Liposoluble Compounds with Antioxidant Activity from Strobilanthes tonkinensis

Strobilanthes tonkinensis Lindau is a genus of perennial plants, of the Acanthaceae, originating in Yunnan Province of China and also distributed in Thailand and Vietnam 1 . It is a natural spice and medicinal plant, sevving as a traditional tea beverage in the Dai ethnic minority of China for a long time 2 . S. tonkinensis has been proved to show a wide spectrum of biological activities, including antiviral, antitumor, antiinflammatory, anticoagulant, and so forth 3 . However, no research has so far been conducted concerning the liposoluble compounds in the hexane extract of S. tonkinensis. In this investigation, the hexane extract of S. tonkinensis has been analyzed by LC-APCI-MS and RP-HPLC. Here the results are presented. The specific fragmentation behavior and relative ion abundances of the extract of S. tonkinensis are presented as following. The peaks appearing from 6 to 25 min were analyzed. Peak 1 (Rt = 7.4 min) showed the signal of a molecular ion at m/z 591.6 with MS2 generating 573.7 [M – H2O] +, 394.6 [M + H – Glu – H2O], and 299.1 [M – Glu – C8H17]. The spectral data indicated that this compound may be -5-avenasteryl-D-glucoside 4 . Peak 2 (Rt = 8.2 min) showed the main signal of the molecular ion [M + H]+ at m/z 429.6 with fragment ions at m/z 411.3 [M + H – H2O] +, 204.8 [M + H – C16H32] +, and 164.7 [M + H – C19H36] +. According to the mass spectral data and the main fragmentation patterns, it is believed that -tocoenol exists at 8.2 min 5 . Peak 3 (Rt = 8.3 min) showed a molecular ion at m/z 593.4. In the respective MS2 spectrum of m/z 593.4, several visible fragment ions were seen at m/z 555.2 [M – 2H2O – 2H] +, 411.3 [M – Glu + H – 3H]+, 409.6 [M – Glu + H – 4H]+, 395.1 [M – Glu – H2O] +, and 255.0 [M – Glu – C10H20 – H2O] +, and the latter four ions were typical fragment ions of stigmasterol 6 . This compound was identified as stigmasteryl-D-glucoside 4 . Peak 4 (Rt = 8.6 min) showed a molecular ion at m/z 595.6 with fragment ions at m/z 580.9 [M – CH3], 409.8 [M – Glu – 6H]+, 397.6 [M – Glu – H2O] +, 339.9 [M – Glu – C3H7 – CH3 – H2O] +, 313.3 [M – Glu – C6H11 – H2O] +, and 271.0 [M – Glu – C9H17 – H2O] +, and the latter five fragment ions were similar to -sitosterol 6 . The compound was identified as a combination of stigmasterol and glucoside 4 . Peak 5 (Rt = 9.8 min) showed a molecular ion [M + H]+ at m/z 447.4 and fragment ions at m/z 429.1 [M + H – H2O] and 359 [M + H – H2O – C5H11] +. It was hypothesized that this compound may be dihydroxystigmasterol 7 . Peak 6 (Rt = 11.2 min) showed the strong signal of the molecular ion [M + H]+ at m/z 431.3 with the MS2 fragment at m/z 204.8 [M + H – C16H34], 164.7 [M + H – C19H38], and 136.7 [M + H – C20H38O]. The MS spectral data indicated this compound was -tocopherol 8 . Peak 7 (Rt = 13.4 min) had [M + H – H2O] + of the highest abundance at m/z 395.2, producing ions [M – 3H]+ at m/z 409.3, [M + 2H – H2O – C7H14] + at m/z 299.1, and [M + 2H – H2O – C9H18] + at m/z 271.0. It was named -5-avenasterol 4 . Peak 8 (Rt = 15.1 min) showed a molecular ion at m/z 412.0 and fragments at m/z 409.2 [M + H – 4H]+, 395.8 [M + H – H2O], and 255 [M – C10H20 – H2O]. The MS spectral data showed that this compound was stigmasterol 4, 9 . In general, fragments indicating a loss of water were observed in the mass of sterol. Peak 9 (Rt = 17.0 min) showed a molecular ion at m/z 414.1 with MS2 at m/z 411.3 [M + H – 4H]+, 410.9 [M + H – 5H]+, 397.5 [M + H – H2O], and 271.1 [M – C9H18 – H2O], proving that this compound was -sitosterol 9 .