Changes in the structure and thermal properties of feather keratin induced by L-Cys extraction followed by enzymatic hydrolysis

Feather waste was valorised into bioactive peptides by extracting high-purity L-Cys-urea solution, followed by enzymatic hydrolysis with commercial proteases selected for their catalytic efficiency. The extraction reduced the α-helix content in favor of the β-sheet leading to less ordered crystal structures as ATR FT-IR and the XRD analyses revealed. These structural changes decreased keratin's thermal stability, resulting in its slower yet more gradual decomposition, as TGA and DSC results showed. The hydrolysis with trypsin, chymotrypsin, pepsin and subtilisin altered the secondary structure and thermal stability of extracted keratin, with changes depending on enzyme specificity. All hydrolysates showed the preserved amide features but reduced α-helix and β-sheet content and crystallinity. Subtilisin induced the most profound structural and molecular transformations, including the secondary structure loss, carboxylate group formation, and short peptide fragment generation, resulting in reduced structural order but enhanced thermal stability attributed to the stabilizing effect of sodium carboxylates.