On the Secondary Structure of Silk Fibroin Nanoparticles Obtained Using Ionic Liquids: An Infrared Spectroscopy Study

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.

A Fast and Reliable Process to Fabricate Regenerated Silk Fibroin Solution from Degummed Silk in 4 Hours

“…This is observed when antiparallel beta-sheets are present. The bands that characterize the amide I band profile of silk fibroin polymorphs have been described elsewhere [ 15 ]. All three regenerated silk fibroin materials show enhanced random coil structures around 1640 cm −1 to 1625 cm −1 ( Figure 7 ).…”

“…This dissolution step is inhibited by the strength of the hydrogen bonds and the hydrophobic nature of the β-sheet crystallites of silk fibroin. Therefore, aqueous or organic salt containing systems with high ionic strength are most suitable for complete dissolution of silk fibroin such as calcium chloride/formic acid (CaCl 2 /FA) [ 12 ], lithium salt solutions such as lithium thiocyanate (LiSCN) [ 3 ], and lithium bromide (LiBr-H 2 O) [ 7 ], calcium nitrate/methanol (Ca(NO 3 ) 2 )/CH 3 OH) mixtures [ 13 ], N-methylmorpholine-N-oxide (NMMO) [ 14 ], ionic liquids [ 15 , 16 , 17 ] and the so-called Ajisawa’s reagent consisting of calcium chloride/water/ethanol (CaCl 2 /H 2 O/C 2 H 5 OH) [ 18 ]. The most used solvent systems for degummed silk fibroin fibers in the literature are 9.3 M LiBr-H 2 O and Ajisawa’s ternary solvent system.…”

Structure of Silk I (Bombyx mori Silk Fibroin before Spinning) -Type II β-Turn, Not α-Helix-

“…So far, many papers, including this one, assigned the silk I structures using IR and Raman spectroscopies by carefully considering type II β-turn model [ 52 , 53 , 100 , 101 , 102 , 103 , 104 , 105 , 106 , 107 , 108 ]. For example, Monti et al [ 52 , 104 ] assigned the strong amide I, II, II, and V bands fell at 1654, 1540, 1240, and 660 cm −1 to type II β-turn structure in the IR spectrum of (AG) 15 with silk I form.…”

Direct Quantification of Drug Loading Content in Polymeric Nanoparticles by Infrared Spectroscopy

“…The silk fibroin (SF) used in this work was extracted from Bombyx mori silk cocoons kindly provided by Instituto Murciano de investigación y Desarrollo Agrario y Alimentario (Murcia, Spain). The SF was purified as reported elsewhere [ 47 ]. In brief, cocoons were shredded in a mill and filter through a 1 mm stainless still mesh and later boiled for 2 h in a 0.2 N Na 2 CO 3 solution to remove sericin, waxes and other impurities.…”