Comparative study of silk residue extraction agents: Evaluating efficiency and investigating structural impact

• Silk residue extractants was compared, vital for optimizing protein recovery and identification. • Calcium-alcohol solution is more efficient than other reagents for non-aged and aged silk residues. • The calcium-alcohol extraction process maintains a more favorable protein conformation. This paper investigates the effectiveness of five different types of extractants for unearthed silk fabric residues. Residual proteins are extracted from non-aged and thermally-aged samples, and the extraction efficiencies are quantified by the bicinchoninic acid (BCA) method. Multivariate analysis of variance (MANOVA) is used for data processing, and sodium dodecyl sulfate - polyacrylamide gel electrophoresis (SDS-PAGE), ultraviolet-visible (UV-vis) absorption spectroscopy, and circular dichroism (CD) spectroscopy are adopted to characterize the influence of the extraction process on the protein structure. The results show that the extraction efficiency of the calcium-alcohol solution for all samples is significantly higher than that of the other four extractants. With longer aging time, the extraction efficiency of each extractant becomes lower, with copper-ethylenediamine (CED) being the least affected by aging. Furthermore, the lithium bromide (LiBr) and 1-allyl-3-methylimidazolium chloride (AmimCl) extraction processes exhibit the least effect on the molecular weight of the residual proteins in the non-aged samples. With increasing aging time, the influence of each extractant on the molecular weight of the residual proteins tends to be the same. The extraction process with calcium-alcohol solution retains more aromatic amino acid residues and yields proteins with a higher percentage of α-helic and random coil conformations. Therefore, it is more advantageous to use calcium-alcohol solution as an extractant for unearthed silk fabric residues.