Determination of sinomenine in Sinomenium acutum by capillary electrophoresis with electrochemiluminescence detection
Introduction
Sinomenine (7,8-didehydro-4-hydroxyl-3,7-dimethoxy-17-methylmorphinan-6-one; SIN) is a principal alkaloid isolated from the stem and root of Chinese medical plant Sinomenium acutum Rehd. et Wils, and its chemical structure is shown in Fig. 1. Due to its analgesic and anti-inflammatory effects, sinomenine has been utilized clinically to treat rheumatoid arthritis and neuralgia [1], [2], [3]. At present, several chromatographic methods including high-performance liquid chromatography (HPLC) [4], [5], [6], [7] and thin-layer chromatography (TLC) [8], [9] have been reported for the analysis of sinomenine. However, a drawback of the methods mentioned above appears to be time-consuming due to necessary extraction, concentration and/or derivatization prior to the analysis although the high sensitivity and the good selectivity have been obtained in such procedures. Therefore, it is necessary to establish rapid and effective methods for the quantitation of sinomenine.
Capillary electrophoresis (CE) is now a widely used separation technique for analysis of alkaloids with various pharmaceutical applications because of its high efficiency, resolution potential, short analysis time and minimal sample volume [10], [11], [12]. Recently, an easy, rapid nonaqueous capillary electrophoresis method has been developed for the determination of sinomenine with a UV detector [13]. In addition, Zhai et al. [14] has also proposed another CE method for sinomenine determination by use of high frequency conductivity detector with a detection limit of 0.2 μg mL−1.
In recent years, there are increasing interests in coupling CE separation with high-sensitive chemiluminescence (CL) detection for alkaloids analysis [15], [16], [17], [18]. Especially, electrochemiluminescence (ECL) detection involving tris(2,2′-bipyridyl) ruthenium(II) (Ru(bpy)32+) offers other merits with wide linear range, no derivatization and good selectivity for nitrogen-containing compounds [19], [20]. Therefore, analytical procedures combining CE separation with Ru(bpy)32+-based ECL detection have been paid more attention to the detection of some alkaloids [21], [22], [23], [24]. However, as far as we know, such CE-ECL procedure has not been reported for the determination of sinomenine.
In this paper, a CE-ECL method based on Ru(bpy)32+ system has been developed for the determination of sinomenine in Chinese herb S. acutum Rehd. et Wils. It is worth mentioning that a europium(III)-doped prussian blue analogue (Eu-PB) film was modified chemically on the surface of a microdisk platinum working electrode to avoid the possible electrode fouling as well as to improve the detection sensitivity.
Section snippets
Reagents and chemicals
All chemicals and reagents were of analytical grade except for specific statements and used without further purification. Tris(2,2′-bipyridyl) ruthenium(II) chloride hexahydrate (98%) was obtained from Aldrich (Milwaukee, WI, USA) and prepared with doubly deionized water. Sinomenine was purchased from National Institute for the Control of Pharmaceutical and Biological Products (Beijing, China) and freshly prepared with 50% methanol (Spectrum analytical grade) just before use. Dried Chinese herb
Effect of the platinum electrode modified with Eu-PB film
In comparison with the response to the oxidation of Ru(bpy)32+ on a bare platinum electrode, the Eu-PB modifying platinum electrode exhibited higher current response, with slight negative shift ca. 20 mV for the direct oxidation peak of Ru(bpy)32+ (see Fig. 3A). Consequently, an enhanced ECL peak of Ru(bpy)32+ was obtained, as shown in Fig. 3B. Thus, the prepared electrode would benefit from the improved sensitivity and give less interfering signals from other electroactive substances in real
Conclusion
A Ru(bpy)32+-based CE-ECL method was studied for identification and determination of sinomenine for the first time. The developed method was found not only a good alternative for the rapid determination of sinomenine in plant extracts with good selectivity, wide linearity and reliable stability, but also an efficient supplementary technique for the preliminary investigation of other quinolizidine alkaloids in Chinese traditional herbs.
Acknowledgements
We are grateful to Natural Scientific Foundation of Gansu Province, China, for supporting the research with Project 3ZS051–A25–097, and more thanks to Scientific Research Foundation of Gansu Ministry of Education, China, for the partial financial aid with Project 0501–07.
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