Optimization of Graptophyllum pictum leaf polyphenols via microwave extraction and antioxidant properties

Abstract

Graptophyllum pictum (L.) Griff. leaves are traditionally used for various ailments and represent a promising source of antioxidant phenolic and flavonoid compounds. However, optimization of microwave-assisted extraction (MAE) to maximize total phenolic content (TPC) and total flavonoid content (TFC) remains limited. This study optimized MAE parameters using response surface methodology with a Box–Behnken design and compared the optimized extract (OEGP) with a conventional reflux extract (EGP). The optimized MAE conditions demonstrated good agreement between predicted and experimental responses, confirming the validity of the developed model under optimal MAE conditions and yielding significantly higher TPC (151.18 ± 1.09 mg GAE/g) and TFC (55.90 ± 1.52 mg QE/g) than reflux extraction (TPC 90.57 ± 1.67 mg GAE/g; TFC 26.09 ± 1.12 mg QE/g). Antioxidant assays demonstrated concentration-dependent activity (31.25–500 µg/mL), with OEGP consistently outperforming EGP. At 500 µg/mL, OEGP achieved 91.49 ± 1.00% DPPH scavenging, comparable to Trolox (p = 0.1102), while ABTS and H₂O₂ scavenging reached 90.55 ± 0.21% and 81.35 ± 0.94%, respectively. LC–HRMS analysis tentatively identified several phenolic acids and flavonoids based on database matching, without authentic standards, including cinnamic, p-coumaric, ferulic, vanillic, gentisic, and salicylic acids, as well as kaempferol, apigenin, cynaroside, apigetrin, and neodiosmin, supporting the enhanced antioxidant capacity. Overall, MAE optimized by RSM–BBD significantly improved phytochemical recovery and antioxidant activity, highlighting G. pictum leaves as a valuable source of bioactive compounds for further pharmacological investigation.