstanding biocompatible polymer for the production of biomaterials. Its impressive combination of strength, flexibility, and degradability are related to the protein’s secondary structure, which may be altered during the manufacture of the biomaterial. The present study looks at the silk fibroin secondary structure during nanoparticle production using ionic liquids and high-power ultrasound using novel infrared spectroscopic approaches. The infrared spectrum of silk fibroin fibers shows that they are composed of 58% β-sheet, 9% turns, and 33% irregular and/or turn-like structures. When fibroin was dissolved in ionic liquids, its amide I band resembled that of soluble silk and no β-sheet absorption was detected. Silk fibroin nanoparticles regenerated from the ionic liquid solution exhibited an amide I band that resembled that of the silk fibers but had a reduced β-sheet content and a corresponding higher content of turns, suggesting an incomplete turn-to-sheet transition during the regeneration process. Both the analysis of the experimental infrared spectrum and spectrum calculations suggest a particular type of β-sheet structure that was involved in this deficiency, whereas the two other types of β-sheet structure found in silk fibroin fibers were readily formed.
Highlights
Silk fibroin (SF) from the silkworm Bombyx mori is a fibrous protein that presents outstanding mechanical properties, which are still hard to imitate by human-made synthetic polymers [1].The protein is produced and stored in the posterior and middle silk gland as a semi-liquid gel, in which the protein is in a water-soluble state [2] with a partially ordered structure [3], commonly known as silk I
The transition from silk I to silk II is achievable in the laboratory and the resulting regenerated silk can be used to produce many kinds of bioengineered materials [7,8] including but not limited to, bone fracture fixation devices [9], scaffolds for tissue engineering [10,11,12], biosensor [13,14], and nanoparticles for drug delivery applications [15,16]
The later has attracted considerable attention due to the SF nanoparticles’ (SFN) ability to load a wide variety of therapeutic compounds [17], Polymers 2020, 12, 1294; doi:10.3390/polym12061294
Summary
Silk fibroin (SF) from the silkworm Bombyx mori is a fibrous protein that presents outstanding mechanical properties, which are still hard to imitate by human-made synthetic polymers [1]. Cryo-electron microscopy is a relatively new and powerful technique for structural determination but does not seem to have been used to obtain atomic resolutions structures of silk These methods are costly, labor-intensive, and the proteins of interest cannot be studied under all the conditions that might be of technological interest. In addition to the experiments, calculations were carried out to simulate the infrared spectrum of different β-sheet structures within SF to confirm the band assignment Using this approach, we studied the secondary structure of SF in fiber state (silk II) dissolved in the ionic liquid 1-ethyl-3-methylimidazolium acetate (EmimAc), and after regeneration into nanoparticle form (SFNP).
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