Metal-assisted hydrogen-bond restructuring in a ternary deep eutectic solvent for near-complete keratin dissolution and circular biomass valorization

Abstract

Metal-assisted restructuring of hydrogen-bond networks within deep eutectic solvents (DESs) offers a promising route to energy-efficient biopolymer dissolution and extraction, yet the molecular principles governing such reactivity remain largely unexplored. Here, we investigate these mechanisms using chicken-feather keratin as a model highly crosslinked protein substrate and demonstrate that coordinated Al³⁺ species fundamentally reorganize the hydrogen-bond lattice of a ChCl–citric acid eutectic matrix. The introduction of AlCl₃·6H₂O generates a coupled Lewis-acid and proton-solvation environment that drives near-complete keratin dissolution (99.62%) under mild conditions (100 °C, 2 h). The optimized ternary formulation (1 : 5 : 1, ChCl–CA–ACH) with 10 wt% water provides an ideal balance of coordination strength and hydrogen-bond flexibility, lowering viscosity and melting point while maintaining solvent stability. Spectroscopic analyses and DFT/RDG–NCI calculations reveal that Al–citrate complexation enhances charge delocalization and selectively polarizes disulfide linkages, enabling S–S cleavage without peptide backbone degradation. The regenerated keratin preserves its β-sheet secondary structure and thermal resistance, confirming targeted depolymerization over nonspecific breakdown. The catalytic DES exhibits >97% extraction efficiency over three reuse cycles through a closed-loop, water-assisted regeneration protocol, highlighting its durability and low environmental burden. This work establishes a mechanistic framework for metal-assisted hydrogen-bond restructuring in DESs and introduces a reactive, self-recovering solvent platform for energy-efficient biopolymer deconstruction and circular biomass valorization.