Large-scale separation of hydroxyanthraquinones from Rheum palmatum L. by pH-zone-refining counter-current chromatography

pH-zone-refining counter-current chromatography was successfully applied to purify four hydroxyanthraquinones, rhein, emodin, aloe-emodin and chrysophanol, from three crude extracts of Rheum palmatum L.. After the two-phase solvent system methyl tert-butyl ether–tetrahydrofuran–water at an optimized volume ratio of 2:2:3 (v/v) was equilibrated, trifluoroacetic acid (10 mM) was added to the organic phase as a retainer and ammonia (10 mM), sodium carbonate (15 mM) and sodium hydroxide (15 mM) were added to the aqueous phase as the eluter, respectively, for three individual runs. Three separation runs of 1.25, 1.53 and 1.41 g of the three crude samples yielded four hydroxyanthraquinones: 0.70 g rhein, 0.81 g emodin, 0.41 g aloe-emodin and 0.94 g chrysophanol at a high purity of over 99.0, 98.5, 98.2 and 97.8% (determined by HPLC), respectively. The structures were identified by electrospray ionization MS–MS and 1H NMR.