Seeking Solvation: Exploring the Role of Protein Hydration in Silk Gelation

In Situ Identification of Secondary Structures in Unpurified Bombyx mori Silk Fibrils Using Polarized Two-Dimensional Infrared Spectroscopy

The mechanical properties of biomaterials are dictated by the interactions and conformations of their building blocks, typically proteins. Although the macroscopic behavior of biomaterials is widely studied, our understanding of the underlying molecular properties is generally limited. Among the noninvasive and label-free methods to investigate molecular structures, infrared spectroscopy is one of the most commonly used tools because the absorption bands of amide groups strongly depend on protein secondary structure. However, spectral congestion usually complicates the analysis of the amide spectrum. Here, we apply polarized two-dimensional (2D) infrared spectroscopy (IR) to directly identify the protein secondary structures in native silk films cast from Bombyx mori silk feedstock. Without any additional peak fitting, we find that the initial effect of hydration is an increase of the random coil content at the expense of the helical content, while the β-sheet content is unchanged and only increases at a later stage. This paper demonstrates that 2D-IR can be a valuable tool for characterizing biomaterials.

Broadband Multidimensional Spectroscopy Identifies the Amide II Vibrations in Silkworm Films

We used two-dimensional infrared spectroscopy to disentangle the broad infrared band in the amide II vibrational regions of Bombyx mori native silk films, identifying the single amide II modes and correlating them to specific secondary structure. Amide I and amide II modes have a strong vibrational coupling, which manifests as cross-peaks in 2D infrared spectra with frequencies determined by both the amide I and amide II frequencies of the same secondary structure. By cross referencing with well-known amide I assignments, we determined that the amide II (N-H) absorbs at around 1552 and at 1530 cm^–1 for helical and β-sheet structures, respectively. We also observed a peak at 1517 cm⁻¹ that could not be easily assigned to an amide II mode, and instead we tentatively assigned it to a Tyrosine sidechain. These results stand in contrast with previous findings from linear infrared spectroscopy, highlighting the ability of multidimensional spectroscopy for untangling convoluted spectra, and suggesting the need for caution when assigning silk amide II spectra.

Insignificant Difference in Biocompatibility of Regenerated Silk Fibroin Prepared with Ternary Reagent Compared with Regenerated Silk Fibroin Prepared with Lithium Bromide

Bombyx mori silk fibroin (SF) is widely used in the field of biomaterials due to its excellent biocompatibility and mechanical properties. However, SF cannot be used directly in many applications and needs to be dissolved first. Lithium bromide (LiBr) is a traditional solvent which is usually used to dissolve SF. However, LiBr has several limitations, e.g., it is expensive, it is toxic to organisms, and it is environmentally unfriendly. Herein, we investigate the possibility of developing a ternary reagent system that is inexpensive, non-toxic to organisms, and environmentally friendly as an alternative for silk fibroin solubilization. The results confirm that regenerated silk fibroin (RSF) prepared using a ternary reagent has the same morphology and amino acid composition as that prepared using LiBr, but the RSF prepared using a ternary reagent still had a small amount of calcium residue even after long-term dialysis. Further research found that the residual calcium does not cause significant differences in the structure and biological performance of the RSF, such as its cytotoxicity, blood compatibility, and antibacterial properties. Therefore, we believe that ternary reagents are an ideal alternative solvent for dissolving SF.