Proteomic Profiling of the Photo-Oxidation of Silk Fibroin: Implications for Historic Tin-Weighted Silk

The stability of silk proteins to ultraviolet light is an issue of significant concern in both the appearance retention of silk‐derived products and the preservation of historic silk textiles. Until now, evaluation of silk degradation has only been performed at the holistic, rather than molecular level. This article describes the first proteomic profiling of silk photo‐oxidation, characterizing protein primary level modification leading to coloration changes, and evaluating the effects of tin weighting on photodegradation. Heavy‐chain fibroin, the main proteinaceous component of the silk thread, is a repetitive, highly crystalline protein with a content rich in tyrosine. Photoproducts of tyrosine were characterized and the levels of oxidative modification at the protein primary structural level correlated with changes in coloration and tensile strength. The effect of tin as a weighting agent used on historical fabrics was examined. Tin‐weighted fabrics were evaluated following two treatments (pink and dynamite) and proteomic analysis revealed a significant increase in oxidatively modified amino acid residues within the pink‐treated silk. These findings offer new insight into the molecular‐level oxidation of silk proteins under UV exposure, and the effects of silk treatments in either exacerbating or ameliorating this degradation. Exposure of silk to UV light affects physical properties of the fabric, such as strength and elasticity and causes yellowing. Photoproducts of tyrosine are characterized and the extend of photodegradation is correlated with changes in coloration by assigning each individual observed oxidative modification within the peptide a score based on the relative level of the modification. Tin weighting was a common method used from the late 18th century to the early 20th century for replacing weight lost during degumming of silk. It causes fibrillation and fractures and intensifies yellowing under UV irradiation.