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.