Isomerization of astilbin and its application for preparation of the four stereoisomers from Rhizoma Smilacis Glabrae.

Astilbin is the most predominant flavonoid in Rhizoma Smilacis Glabrae (RSG) with many bioactivities. The interconversion of the astilbin and its three stereoisomers was found with incubation of RSG extract at different temperatures, and the equilibrium ratios were calculated. Under certain conditions, neoastilbin would replace astilbin and become the predominant flavonoid in RSG extract. The effects of ascorbic acid, sucrose, sodium benzoate, β-cyclodextrin (β-CD) and common metal ions on the isomerization and decomposition of astilbin were studied. Ascorbic acid showed the best protective effect on the decomposition of astilbin and its isomers, which may be attributed to its reducing and radical scavenging ability. Besides, ascorbic acid also accelerated the isomerization of astilbin. β-CD suppressed both isomerization and decomposition of astilbin through complexation between them. Most metal ions had inhibition effects on the isomerization of astilbin. Al3+ could almost completely inhibit the isomerization. The presence of Fe3+ caused the rapid decomposition of astilbin, and Cu2+ also showed weak effects. Based on the isomerization study, a novel and simple method for preparative separation of astilbin and neoastilbin from RSG sample was developed. Astilbin and neoastilbin with purity of 93% and yield of 0.86% and 0.48% were obtained, respectively, which represent 46.8% of total flavonoids in RSG sample. By controlling the isomerization conditions, astilbin and neoastilbin could be used as the initial reactants to produce neoisoastilbin and isoastilbin, respectively.