Antioxidant and LC-QToF-MS/MS analysis of polyphenols in polar and non-polar extracts from Strobilanthes crispus and Clinacanthus nutans

Strobilanthes crispus and Clinacanthus nutans are popular herbal plants in the Southeast Asian region. The present work was aimed at determining the antioxidant activities and the associated components in the leaf extracts of both species using polar and non-polar solvents namely water, methanol, ethyl acetate, and hexane. The total phenolic content (TPC) and total flavonoid content (TFC) were higher in the leaf extracts of S. crispus as compared to C. nutans. Among the solvents, methanol was the best solvent in extracting the antioxidant components for S. crispus (TPC: 159.85 ± 0.89 mg GAE/g extract and TFC: 955.47 ± 2.66 mg RE/g extract). However, for C. nutans, its methanolic extract yielded the highest TPC (36.39 ± 0.17 mg GAE/g extract), whereas ethyl acetate yielded the highest TFC (229.61 ± 7.81 mg RE/g extract). The high levels of both TPC and TFC contributed to the antioxidant activities of S. crispus extract as reflected in the methanolic extract attaining the highest level of antioxidant activities, measured by ferric reducing antioxidant power (FRAP) (6.84 ± 1.12 mmol Fe²⁺/g extract), DPPH radical scavenging (IC₅₀: 203.60 ± 7.28 μg/mL), and Trolox equivalent antioxidant capacity (TEAC) (1.01 ± 0.01 mmol TE/g extract) assays. This contrasted with C. nutans which showed lower antioxidant activities owing to its lower TPC and TFC. Correlation analysis revealed significant correlations (p < 0.05, r = 0.915 - 0.985) between both TPC and TFC in S. crispus and antioxidant activities. However, only TPC of C. nutans showed a significant correlation with FRAP values (r = 0.934). Further tentative identification of the constituents in the extracts using HPLC-ESI-QToF-MS/MS revealed the existence of 20 polyphenolic compounds in both S. crispus and C. nutans, which were likely responsible for their antioxidant activities. In addition, 15 polyphenolic compounds classified as chalcones, isoflavanoids, flavones, and flavonols have not been previously reported in both species. The methanolic extracts of both species yielded a higher content of antioxidants, with S. crispus offering a richer source of dietary antioxidants as compared to C. nutans. However, further study is needed to identify their bioactivities in relation to their bioactive components.