Diterpenoid Alkaloids and One Lignan from the Roots of Aconitum pendulum Busch

Abstract Diterpenoid alkaloids have neroprotective activity. Herein, three napelline-type diterpenoid alkaloids 1–3, two aconitine-type diterpenoid alkaloids 4–5, and one isoquinline-type alkaloid 6, as well as one lignan glycoside 7, have been isolated from the roots of Aconitum pendulum Busch. Compounds 1 and 7 were new compounds, and their chemical structures were determined on the basis of nuclear magnetic resonance (NMR) spectra and mass spectrometry analysis. A ThT assay revealed that compound 2 showed significant disaggregation potency on the Aβ1−42 aggregates. Graphical Abstract Electronic supplementary material The online version of this article (10.1007/s13659-019-00227-y) contains supplementary material, which is available to authorized users.


Introduction
The plant Aconitum pendulum Busch, belonging to the Ranunculaceae family, is mainly distributed in Northwestern China at the altitude of 3000-4000 m [1]. The roots of A. pendulum have long been used as a traditional herb "Tie Bang Chui" for treating traumatic injury, fracture, rheumatism, and chilblains [2]. Previous studies showed that this plant mainly produced diterpenoid and norditerpenoid alkaloids [3], such as aconitine (AC), deoxyaconitine (DA), and mesaconitine (MA) [4], which were considered as the toxic components of this kind of folk medicine. These alkaloids have shown to cause widespread membrane excitation in cardiac, neural, and muscular tissues because of their significant activation on sodium channels [5][6][7][8]. Besides, they were also reported to show the potential as drug leads in Alzheimer diseases by targeting the neuronal nicotinic acetylcholine receptor [9][10][11].

3
Our research group has long been focused on the discovery of bioactive natural compounds from the traditional herbs cultivated in Northwestern China [12][13][14][15][16]. As part of this ongoing program, a phytochemical study on the roots of A. pendulum collected from Gansu province has been conducted. Three napelline-type diterpenoid alkaloids 1-3, two aconitine-type diterpenoid alkaloids 4-5, and one isoquinline-type alkaloid 6, as well as one lignan glycoside 7, were isolated (Fig. 1). Herein, we report the isolation, structural determination, and biological activity of these isolates.

Results and Discussion
Compound 1 was isolated as a colorless gum with a small value of specific optical rotatory value. Its chemical formula C 22 [17], and three aliphatic quaternary carbons (δ C 54.7, 52.3, 47.1). The data indicated that this compound was a C 20 -diterpenoid alkaloid possessing an iminium methine moiety [17]. Further HMBC and HSQC experiments determined that compound 1 shared the same molecular structure as that of aconicarmichinium A except for the relative configuration of OH-12 [17]. Specifically, three hydroxy groups were substituted at C-1, C-12, and C-15 based on the HMBC correlations from H-1 (δ H 4.02) to C-2 (δ C 31.9), C-3 (δ C 35.5), C-5 (δ C 45.0) and C-10 (δ C
Due to the available amounts, compounds 1-4 were evaluated for their anti-AD potential based on their effect on the copper-mediated Aβ 1−42 disaggregation by using ThT assay [25] with resveratrol as positive control. All compounds were treated at 25 µM. As shown in Table 1, the disaggregate potency of compounds 1-4 were ranging from 10.2 ± 3.8 to 28.6 ± 2.9%, while the datum for the positive control resveratrol was 46.9 ± 4.6%. Compound 2 showed the most significant disaggregation effect on the Aβ 1−42 aggregates.

General
The optical rotation values were measured on a 241 polarimeter (Perkin-Elmer). The infrared spectra were measured by a FTS 165-IR instrument (Bio-Rad, USA). A Varian INOVA-400 FT-NMR spectrometer (USA) and a Bruker APEX II spectrometer were used to record the NMR and HRESIMS data, respectively. Different types of chromatographic materials were used for the fractionation of natural compounds, including Sephadex LH-20 (Amersham Biosciences), silica gel (200-300 mesh, Qingdao Haiyang Chemical Co., Ltd), and ODS (YMC Co., Ltd). Prep-HPLC separation was performed on a prep-HPLC manufactured by Hanbon Sci & Tech of China using a Megres C18 column (250 mm × 20 mm).

Plant Materials
The roots of A. pendulum Busch (Ranunculaceae), collected from Gansu Province in China, were purchased from Lanzhou Huanghe Herbal Medicines Market in 2017. The materials were identified by Dr. Huan-Yang Qi at Lanzhou Institute of Chemical Physics (LICP), and a voucher specimen (ZY2017HTBC) was deposited at the CAS Key Laboratory of Chemistry of Northwestern Plant Resources.

Acid Hydrolysis of Compound 7
This process was performed according to the literature [26]. The detailed process can be found in Supporting Information.

ThT Assay
The procedures of ThT assay were reported in the literature [25]. The detailed information can be found in Supporting Information.
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