Deep eutectic solvent-mediated extraction of collagen peptides from sturgeon skin: Molecular-weight control and...

A choline-oxalic acid deep eutectic solvent (DES) was developed for the molecular-weight-controlled extraction of collagen peptides from sturgeon (Acipenser spp.) skin, a collagen-rich fish-processing by-product containing 78.28% protein and 63.47% collagen. Among three choline-based organic-acid DESs, choline-oxalic acid exhibited the best extraction performance and produced lower-molecular-weight peptide fractions than choline-lactic acid and choline-acetic acid systems. Under the optimized conditions of 70 °C, a solid-to-liquid ratio of 1:80, and 2 h, the extraction rate reached 98.45%. Time-dependent extraction enabled controllable peptide production: a 2 h treatment mainly generated collagen polypeptides in the 1-10 kDa range, whereas extending the reaction to 4 h produced oligopeptides predominantly in the 0.5-1 kDa range. For downstream recovery, isopropanol showed the highest precipitation efficiency, achieving approximately 92% peptide recovery at an extract-to-solvent ratio of 1:6. UV-Vis analysis showed a characteristic collagen peptide absorption band near 230 nm, while FT-IR spectra displayed typical amide bands, including amide I at approximately 1630 cm^-1 and amide III at 1240-1300 cm^-1, confirming the collagen-derived nature of the products. Molecular simulation further suggested that oxalic acid competed for backbone hydrogen bonding, while chloride ions interacted with hydroxyproline residues, as indicated by a Hyp-Cl^- radial distribution peak at 0.328 nm, promoting collagen swelling, triple-helix loosening, and controlled depolymerization. Overall, this DES-based strategy provides a rapid, tunable, and potentially sustainable route for producing collagen peptides from aquatic by-products.