Degradation pathways and mechanisms insight of indigo and shikonin with experiments and quantum chemical calculations

The silk relics have been severely faded after being discovered due to the degradation of dyes and pigments, which presents significant challenges to the identification and preservation of the silk relics. Herein, indigo and shikonin were chosen as the representative of natural dyes to look into the degradation pathways and mechanisms under UV and thermal exposure with experiments and quantum chemical calculations. The fading processes and degradation products of indigo and shikonin were analyzed by color metrics, UV-Vis, UPLC and UPLCHRMS, respectively. In total, eight compounds in indigo and five compounds in shikonin were identified. In addition, a quantum chemical calculation (QCC) was employed to investigate the molecular frontier orbitals, reaction sites, transition states and Gibbs free energy for interpreting the & BULL;OH-mediated oxidation of two dyes under UV exposure. These results help to give us insights into the degradation pathways and mechanisms of dyes, providing a theoretical basis for the identification and protection of silk cultural relics.