Purpose: To identify the main flavonoid in Adinandra nitida leaf, employ response surface methodology to optimize its ultrasonic-assisted extraction, and determine the antioxidant activity of the obtained extract.

Methods: The main flavonoid in Adinandra nitida leaf was obtained by traditional solvent extraction and recrystallization methods, and identified by ultraviolet-visible spectroscopy (UV), Fourier transform infrared spectroscopy (FT-IR), electrospray ionization-mass spectrometry (ESI-MS), nuclear magnetic resonance spectroscopy (NMR). By using response surface methodology, the effects of extraction time, ethanol concentration and ultrasonic frequency on flavonoid yield were investigated and optimal conditions determined. 1,1-Diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity and reducing power of the obtained extract was also examined.

Results: The main flavonoid in Adinandra nitida leaf was identified as camellianin A. Optimal extraction conditions were as follows: extraction time, 30.25 min; ethanol concentration, 63.84 %; and ultrasonic frequency, 45 KHz. The mean experimental flavonoid yield under optimum conditions was 84.52 ±1.65 %, which agreed with the predicted value of 83.02 %. The obtained flavonoid extract was an effective scavenger of DPPH radicals with IC₅₀ of 0.02 mg/mL. A linear correlation between concentration of the flavonoid extract and reducing power was observed with a coefficient of r² = 0.9867.

Conclusion: By using ultrasonic-assisted extraction, the main flavonoid in Adinandra nitida leaf can be obtained at a high yield. The flavonoid displays a strong DPPH radical scavenging activity and reducing power, which makes it potentially useful in the food and pharmaceuticals industries.