Thermal and Structural Properties of Silk Biomaterials Plasticized by Glycerol

Abstract

The molecular interactions of silk materials plasticized using glycerol were studied, as these materials provide options for biodegradable and flexible protein-based systems. Plasticizer interactions with silk were analyzed by thermal, spectroscopic, and solid-state NMR analyses. Spectroscopic analysis implied that glycerol was hydrogen bonded to the peptide matrix, but may be displaced with polar solvents. Solid-state NMR indicated that glycerol induced β-sheet formation in the dried silk materials, but not to the extent of methanol treatment. Fast scanning calorimetry suggested that β-sheet crystal formation in silk-glycerol films appeared to be less organized than in the methanol treated silk films. We propose that glycerol may be simultaneously inducing and interfering with β-sheet formation in silk materials, causing some improper folding that results in less-organized silk II structures even after the glycerol is removed. This difference, along with trace residual glycerol, allows glycerol extracted silk materials to retain more flexibility than methanol processed versions.

Experimental Methods for Characterizing the Secondary Structure and Thermal Properties of Silk Proteins

“…[4,20] More recently, 13 C-CP/MAS NMR was used to study the effect of plasticizers on silk, demonstrating that the addition of 30% glycerol to silk fibroin films resulted in a β-sheet fraction between that of untreated silk films and methanol treated silk films. [50] …”

“…Results revealed that the silk films caused a shift in the water and glycerol evaporation peaks, suggesting that the silk provides some stabilization (Figure 11). [50] TGA analysis has also been used in the study of spider silk supercontraction, a phenomena where fibers contract up to 50% of their length upon exposure to water. [115,116] Dragline silk from N. clavipes spiders was tested both in the native form and after supercontraction in water; based on TGA results, it was concluded that supercontraction altered the molecular organization of the fibers.…”

Skin‐Friendly Electronics for Acquiring Human Physiological Signatures

“…The chemistry of the water-triggered decomposition at the molecular level is shown Figure 3a. [38] Notably, compared with the soluble unplasticized initial hybrid copolymer, the increased crystalline domains render the GEPC film water-insoluble and greatly improve the mechanical and thermal robustness of GEPC film, while maintaining the outstanding transparency and biocompatibility, so that it is capable to interface with skin tissues and dope with other biological molecules. [36,37] It is hypothesized that the glycerol interaction occurs predominantly with serine and tyrosine residues due to their polar side chains.…”

“…In principle, glycerol molecules act as the plasticizer to replace the incorporated water in protein hydration and to form intensified hydrogen bonds with the peptide matrix, resulting in the initial stabilization of α-helical structures in the films, as opposed to random coil or β-sheet structures ( Figure S21, Supporting Information). [38] The water-triggered physical transiency is initiated by introducing large amount water into the GEPC matrix. [38] Notably, compared with the soluble unplasticized initial hybrid copolymer, the increased crystalline domains render the GEPC film water-insoluble and greatly improve the mechanical and thermal robustness of GEPC film, while maintaining the outstanding transparency and biocompatibility, so that it is capable to interface with skin tissues and dope with other biological molecules.…”

Solvent‐Free Plasticity and Programmable Mechanical Behaviors of Engineered Proteins

“…Typically, external stimuli to actively trigger such plasticity include optical‐, electrical‐, thermo‐ as well as solvent‐treatments . However, in biological systems, the applied inputs such as ultraviolet light, high temperature or organic solvents involved in the responsive processes may significantly destabilize sophisticated structures of biopolymers (e.g., helix of DNA or folding of proteins), leading to the impaired mechanical behaviors of biomaterials . Thus, it is challenging to realize high plasticity in biomacromolecular materials with preserved structural stability and functional integrity.…”