Aroma volatile compounds in Mussaenda pubescens

Abstract

The whole plants of Mussaenda pubescens W. A. Aiton (Rubiaceae) have been used in Chinese folk medicine against laryngopharyngitis, acute gastroenteritis, and dysentery, and also as a contraceptive agent. It is a liana-like shrub, distributed widely in shady hillsides, valleys, and shrub jungles of east, south, and southwest China. Phytochemical research showed that triterpenes, triterpenoid saponins, iridoid glycosides, and organic acids are the main components in M. pubescens [1–5]. However, to the best of our knowledge, a literature search showed no studies on the essential oils of M. pubescens. In this paper, we report on the essential oil of M. pubescens using the HS-SPME technique subsequently analyzed by GC-MS for the first time. The air-dried leaves of M. pubescens were collected in Guizhou, China, in June 2010. They were identified by Prof. Deyuan Chen. Voucher specimens were deposited at the Institute of Chinese Materia Medica, Henan University (No. 20100816). Volatile organic compounds were extracted using a manual SP-ME holder together with 5 mL vials and PDMS-DVB fiber (polydimethylsiloxane-divinylbenzene) purchased from Supelco Inc. (Bellefonte, USA). The powder of M. pubescens leaves (about 0.7 g) was placed in vials (5 mL); then the SPME fiber was exposed in the upper space of the sealed vial for 30 min at 80 C to adsorb the analytes. After that, the fiber was withdrawn and directly inserted into the GC-MS inlet for desorption of the volatiles for 1 min. The volatile constituents were analyzed by HS-SPME-GC-MS analysis, which was carried out using an Agilent 6890 N gas chromatograph equipped with a capillary column HP-5 MS (5% phenylmethylsiloxane, 30 m 0.25 mm, film thickness 0.25 m, Agilent Technologies, USA) and coupled with a 5975B mass selective detector spectrometer from the same company. The front inlet was kept at 250 C in the splitless mode. The temperature program was as follows: initial column temperature 50 C, held for 1 min, then programmed to 120 C at a rate of 3 C min–1 and held for 2 min, and finally programmed to 210 C at a rate of 4 C min–1 and held at 210 C for 10 min. As a carrier gas, helium at 1.0 mL·min–1 was used. MS conditions: the MS detector was used in the EI mode with an ionization voltage of 70 eV. The ion source temperature was at 230 C. The transfer line was at 280 C.The spectra were collected at 3 scans/s over the mass range (m/z) 30–440. Retention indices were calculated by using the retention times of n-alkanes (C6–C26) that were injected under the same chromatographic conditions [6]. The volatile constituents (Table 1) were identified by comparison of their linear retention indices (relative to C6–C26 alkanes on the HP-5MS column) and from mass spectral database search (NIST 08.L library). Twenty-nine compounds in the leaves of M. pubescens were identified (Table 1), which comprised 97.2% of the volatile fraction. The main constituents of the oil were (E)-hexenoic acid 2-butyl ester (25.23%), (E,E)2-hexenoic acid 2-hexenyl ester (15.1%), and -ionone (7.2%). Monoterpenoids and sesquiterpenes, such as -ionone (7.2%), linalool (5.6%), and limonene (3.6%), were the main aromatic constituents in M. pubescens.