Silk proteins in archaeological soils: mass transfer, structural evolution, and an identification framework

This study investigates the retention behavior of silk-derived proteins in archaeological soils and seeks to establish an analytical framework for the identification of silk residues preserved in burial environments. Silk fabric ageing and degradation were simulated using soil collected from the Tianzifen Site in Wulong, Chongqing. A multi-dimensional analytical framework combining multiple testing methods was used to characterize the transfer and preservation of silk proteins in soil. The results showed that silk degradation followed a clear staged trajectory. In the intermediate stage, fibers exhibited axial splitting and fibrillation with deposition of Al, Si and Ca. At the late stage, the fabric structure became visibly loosened, with extensive fiber fracture, and the mass loss rate reached 24.84%. At the molecular level, the relative proportion of β-sheet structures first increased and then decreased, reflecting sequential degradation of amorphous and crystalline regions. Soil protein concentration rose markedly with the continuous release of silk-derived proteins. Proteomic analysis identified two intrinsic silk protein components, the fibroin heavy chain and fibroin light chain. The fibroin heavy chain showed greater detection continuity across the ageing series, whereas the light chain was detected less consistently. The peptide "DASGAVIEEQITTK" was identified as a promising characteristic peptide for silk residue identification. This study extends silk protein identification from the textile itself to archaeological soils, providing a new technical pathway for exploring the origin, transmission, and technological development of silk production.