In a continuation of our work on quinones that occur widely in the plant kingdom and that may have biological activities, we have developed a simple cyclodextrin modified capillary zone electrophoresis (CD-CZE) to separate and determine emodin, chrysophanol, aloe-emodin, rhein, emodin-8-b-D-glucoside, chrysophanol-8-b-D-glucoside, rhein-8-b-Dglucoside, sennoside A, and sennoside B of Rhei Rhizoma. Rhei Rhizoma is an old-and well-known herbal medicine known for its purgative effect due to anthracene derivatives, especially their glycosides. The analysis of these compounds and anthraquinones in pharmaceutical preparations or in complex matrices such as crude plant extracts is, therefore, of special interest.1,2) Some naturally-occurring anthraquinones have already been examined by thin layer chromatography, 3) HPLC methods, [4][5][6][7][8] and capillary electrophoresis (CE), [9][10][11][12][13][14] however, only a single component or limited components have been determined in the majority of them. A few simultaneous analyses of different types of components have been conducted. 6,8,14) Studies related to simultaneous determination by CE are especially rare. Rhei Rhizoma contains a wide variety of components, including monomeric anthraquinones, their glycosides, and dimeric anthrone glycosides, and thus simultaneous determination of these different groups of components showing large differences in polarity is desirable for the quality control of the herb. For this purpose, we have developed a simple and rapid CD-CZE method (except for rhein) using 0.005 M a-CD in 0.03 M borate buffer (pH 10.0) containing 20% acetonitrile.
ExperimentalReagents and Materials Sodium tetraborate, sodium dodecyl sulphate (SDS), sodium hydroxide, hydroxypropyl-b-CD (HP-b-CD), g-CD, and HPg-CD were purchased from Wako Pure Chemicals (Osaka, Japan), a-CD, b-CD, and sennoside A (8) from Nacalai Tesque (Kyoto, Japan), sennoside B (9) from Funakoshi (Tokyo, Japan), and boric acid from Fluka (Buchs, Switzerland). Acetonitrile, methanol, and water were of HPLC grade. Emodin (1) and chrysophanol (2) were isolated from the root bark of Cassia siamea.15) Emodin-8-b-D-glucoside (5) was isolated from the root of Polygonum cuspidatum SIEB. et ZUCC. 16,17) and chrysophanol-8-b-D-glucoside (6) from Cascara Sagrada (the root of Rhamnus purshiana DC.).18,19) Aloeemodin (3), rhein (4), and rhein-8-b-D-glucoside (7) were isolated from Rhei Rhizoma 20) (Fig. 1). Rhei Rhizoma (Rheum palmatum L., Rheum tanguticum MAXIM.) was purchased from The Iguchi Pharmacy (Kobe, Japan).Procedure for CE The CE analyses were carried out using a Beckman MDQ and P/ACE System 5000 apparatus (Fullerton, CA, U.S.A.) equipped with a UV detector set at 254 nm, a diode array detector, and a Beckman untreated fused-silica capillary (570 mmϫ75 mm i.d.; 500 mm effective length). The analytical conditions were as follows: sampling time, 5 s (hydrodynamic mode; 0.5 p.s.i.); applied constant voltage, 20 kV; column temperature, 20°C. The CD-CZE electrolyte was a buffer solution prepared b...