A reductive natural deep eutectic solvent based on citric acid and l-cysteine for extracting keratin from wool

Keratin, a naturally occurring polymer material, is primarily derived from protein-based biomass such as hair, wool, and feathers. The development of sustainable dissolution technologies is critical for achieving the high-value applications of keratin-rich biomass. This study reports a reductive natural deep eutectic solvent (NADES) system composed of citric acid (CA) and l-Cysteine (l-Cys) for efficiently extracting keratin from wool. Molecular dynamics (MD) simulations revealed key interactions between the solvent components, while experimental evaluations assessed wool dissolution efficiency, keratin yield, and the properties of regenerated keratin. Notably, a 2:1 M ratio of CA to l-Cys enabled complete wool dissolution at 90 °C within 3 h, yielding ∼76 % keratin, comprising 58 % water-insoluble keratin (WIK) and 18 % water-soluble keratin (WSK). SDS-PAGE analysis showed a broad molecular weight distribution for WIK, whereas WSK was primarily below 20 kDa. In situ experiments showed that the optimal solvent system can completely destroy the wool structure at around 1.5 h. X-ray diffraction (XRD) results confirmed a pronounced reduction in the α-helix content of WSK relative to natural wool and WIK. Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and elemental analysis indicated trace CA and l-Cys residues in WSK, which nonetheless exhibited good cytocompatibility. These findings demonstrate a simple, green, and efficient approach for extracting keratin from wool, providing a viable pathway for upcycling keratin-rich waste biomass.