Antioxidant constituents and a new triterpenoid glycoside from Flos Lonicerae

As a component of our continuing investigations into herb-derived antioxidant agents, we have evaluated the antioxidant effects of Flos Lonicerae (Lonicera japonica flowers), via 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical, total reactive oxygen species (ROS), hydroxyl radical (·OH), and peroxynitrite (ONOO^-) assays. Among the methanolic extract and the dichloromethane, ethyl acetate, n-butanol, and water fractions, the EtOAc fraction of Flos Lonicerae exhibited marked scavenging/inhibitory activities, as follows: IC₅₀ values of 4.37, 27.58 ± 0.71, 0.47 ± 0.05, and 12.13 ± 0.79 μg/mL in the DPPH, total ROS, ONOO_-, and ·OH assays, respectively. Via a bioactivity-guided fractionation approach, a new triterpenoid glycoside, oleanolic acid 28-O-α-L- rhamnopyranosyl-(1→2)-[β-D-xylopyranosyl(1→6)] -β-D-glucopyranosyl ester (12), along with eleven known compounds, including chrysoeriol (1), luteolin (2), 5-hydroxymethyl-2-furfural (3), caffeic acid (4), protocatechuic acid (5), chrysoeriol 7-O-β-D-glucopyranoside (6), isorhamnetin 3-O-β-D-glucopyranoside (7), kaempferol 3-O-β-D-glucopyranoside (8), quercetin 3-O-β-D-glucopyranoside (9), hederagenin 3-O-α-L-arabinopyranoside (10), and luteolin 7-O-β-D-glucopyranoside (11), were isolated from the EtOAc fraction. The structures of isolated compounds 1-12 were elucidated via spectroscopic analyses. Compound 12 was isolated from a natural source for the first time. Compounds 2, 4, 5, 7, 9, and 11 evidenced marked scavenging activities, with IC₅₀ values of 2.08-11.76 μM for DPPH radicals, and 1.47-6.98 μM for ONOO^-.