Spectrum–Effect Relationships Guided Discovery of Antioxidant Q‐Markers in Two Mature Stages of <i>Chimonanthus salicifolius</i> Leaves

Chimonanthus salicifolius leaves (CSL), a traditional She medicine, are used both as food and medicine. Conventionally, tender leaves are used as tea beverages, while mature leaves serve as medicinal materials. However, their difference in chemistry and activity remains unknown. In this study, HPLC fingerprints of 20 batches of both tender and mature CSL were established, revealing a shared metabolite backbone, yet hierarchical cluster analysis clearly split the two stages into discrete chemotypes. DPPH and ABTS assays demonstrated that mature leaves possessed significantly higher antioxidant capacity than tender leaves. Integration of Pearson correlation analysis and partial least squares regression analysis uncovered the spectrum-bioactivity relationship, guiding three flavonoid glycosides, isoquercitrin (IQ), kaempferol-7-O-rutinoside, and nicotiflorin, as antioxidant quality markers (Q-markers). Moreover, the quantitation of Q-markers illustrated that IQ emerged as the pivotal differential compound: Its abundance in mature leaves was 1.7-fold higher than in tender leaves, whereas the content of the three glycosides was nearly twice as high in tender leaves. The established strategy for identifying antioxidant Q-markers, based on HPLC fingerprinting and spectrum-effect relationships, effectively reveals quality differences between mature and tender leaves and provides a robust scientific basis for the tailored exploitation of CSL in both medicinal and nutraceutical applications.