Rubiacordone A: A new anthraquinone glycoside from the roots of Rubia cordifolia

Article abstract of DOI:10.3390/molecules14010566

A new anthraquinone, Rubiacordone A (1) (6-acetoxy-1-hydroxy-2- methylanthraquinone-3-O-α-L-rhamnopyranoside), was isolated together with the known anthraquinone, 1-acetoxy-6-hydroxy-2-methylanthraquinone-3-O-[α- L-rhamnopyranosyl-(1 → 2)-β-D-glucopyranos

Full text of DOI:10.3390/molecules14010566

Molecules 2009, 14, 566-572; doi:10.3390/molecules14010566
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Communication
Rubiacordone A: A New Anthraquinone Glycoside from the
Roots of Rubia cordifolia
Xiang Li 1,†, Zhi Liu 2, Yun Chen 3, Li-Juan Wang 1, Yi-Nan Zheng 1, Guang-Zhi Sun 2 and
Chang-Chun Ruan 2,*
1
College of Chinese Medicinal Materials, Jilin Agricultural University, Changchun, 130118, P.R.
China; E-mails: drxiang@hotmail.com (X. L.); ljwang@yahoo.com (L-J. W.);
zhenyinan@tom.com (Y-N. Z.)
2
Institute of Agricultural Modernization, Jilin Agricultural University, Changchun, 130118, P.R.
China; E-mails: jlndxdhs@126.com (Z. L.); gzsun1967@yahoo.com (G-Z. S.)
3
Research Centre for Medical and Structural Biology, School of Basic Medical Science, Wuhan
University, Wuhan, 430071, P.R. China; E-mail: yunchen@whu.edu.cn (Y. C.)
†
Present address: Agriculture and Agri-Food Canada, Saskatoon Research Center, 107 Science Place,
Saskatoon, SK, S7N 0X2, Canada
* Author to whom correspondence should be addressed; E-mail: yuan.changchun1@gmail.com;
Tel.: +86-431-8451-0949; Fax: +86-431-8451-0409.
Received: 20 November 2008; in revised form: 13 January 2009 / Accepted: 15 January 2009 /
Published: 23 January 2009
Abstract: A new anthraquinone, Rubiacordone A (1) (6-acetoxy-1-hydroxy-2-
methylanthraquinone-3-O-α-L-rhamnopyranoside), was isolated together with the known
anthraquinone, 1-acetoxy-6-hydroxy-2-methylanthraquinone-3-O-[α-L-rhamnopyranosyl-
(1→2)-β-D-glucopyranoside] (2), from the dried roots of Rubia cordifolia. Their structures
were elucidated on the basis of extensive 1D and 2D-NMR, as well as HRESI-MS
spectroscopic analysis. Metabolites 1 and 2 showed considerable antimicrobial activity
against Gram-positive bacteria.
Keywords: Rubia cordifolia; Rubiacordone A; Anthraquinone; Antimicrobial.

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Introduction
Rubia cordifolia Linn (Rubiaceae) is a slender, branched, climbing plant, with very long cylindrical
roots, widely distributed in China, India and tropical Australia [1]. In the traditional Chinese system of
medicine, Rubia cordifolia is used for the treatment of vertigo, insomnia, rheumatism, tuberculosis,
hematemesis, menstrual disorders and contusions. The plant was recently reported to possess
antimicrobial activity [2-3] and previous phytochemical examinations have shown that it produces
triterpenoids, anthraquinones, cyclopeptides and phenolics [4-12]. During our investigations on plants
used for antimicrobial purposes in Chinese medicine, a 70% methanolic extract of the dried roots of
Rubia cordifolia showed considerable activity against Gram-positive and Gram-negative bacteria.
Purification of the bioactive extract results in the isolation and identification of a novel anthraquinone,
designated with the common named Rubiacordone A (1), and the previously reported anthraquinone 2,
both of which showed antimicrobial activity against Gram-positive bacteria when tested using the disc-
diffusion method.
Results and Discussion
Characterization of compound 1
Anthraquinone 1 was isolated as an orange-colored amorphous powder, and its molecular formula
was determined as C23H22O10 on the basis of its HR-ESI-MS (m/z 459.1303 [M+H]+, calcd. 459.1291)
and NMR data (Table 1). While, the IR spectra of 1 exhibited absorption bands at 3,438, 1,739 and
1,630 cm-1, indicating the existence of hydroxyl, acetyl and ketone functionalities in the structure, the
13
UV spectra showed typical anthraquinone maxima values at 269, 305, 410 nm. Analysis of the C-
NMR spectrum and the DEPT experiment, allowed the identification of three methyl groups, nine
methines and nine quaternary carbons. The chemical shifts of the protons and carbons in the phenolic
region of 1 (Table 1) were similar to those of the previously reported 1-acetoxy-6-hydroxy-2-
methylanthraquinone-3-O-[α-L-rhamnopyranosyl-(1→2)-β-D-glucopyranoside (2), an anthraquinone
also isolated from Rubia cordifolia [8]. The main differences between the two metabolites concerned
the location of the acetyl group and the nature of the sugar moiety; with the acetyl group located at C-1
in 2 and to C-6 in 1, and the sugar moiety being α-L-rhamnopyranoside in the novel anthraquinone (1),
instead of the α-L-rhamnopyranosyl-(1→2)-β-D-glucopyranoside present in the known anthraquninone
13
2. This was supported by the C-NMR data of 1, which showed only one set of “sugar” signals at δC
103.3 (d, C-1'), δC 71.7 (d, C-2'), δC 72.9 (d, C-3'), δC 72.0 (d, C-4'), δC 73.2 (d, C-5'), and δC 18.6 (q,
C-6'''). The location of the sugar moiety at C-3 was established taking into account the correlation
observed between H-1’ (δ 5.48) and C-3 (δ 156.8) in the HMBC experiment of 1 (Figure 1). The sugar
moiety was identified as rhamnose when the acid hydrolysis of 1 produced a component having the
same Rf value on TLC as that of an authentic sample of rhamnose. The coupling constant between the
H1' and H2' (1.5 Hz) protons in the 1H-NMR of 1 indicated an α-rhamnose; the absolute configuration
of the α-rhamnose was further determined to be α-L-rhamnose by chiral GC analysis. Finally, the
location of the acetyl group was established on the basis of the HMBC correlation between the acetyl’s
methyl group (δ 2.10) and the C-6 carbon (158.7) (Figure 1). On the basis of this data, metabolite 1

Molecules 2009, 14
568
was identified as 6-acetoxy-1-hydroxy-2-methylanthraquinone-3-O-α-L-rhamnopyranoside, to which
we have given the common name Rubiacordone A.
Table 1. 1H- (500 Hz) and 13C- (125 Hz) NMR data for Rubiacordone A (1)a.
Position
1H-NMR
J (Hz)
13C-NMR DEPT
COSY
HMBC
1
-
-
162.0
123.4
156.8
104.6
132.7
113.1
158.7
120.5
129.1
124.5
N.D. c
108.6
N.D. c
131.7
169.2
24.3
C
C
-
-
2
-
-
-
-
3
-
-
C
-
-
4
7.35, s
-
CH
C
-
C-2, C-4a, C-9a, C-10
4a
-
-
-
-
5
7.45, d
2.4
CH
C
H-7
C-7, C-8a, C-10
6
-
-
-
-
7
8
7.21, dd
8.0, 2.4
CH
CH
C
H-5, H-8
C-5, C-8a
7.95, d
8.0
H-7
C-6, C-9, C-10a
8a
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
9
-
-
-
9a
-
C
-
10
-
-
-
10a
Ac-C=O
Ac-Me
2-CH3
Rha
1’
-
C
-
-
C
-
2.10, s
1.05, s
CH3
CH3
Ac-C=O, C-6
C-1, C-2, C-3
8.7
5.48, d
4.22, dd
3.79, dd
3.36, dd
3.45, m
0.88, d
1.5
1.5, 3.4
3.4, 10.0
9.5, 10.0
-
103.3
71.7
72.9
72.0
73.2
18.6
CH
CH
CH
CH
CH
CH3
H-2’
C-5’, C-3’
C-3’, C-4’
2’
H-1’, H-3’
H-2’, H-4’
H-3’, H-5’
H-4’, H-6’
H-5’
3’
C-1’, C-5’
4’
C-2’, C-5’ C-6’
C-1’, C-3’
5’
6’
6.0
C-4’, C-5’
a Measured in DMSO-d6; chemical shifts are expressed in ppm.
Figure 1. HMBC and COSY correlations of compound 1.

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Bioactivity Results
The antimicrobial activities of 20 μg of the isolated metabolites (in 50 μL DMSO) against five
Gram-positive bacteria (S. aureus, B. subtilis, S. epidermidis, S. faecalis, and B. cereus) and four
Gram-negative bacteria (V. parahaemolyticus, P. aeruginosa, S. typhimurium, and E. coli) were
determined using the agar-disc diffusion method [13]. Metabolites 1 and 2 showed a similar pattern on
antimicrobial activity, with both of them showing activity towards the Gram-positive bacteria B.
subtilis, S. faecalis and B. cereus (Table 2).
Table 2. Antimicrobial activities of 70% methanolic extract and isolated compounds a.
Inhibition zone diameter (mm)
Microorganism
CEb
200 μg/disc 20 μg/disc 20 μg/disc
1
2
Amracin
10 μg/disc
32 ± 0.5
41 ± 0.3
36 ± 0.6
35 ± 0.5
35 ± 0.5
24 ± 0.6
21 ± 0.4
32 ± 0.5
28 ± 0.4
S. aureus
6 ± 0.5
16 ± 0.6
7 ± 0.3
13 ± 0.3
26 ± 0.5
14 ± 0.4
29 ± 0.2
24 ± 0.6
8 ± 0.5
9 ± 0.5
12 ± 0.2
10 ± 0.4
19 ± 0.6
27 ± 0.3
11 ± 0.5
22 ± 0.5
26 ± 0.5
11 ± 0.3
8 ± 0.6
B. subtilis
S. epidermidis
S. faecalis
14 ± 0.6
14 ± 0.3
10 ± 0.6
7 ± 0.5
B. cereus
V. parahaemolyticus
P. aeruginosa
S. typhimurium
6 ± 0.4
8 ± 0.3
E. coli
a
13 ± 0.4
11 ± 0.5
Mean value ± SD, n = 3 (the zone of inhibition including disc of 6 mm in
diameter). b CE: 70% MeOH Crude Extract.
Conclusions
In summary, we have isolated a novel anthraquinone glycoside, rubiacordone A (1), from the dried
roots of Rubia cordifolia together with the known compound 2. The isolated compounds 1 and 2 were
tested for their antimicrobial activities against Gram-positive and Gram-negative bacteria by the disc
diffusion method. Both compounds were found to have antimicrobial effects against the Gram-positive
bacteria B. subtilis, S. faecalis and B. cereus.
Experimental
General
1
13
The H- and C-NMR spectra were measured on a Bruker Avance DRX 500 NMR spectrometer
(Bruker Bio-Spin, Rheinstetten, Germany). Chemical shifts (δ) are expressed in parts per million
(ppm) and coupling constants (J) in Hertz (Hz). The ESIMS spectra were recorded on a triple
quadrupole mass spectrometer Quattro (VG Biotech, Altrincham, England) and the HRESI-MS spectra
on a Bruker FT-ICRMS spectrometer. IR spectrum was recorded on a Thermo Nicolet Nexus 470 FT-

Molecules 2009, 14
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IR spectrometer. Gel permeation column chromatography purifications were carried out using
Sephadex LH-20 (Pharmacia); HPLC analyses were performed with an Agilent 1100 instrument using
a Zorbax C18 column (150 x 25 mm, Phenomenex, Torrance, CA). UV absorption data (λ280) were
obtained with an Agilent Chemstation Ver 8.01. All solvents used in this study were HPLC grade,
purchased from Yu-dong Biotech. Co. Ltd., Shanghai, P.R. China. Other chemicals were purchased
from the Chinese Chemical Group, Beijing, P.R. China.
Extraction and isolation
The dry roots of Rubia cordifolia were purchased from Hu-jiao Chinese Medicinal Material
Company, An-Hui, China and identified by one of the authors, Prof. Yi-Nan Zheng. A voucher
specimen (Qian-Cao-001) was deposited in the Laboratory of Medicinal Chemistry, JLAU. The
powdered roots (1.1 kg) of roots of Rubia cordifolia were extracted three times for 48 h with 4 L of
70% MeOH at room temperature; the extract was concentrated to give a dark brown residue (273 g),
which was re-dissolved in 50% MeOH and subjected to gel permeation column chromatography,
eluting with MeOH: H2O: AcOH (10:10:1), to yield six fractions (FA – FF). The third fraction (FC) was
purified by semipreparative HPLC (gradient elution of 5% aqueous MeOH to 100% MeOH) to yield
metabolite 1 (3 mg). Purification of the fourth fraction (FD) by silica gel column chromatography,
eluting with CHCl3-MeOH-H2O (4:4:1), resulted in five new fractions (FD-1 to FD-5). FD-2 was purified
by semipreparative HPLC, using the same elution conditions as mentioned above, to afford metabolite
2 (81 mg).
Acid hydrolysis of compound 1
Anthraquinone 1 (2.0 mg) was refluxed with 6 N HCl (5 mL) at 100 ºC for 2 h. The reaction
mixture was extracted with CH2Cl2 to obtain the corresponding aglycone; the aqueous layer was
neutralized with Na2CO3, filtered, and dried under vacuum. The resulting residue was re-dissolved in
H2O for sugar analysis on TLC, eluting with n-BuOH–HOAc–H2O (6:1:2). The sample spots were
detected by spraying aniline hydrogen phthalate reagent (100 mL n-BuOH saturated by H2O, 0.96 g
aniline and 1.66 g phthalic acid) and heating. Glucose, xylose and rhamnose were used as authentic
standards.
Absolute configuration of rhamnose
The absolute configuration of rhamnose was determined by chiral GC analysis using a SatoChrom
GC and a 0.25 mm x 25 m Hydrodexb-6-TBDM chiral capillary column (Macherey-Nagel, Germany),
and α-L-rhamnose as an authentic GC standard. To prepare the sample for analysis, the aqueous
residues mentioned above was re-suspended in dichloromethane (1 mL) and treated with
trifluoroacetic anhydride (50 µL). The mixture was allowed to react at room temperature for 16h and
then dried under a stream of nitrogen at room temperature. The sugar derivative was analyzed using
the following temperature program: inlet temperature was set at 240 °C, with hydrogen carrier gas and
a 1/20 split, using nitrogen makeup gas. Column temperatures starting at 120 °C, climbing to 220 °C at
50 °C min–1 and maintaining the temperature for 12 min.

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Rubiacordone A (1). Obtained as an orange-colored amorphous powder, mp. 251 – 252 ºC; UV
(MeOH) λmax: 269, 305, 410 nm; IR ν (KBr) cm-1: 3,438 (OH), 1,739 (C=O), 1,630 (C=O); HRESI-MS
[+]: m/z = 459.1303 [M+H]+ (calcd. for C23H23O10, 459.1291); 1H and 13C-NMR data, see Table 1.
Antimicrobial assay
The antimicrobial activity of samples was determined using the agar-disc diffusion method [13]
with a slight modification. Five Gram-positive bacteria (S. aureus, B. subtilis, S. epidermidis, S.
faecalis, and B. cereus) and four Gram-negative bacteria (V. parahaemolyticus, P. aeruginosa, S.
typhimurium, and E. coli) originally isolated from various sources were kindly provided and identified
by Professor Zhao-yang He (Animal Science Department, Jilin Agricultural University). The nutrient
agar plates were prepared with 25 mL nutrient broth containing a 2% agar (Difco). Five milliliters of
top nutrient agar (1% agar, 45 °C) containing a 10% inoculums of microbial strains with an OD600nm
=
1 was poured and uniformly spread on each agar plate. After the solidification of the top agar, the
sterile paper discs were placed onto the plate, and the sample solution (20 μg/50 μL DMSO) was then
applied to each paper disc. Each plate was incubated at 37 °C for 24 h and the growth inhibition zones
were measured. Amracin (10 μg /disc) (Sigma-Aldrich) was used as positive control.
Acknowledgements
We are grateful to Professor Dr. Zhi-wei Deng (Analytical Center, Beijing Normal University) for
measuring the NMR spectra. This work was partially supported by grant from Shanghai Bio-sun
Biotechnology Co. Ltd.
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Sample Availability: Samples are available from the authors.
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