Bioactive Quinic Acid Derivatives from Ageratina adenophora

A novel quinic acid derivative, 5-O-trans-o-coumaroylquinic acid methyl ester (1), together with three known ones, chlorogenic acid methyl ester (2), macranthoin F (3) and macranthoin G (4), were isolated from the aerial parts of the invasive plant Ageratina adenophora (Spreng.). The structure of new compound 1 was elucidated on the basis of extensive spectroscopic analysis, including 1D- and 2D-NMR techniques. Compounds 2–4 were isolated from plant A. adenophora for the first time. All the compounds showed in vitro antibacterial activity toward five assayed bacterial strains, especially 3 and 4, which showed in vitro antibacterial activity against Salmonella enterica with MIC values of 7.4 and 14.7 μM, respectively. Compound 1 was further found to display in vitro anti-fungal activity against spore germination of Magnaporthe grisea with an IC50 value 542.3 µM. These four compounds were also tested for their antioxidant activity against DPPH (1,1-diphenyl-2-picrylhydrazyl) radical.


Introduction
Ageratina adenophora (Spreng.) King & Robinson, native to Mexico and Costa Rica, is a perennial, herbaceous invasive plant which has invaded around 30 countries in tropical and subtropical zones of the world [1,2]. This plant was first introduced to Yunnan Province of China in the 1940s, and by now it has rapidly spread across a large area of southwest China, including Yunnan, Guizhou, Guangxi, Sichuan, Chongqing and Xizang provinces [3]. The rapid spread of A. adenophora in China has caused serious economic losses to agriculture, forestry and livestock, and damaged the ecology and environment of China's native habitat [4,5].
A. adenophora is seldom attacked by bacteria, fungi and insects, suggesting that rich bioactive secondary metabolites that might be defense related, might exist in this plant. Previously, structurally diverse chemicals including (mono-, sesqui-, di-, and tri-) terpenoids, phenylpropanoids, flavonoids, coumarins, sterols and alkaloids were reported from this species [6][7][8], some of which were shown to possess allelopathic [9,10], phytotoxic [11] and antifeedant [12] activities. Our study reported herein has further led to the isolation of a novel compounds, 5-O-o-coumaroylquinic acid methyl ester (1), and three known quinic acid derivatives 2-4 from this species (Figure 1). We report the isolation and structural elucidation of these compounds, as well as their antimicrobial and DPPH radical scavenging activities.

Results and Discussion
Compound 1 was obtained as a yellowish gum. HR-ESI-MS (positive mode) showed a [M+Na] + ion at m/z 375.1049, corresponding to the molecular formula C 17 H 20 O 8 (calcd. for C 17 H 20 NaO 8 , 375.1056). IR absorptions at 3411 cm −1 , 1733 cm −1 and 1654 cm −1 , implied the existence of hydroxyl and carbonyl groups. In its 13 C-NMR and DEPT spectra, the seventeen carbon signals of the molecule (1×C, 3×CH, 2×CH 2 , 1×CH 3 , two carbonyl group C-atoms and eight sp 2 C-atoms) could all be assigned (see Table 1). The presence of a quinic acid methyl ester moiety in the molecule was suggested by the presence of carbon signals at δ C 75. 8  all observed. Careful analysis of the 1 H-and 13 C-NMR spectra revealed that the NMR data of 1 were closely related to those of chlorogenic acid methyl ester [13], a known quinic acid derivative which was also obtained in the present study (compound 2, see Table 1). The major difference was that the resonances for the substituted caffeoyl moiety in 2 were replaced by the signals for an o-coumaric acid moiety in 1. These data indicated that 1 has the same quinic acid methyl ester moiety as that in 2 and further supported us to preliminarily establish the whole structure of 1 as 5-O-trans-o-coumaroylquinic acid methyl ester, of which the connectivity and the stereochemistry of the substituted o-coumaric acid moiety still needed to be determined. In the HMBC spectrum, the observation of 1 H-13 C long-range correlations of H-7' (δ H 7.91) with C-2' (δ C 158.4) and C-6' (δ C 130.4), H-8' (δ H 6.58) with C-1' (δ C 122.5) and C-9' (δ C 168.6) indicated the direct linkage of C-1' with C-7', and C-8' with C-9' (Figure 2). In the 1 H-NMR spectrum, the presented coupling constants between H-7' and H-8' olefeinic protons (J 7',8' = 16.0 Hz) revealed that the double bond in the o-coumaric acid moiety was in the E geometry. The ester bond linkage between C-5 and C-9' was revealed by the observation of a significant HMBC correlation of δ H 5.28 (H-5) with δ C 168.6 (C-9'). In addition, the location of the hydroxyl group at C-2' 2) indicated that the hydroxyl group at C-1 of 1 was an α-(axial) configuration [14]. Thus, 1 was elucidated as 5-O-transo-coumaroylquinic acid methyl ester as shown in Figures 1 and 2. All the spectral data supported this structure. The three known quinic acid derivatives were determined as chlorogenic acid methyl ester (2) [13], macranthoin F (3) and macranthoin G (4) [15], by interpretation of their spectroscopic data and comparison with literature values. They were isolated from A. adenophora for the first time.
These four compounds were tested for their in vitro antibacterial activities against five bacterial strains, including two Gram-positive (Staphylococcus aureu and Bacillus thuringiensis) and three Gram-negative (Escherichia coli, Salmonella enterica and Shigella dysenteria) bacterial species. The experimental results obtained from the bioassay ( Table 2) revealed that 1-4 were all active compounds toward the five test bacterial strains, especially for 3 and 4, which showed in vitro bacteriostatic activity against S. enterica with MIC values (7.4 and 14.7 μM, respectively) close to that of the positive control compound kanamycin (MIC 3.4 μM to S. enterica). These compounds were also tested for their antifungal activity against spore germination of the rice pathogenic fungus Magnaporthe grisea and their antioxidant activity against DPPH radical by using the bioassay methods as indicated in the Experimental section. Compound 1 was found to show in vitro anti-fungal activity against spore germination of M. grisea with IC 50 542.3 µM. Compounds 2 and 4 showed scavenging activity against DPPH radical, with SC 50 values 212.2 and 150.2 µM, respectively, but they were much weaker than the positive control resveratrol (SC 50 42.1 µM).
Among these compounds, 1 is a novel chlorogenic acid derivative ester bond linked with an o-coumaric acid (2-coumaric acid). Generally, chlorogenic acid related compounds are formed by a quinic acid unit ester linked with one or more caffeoyl or p-coumaroyl unit(s) [16][17][18]. It is rather rare for this group of natural products to contain an o-coumaric acid unit in the structure.
A. adenophora is a well-known invasive plant which has spread rapidly and caused great economic loss in China. It has been suggested that allelopathy could be an important strategy for this plant to achieve its invasive success [19,20]. Recent study revealed that o-coumaric acid is phytotoxic and richly abundant in A. adenophora, which was suggested to be probably the most important allelochemical in this invasive species [11]. Taking the structural features into consideration, it is reasonable to predict that 1 might play a role in regulating the allelopathy of A. adenophora by functioning as a storage form of the strongly phytotoxic compound o-coumaric acid.

Plant Materials
The aerial parts of A. adenophora were collected in a suburb of Kunming, Yunnan Province, China, in July 2009, and authenticated by Prof. Fu-Wu Xing, South China Botanical Garden, Chinese Academy of Sciences. A voucher specimen (No.20090702) was deposited at the Laboratory of Phytochemistry at the South China Botanical Garden, Chinese Academy of Sciences.

Antibacterial Assay
The antibacterial activities of 1-4 were tested by using a microdilution method as reported in literature [21], with modification in determination of the minimum inhibitory concentration (MIC) values [22]. In the test, indicator solution (resazurin, 100 μg/mL, 100 μL) was first placed into each control wells (11th column) in 96-well microplates for the assay. Subsequently, indicator solution (100 μg/mL, 7.5 mL) was mixed with test organism (10 6 cfu/mL, 5 mL) followed by transferring (100 μL, each) to growth control wells (12th column) and all test wells (1-10th column) in the 96-well microplates. Then, each of the sample solutions (1.0 mg/mL of test compounds in methanol, 100 μL) and positive control solution (1.0 mg/mL of kanamycin sulfate in methanol) as well as negative control sample (pure MeOH) were applied to the wells in the 1st column of the plates. In each test microplate, the four compound samples along with a positive control and a negative control samples were applied. Once all samples and controls were properly applied to the 1st column of wells in the microplates, half of the homogenized content (100 μL) from these wells was then transferred parallel to the 2nd column of wells, and each subsequent column of wells was treated similarly (doubling dilution) up to the 10th column, followed by discarding the last 100 μL aliquot. Finally, the plates were incubated at 37 °C for 5-6 h until the color of growth control wells change to pink. The lowest concentration for each test compound at which color change occurred was recorded as its primary MIC value. The averages of primary values from three individual tests were calculated and that was taken as the final MIC values for the test compounds [23]. Two Gram-(+) bacteria strains, S. aureus and B. thuringiensis, and three Gram-(−) bacterial species, E. coli, S. enterica and S. dysenteria, were used in the assay. MIC values for test compounds were displayed in Table 2.

Antifungal Assay
The inhibitory activities of test compounds against spore germination of M. grisea were tested by a microdilution assay. Briefly, the mixed solution containing fungal spore suspension solution (10 6 spores/mL, 40 µL), test compound solution (5 µL) and 10% glucose solution (5 µL) were incubated on a concave glass at 28 °C in darkness for spore germination for 2.5 h. The germinated spores were then checked and recorded under a microscope. The solution concentrations of each test compound were set in the range of 2-200 μg/mL, and each of the test compounds were assayed in triplicate. A mixed solution for incubation without test compounds was used as negative control, and ketoconazole was used as a positive reference compound. The reported IC 50 value represents the concentration of a test compound required to inhibit 50% of spore germination.

Determination of Antioxidant Activities
The antioxidant activities of test compounds were determined by the DPPH assay as previously described [24]. Briefly, the reaction mixture containing sample solution (20 μL) and DPPH (180 μL, 150 μM) in ethanol was placed in a 96-well microplate and incubated at 37 °C for 30 min. The absorbance was measured at 517 nm by a microplate reader. SC 50 value represents the concentration of a compound to scavenge 50% of DPPH radicals. Resveratrol was used as positive control.

Conclusions
A new quinic acid derivative, 5-O-trans-o-coumaroylquinic acid (1), was isolated from the aerial parts of the invasive plant A. adenophora (Spreng.), along with three known ones 2-4. The three known compounds were all found in this plant species for the first time. Compound 1 is a chlorogenic acid derivative ester bond linked with an o-coumaric acid unit in the molecule, which is rather rare in Nature. Antibacterial assays revealed that compounds 1-4 were all active toward the five assayed bacterial strains, especially compounds 2 and 4, which showed in vitro antibacterial activity against S. enterica with MIC values (7.4 and 14.7 μM) very close to that of the positive control kanamycin (MIC 3.4 μM). Compound 1 was further found to display obvious in vitro anti-fungal activity against spore germination of M. grisea, with an IC 50 of 542.3 µM. A DPPH radical scavenging assay demonstrated that 2 and 4 are slightly active, but much weaker than the famous polyphenol resveratrol.