Online DPPH Assay Coupled with LC-QTOF-MS for Rapid Identification of Antioxidant Constituents from Zelkova serrata

Zelkova serrata (Thunb.) Makino is a deciduous tree commonly distributed in East Asia. Several investigations have documented the biological activity of extracts of this species, igniting increased interest in its therapeutic potential. However, the phytochemical constituents accountable for these activities remain largely unexplored. In this study, we focused on the antioxidant potential of the leaves and twigs, utilizing bioactivity-guided fractionation combined with LC-QTOF-MS online DPPH screening to systematically identify the active compounds. This approach led to the isolation of eight compounds, three of which (compounds 1, 3, and 7) have not previously been documented in the genus Zelkova. Among the isolates, 4 emerged as the most potent antioxidant, exhibiting significant radical scavenging activity in both DPPH (IC₅₀ = 13.67 ± 1.21 μM) and ABTS (IC₅₀ = 2.69 ± 0.48 μM) assays, exceeding ascorbic acid (IC₅₀ = 61.51 ± 10.22 μM) and trolox (IC₅₀ = 8.22 ± 2.16 μM), respectively. Our findings not only enrich the phytochemical profile of Z. serrata but also highlight the effectiveness of LC-MS-DPPH as an effective approach for rapid antioxidant discovery.