3. Methods for Isolation and Characterization of Xanthones
Plants xanthones are commonly isolated by column chromatography on silica gel using different solvent mixtures with increasing polarity [52–55]. Xanthone glycosides are usually crystallized from MeOH. They may also be separated and identified using TLC [56] and HPLC [57–61] by comparison with authentic samples. The structure of xanthones has been established on the basis of UV, IR, MS, and NMR data [13, 62–72]. Preparative TLC on silica gel using AcOEt, MeOH, and H2O (21 : 4 : 3) as mobile phase has been used in instances of difficult separation. Frequently used solvents in TLC are on polyamide, MeOH-H2O (9 : 1) and MeOH-H2O-AcOH (90 : 5 : 5); on cellulose, HOAc (5–30%); on silica gel, Py-H2O-AcOEt-MeOH (12 : 10 : 80 : 5) and AcOEt-MeOH-H2O (21 : 4 : 3) and chromatoplates are viewed in UV light. In certain cases, spraying with 5% KOH in MeOH or 5% aqueous H2SO4 has been advantageous [33]. Polyamide columns are frequently applied for the separation of xanthone glycosides. Purification of xanthones on Sephadex LH20 column has also been carried out [2]. Xanthones are also isolated from resin of Garcinia hanburyi [73] and from the fermentation products of an endophytic fungus Phomopsis [74].