2.4.2. ABTS Radical Scavenging Assay

The experiment was conducted as previously described [39]. In brief, 3.5 mM of potassium persulfate aqueous solution was used to prepare 0.2 mM of ABTS diammonium salt. The solution was diluted 10-fold using distilled water and allowed to produce ABTS radicals (ABTS⁺) by keeping the solution in the dark for 14 h at room temperature. Thereafter, 10 µL of sample (prepared in MeOH, extract 25–400 µg/mL, compounds 31.25–1000 µM) was mixed with 290 µL of ABTS⁺ solution in a 96-well plate and incubated in the dark for 10 min at 25 °C, which was followed by measuring the absorbance at 750 nm using the same microplate reader. Trolox, a reference antioxidant, was used to make a calibration curve derived from the ABTS⁺ scavenging activity (%) against the final concentrations of Trolox (1.04–16.67 µM), and the results (n ≥ 3) were presented as Trolox equivalent antioxidant capacity (TE, µM Trolox/µg extract or µM Trolox/µM compound). Equation (1) was used to calculate the ABTS⁺ scavenging activity (%), where A_control is the absorbance of ABTS⁺ solution free of samples, A_sample is the absorbance of ABTS⁺ solution incubated with a sample, A_blank1 is the absorbance of the test sample free of ABTS⁺ solution, and A_blank2 is the absorbance of the diluted potassium persulfate solution.