Abstract:

It is difficult to extract bound flavonoids directly from Coreopsis tinctoria Nutt owing to their binding to the
cell wall material by covalent bond. The bound compounds were released using an alkaline digestion method in the current
study and the process parameters were optimized. A crude extract was obtained from the digestion products using the
selected optimum solvent and pure flavonoid compounds were isolated from the crude extract by open silica gel column
chromatography and Sephedex LH-20 gel column chromatography. The antioxidant activities of the purified compounds
were evaluated by DPPH, FRAP and ABTS assays. The results revealed that the optimum alkaline degradation conditions
were 50 ℃, 2 h and 2 mol/L for temperature, time and NaOH concentration, respectively, and n-butanol was selected as the
optimum extraction solvent for flavonoids. Two pure compounds were isolated from the crude extract and their chemical
structures were elucidated as okanin-4’-O-β-D-glucopyranoside and 3’,4’,7,8-tetrahydroxyflavanone by nuclear magnetic
resonance (NMR) and electro-spray ionization mass spectrometry (ESI-MS). Antioxidant experiments showed that the
total antioxidant capacity of these two compounds was higher than that of the positive control vitamin E. The IC50 values
of compounds 1 and 2 in the DPPH assay were (17.29 ± 0.17) and (70.09 ± 0.09) μmol/L, respectively, while those in the
ABTS assay were (22.86 ± 0.21) and (33.4 ± 0.18) μmol/L, respectively, which were far lower than that of vitamin E (the
IC50 values were (78.08 ± 1.63) and (38.54 ± 0.42) μmol/L for DPPH and ABTS+ radicals, respectively), suggesting that both
flavonoids possess powerful antioxidant activities.

Key words: Coreopsis tinctoria Nutt, bound flavonoid compounds, structure identification, antioxidant activities