Nucleoside derivatives from the marine-derived fungus Aspergillus versicolor

Article abstract of DOI:10.1080/14786419.2014.891114

Four nucleoside derivatives (1-4) were isolated from the fungus Aspergillus versicolor derived from the gorgonian Dichotella gemmacea collected in the South China Sea. Their structures were elucidated by comprehensive spectroscopic method of NMR and MS an

Full text of DOI:10.1080/14786419.2014.891114

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Nucleoside derivatives from
the marine-derived fungus
Aspergillusversicolor
Min Chena, Xiu-Mei Fua, Chui-Jian Konga & Chang-Yun Wangab
a Key Laboratory of Marine Drugs, The Ministry of Education of
China, School of Medicine and Pharmacy, Ocean University of
China, 5 Yushan Road, Qingdao 266003, People's Republic of China
b Institute of Evolution & Marine Biodiversity, Ocean University of
China, Qingdao 266003, People's Republic of China
Published online: 26 Mar 2014.
To cite this article: Min Chen, Xiu-Mei Fu, Chui-Jian Kong & Chang-Yun Wang (2014) Nucleoside
derivatives from the marine-derived fungus Aspergillusversicolor, Natural Product Research:
Formerly Natural Product Letters, 28:12, 895-900, DOI: 10.1080/14786419.2014.891114
To link to this article: http://dx.doi.org/10.1080/14786419.2014.891114
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Natural Product Research, 2014
Vol. 28, No. 12, 895–900, http://dx.doi.org/10.1080/14786419.2014.891114
Nucleoside derivatives from the marine-derived fungus
Aspergillus versicolor
Min Chena, Xiu-Mei Fua, Chui-Jian Konga and Chang-Yun Wangab*
aKey Laboratory of Marine Drugs, The Ministry of Education of China, School of Medicine and Pharmacy,
Ocean University of China, 5 Yushan Road, Qingdao 266003, People’s Republic of China; Institute of
Evolution & Marine Biodiversity, Ocean University of China, Qingdao 266003, People’s Republic of China
b
(Received 27 November 2013; final version received 1 February 2014)
Four nucleoside derivatives (1–4) were isolated from the fungus Aspergillus versicolor
derived from the gorgonian Dichotella gemmacea collected in the South China Sea.
Their structures were elucidated by comprehensive spectroscopic method of NMR and
MS analysis. All isolated metabolites were evaluated for their cytotoxicity,
antibacterial activity and lethality towards brine shrimp Artemia salina. Compounds
1/2 exhibited selective antibacterial activity against Staphylococcus epidermidis with
an MIC value of 12.5 mM. It should be noted that 1 and 2, whose structures were listed
in SciFinder Scholar, had no associated reference. This is the first report about their
isolation, structure elucidation and biological activities.
Keywords: marine-derived fungus; Aspergillus versicolor; nucleoside derivative;
transesterification; antibacterial activity
1. Introduction
Marine-derived fungi, especially those belonging to the genus Aspergillus, are rich source of
structurally novel and biologically potent natural products (Fremlin et al. 2009; Yamada et al.
2010; Blunt et al. 2013). During our search for bioactive secondary metabolites from marine-
derived fungi in the South China Sea, we reported the isolation of several new compounds with
antibacterial, cytotoxic and antifouling activities (Shao et al. 2011; Sun et al. 2012; Yang et al.
2012; Shao et al. 2013). Recently, the secondary metabolites were investigated for an
Aspergillus versicolor strain isolated from the gorgonian Dichotella gemmacea collected from
the Xisha Islands coral reef. Four nucleoside derivatives (1–4) were isolated from the
fermentation extract (Figure 1). Although the nucleoside derivatives are ubiquitous, the reports
on naturally occurring aroyl uridine derivatives are relatively uncommon. Especially,
compounds 1 and 2, whose structures were listed in SciFinder Scholar, had no associated
reference. In this article, we report for the first time the isolation, structural elucidation and
biological activities of two aroyl uridine derivatives (1 and 2), together with two known
analogues (3 and 4). The mechanism of the interconversion observed between 1 and 2 are also
discussed.
2. Results and discussion
The fungal strain of A. versicolor was cultivated statically in a rice medium for five weeks at
room temperature (RT). The culture was harvested by extraction with EtOAc to obtain a crude
*Corresponding author. Email: changyun@ouc.edu.cn
q 2014 Taylor & Francis

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M. Chen et al.
Figure 1. Structures of compounds 1–4, 3a and 4a.
extract, which was chromatographed on silica gel (Qing Dao Hai Yang Chemical Group Co.,
Qingdao, China) column, Sephadex LH-20 column (Amersham Biosciences Inc., Piscataway,
NJ, USA) and semi-preparative HPLC (Waters Corp., Milford, MA, USA) to obtain 1–4.
Interestingly, a transesterification reaction was observed in which both 1 and 2 quickly
rearranged to form each other, giving a mixture of 1/2 at a ratio of 7:10 approximately in a
solution of MeOH/H2O at RT. Although 1 and 2 could be separated by HPLC using MeOH/H2O
as the mobile phase, both 1 and 2 changed into a mixture of 1/2 due to the rapid
transesterification reaction (see later). The respective spectroscopic data of 1 and 2 were
resolved from the spectra of 1/2. The similar transesterification reaction was also observed for
kipukasins E and D (3 and 4), resulting in a mixture of 3/4. It should be noted that the
constitutions of 1 and 2 were listed in SciFinder Scholar with the CAS Registry Numbers of
1136789-16-6 and 1175827-47-0, respectively; however, no reference and experimental data
were found. Thus, we report the structural elucidation and give the trivial names kipukasins H
and I for 1 and 2, respectively.
Kipukasin H (1) was isolated as a colourless oil and has the molecular formula C18H20N2O9
(10 degrees of unsaturation) as determined by HR-ESI-MS. Careful comparisons of the 1H and
13C NMR spectra of 1 with those of kipukasin E (3), a nucleoside derivative from the basidioma
of Gandoderma australe deriving A. versicolor fungus (Jiao et al. 2007), suggested that 1 was
very similar to 3. The most obvious difference in the 1H NMR spectrum was a methoxy signal at
dH 3.64 in 1 instead of the two methoxy signals in 3, indicating that 1 was the demethylated
derivative of 3. The structure of 1 was independently assigned by analysing the HMQC, 1H–1H
COSY and HMBC data (Figure S20). The selective NOE experiment was employed to assign the
only one methoxy group for 1. The irradiation of the methoxy signal of 1 resulted in obvious
enhancement for the aromatic H-300 but no obvious enhancement for the H-500, unambiguously
supporting that the methoxy group was anchored at C-200, rather than C-400 in 1.
Kipukasin I (2) was also isolated as a colourless oil, possessing the same molecular formula
C18H20N2O9 as 1. The 1H and 13C NMR spectra of 2 were very similar to those of 1, with only
a subtle difference in the chemical shifts and splitting patterns for partial of the ribose
oxymethine proton signals in the 1H NMR spectrum. The signal for H-20 (dH 4.36; dd, J ¼ 6.6,
5.4 Hz) was shifted upfield, while the H-30 resonance (dH 5.25; dd, J ¼ 5.4, 2.4 Hz) was shifted
downfield in 2 compared with those of 1, leading to the assignment of the aroyl group at C-30 in
2. The HMBC correlation from H-30 to C-700 supported this connection for 2. Therefore, 2 is the
regioisomer of 1.
The absolute configurations of 1 and 2 were resolved based on biogenetic considerations.
Because the absolute configurations of the uridine moiety contained in the known compounds
kipukasins E (3) and D (4) have been previously assigned as uracil-1-b-D-ribofuranoside (Jiao
et al. 2007), the uridine unite in the co-isolated compounds 1 and 2 could also be established as
uracil-1-b-D-ribofuranoside, identical to their methylated derivatives 3 and 4.

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Figure 2. Mechanism of the interconversion for 1 and 2 in MeOH–H2O.
20/30-Transesterification has been known to occur in some ribose derivatives (Hasegawa et al.
2001; Sauve et al. 2001; Jackson & Denu 2002). The interconversion between 1 and 2 was
observed during the extraction and purification processes. The 20-O-acyl ribofuranoside or 30-O-
acyl ribofuranoside was labile and reactive in MeOH–H2O solution and underwent the
intramolecular acyl migration to produce isomers. The aroyl in 1 was transferred from the 20-OH
position to the neighbouring 30-OH on the ribose moiety via an ortho-acid ester intermediate
(Spahn-Langguth & Benet 1992), yielding the regioisomer 2 (Figure 2). This aroyl migration
was reversible, which finally achieved a dynamic equilibrium (n1:n2 ¼ 7:10, approximately).
The same mechanism was responsible for the interconversion between 3 and 4. In addition, the
acetylation of 3 and 4 at the hydroxyl groups on the ribose unit gave the diacetylated derivatives
3a and 4a, respectively, which exhibited no interconversion phenomenon. The acquisition of
pure samples of 3a and 4a confirmed the interconversion mechanism discussed earlier.
Compounds 1–4 were evaluated for antibacterial activity against seven strains of pathogenic
bacteria. Compounds 1/2 exhibited selective antibacterial activity against Staphylococcus
epidermidis with an MIC value of 12.5 mM, while their methylated derivatives 3/4 were found to
be inactive against the tested pathogenic bacteria at a concentration of 50 mM. It indicated that
the hydroxyl at C-400 may have a positive contribution to the antibacterial activity. In addition,
compounds 1–4 were also tested for their lethal activity towards brine shrimp Artemia salina.
Only compounds 1/2 were active with an LC50 value of 8.4 mg/mL. Compounds 1/2 and 3/4
exhibited no cytotoxic activity against human erythroleukaemia K562 and human promyelocytic
leukaemia HL-60 cell lines.
3. Experimental
3.1. General experimental procedures
UV spectra were obtained on a Beckman DU 640 spectrophotometer (Beckman Coulter, Inc.,
Brea, CA, USA). IR spectra were recorded on a Nicolet-Nexus-470 spectrometer (Nicolet Corp.,
Madison, WI, USA) using KBr pellets. NMR spectra were acquired using a JEOL JEM-ECP
1
NMR spectrometer (JEOL Ltd., Tokyo, Japan) (600 MHz for H and 150 MHz for 13C), using

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M. Chen et al.
TMS as internal standard. ESI-MS spectra were obtained from a Micromass Q-TOF
spectrometer (Waters Corp., Milford, MA, USA). HPLC was performed on a Waters 1525
system coupled with a Waters 2996 photodiode array detector, using a C18 column (Kromasil,
5 mm, 250 mm £ 10 mm) for semi-preparation HPLC. Silica gel (Qing Dao Hai Yang Chemical
Group Co., Qingdao, China 200–300 mesh) and Sephadex LH-20 (Amersham Biosciences Inc.)
were used for column chromatography (CC). Precoated silica gel plates (Yan Tai Zi Fu
Chemical Group Co., Yantai, China G60, F-254) were used for thin layer chromatography.
3.2. Fungal material
The marine-derived fungus A. versicolor was isolated from the inner part of a fresh gorgonian D.
gemmacea, which was collected from the Xisha Islands coral reef of the South China Sea in
December 2009. The strain was deposited at the Key Laboratory of Marine Drugs, The Ministry
of Education of China, School of Medicine and Pharmacy, Ocean University of China, Qingdao,
People’s Republic of China. This fungus strain was identified as A. versicolor according to its
morphological traits and a molecular biological protocol by amplification and sequencing of the
DNA sequences of the ITS region of the rRNA gene. The 565 base-pair ITS sequence had 99%
sequence identity to that of the A. versicolor strain ATCC 9577 (NCBI GenBank accession
number AY373880).
3.3. Fermentation, extraction and isolation
The fungal strain of A. versicolor was cultivated statically in a rice medium (100 mL seawater,
100 g rice) in 1-L Erlenmeyer flasks for five weeks at RT. The fermented rice substrate (20
flasks) was extracted with EtOAc (3 £ 300 mL for each flask), and the solvent was combined
and concentrated in vacuo to afford a residue (17.5 g). The residue was separated into five
fractions (Fr.1–Fr.5) by silica gel CC using a step-gradient elution with MeOH–CH2Cl2 (0–
100%). Fr.3 was isolated by silica gel CC eluting with CH2Cl2–MeOH (10:1, v/v), and was
further purified by using semi-preparative HPLC eluting with 35% MeOH–H2O to yield 3
(80 mg) and 4 (96 mg). Fr.4 was subjected to Sephadex LH-20 CC eluting with a mixture of
CH2Cl2–MeOH (1:1, v/v), and was further purified by semi-preparative HPLC (20% MeOH–
H2O) to obtain 1 (12 mg) and 2 (15 mg). All the isolated compounds turned to be a mixture
rapidly.
3.3.1. Kipukasins H and I (1/2)
Colourless oil. UV (MeOH) lmax (log 1): 205 (2.56), 260 (1.84) nm. IR (KBr) nmax 3774,
3445, 1679, 1649, 1558, 1539, 1513, 1397, 1085 cm21. ESI-MS m/z 409.2 [M þ H]þ, 431.2
[M þ Na]þ. HR-ESI-MS m/z 409.1237 [M þ H]þ (calcd for C18H21N2O9, 409.1242).
1
Kipukasin H (1): H NMR (dimethyl sulfoxide (DMSO)-d6, 600 MHz, d, ppm): d 11.39 (1H,
brs, 3-NH), 9.83 (1H, brs, 400-OH), 7.94 (1H, d, J ¼ 7.8 Hz, H-6), 6.27 (1H, d, J ¼ 2.4 Hz, H-
300), 6.22 (1H, d, J ¼ 2.4 Hz, H-500), 6.08 (1H, d, J ¼ 6.6 Hz, H-10), 5.70 (1H, d, J ¼ 7.8 Hz, H-
5), 5.55 (1H, d, J ¼ 5.4 Hz, 30-OH), 5.31 (1H, dd, J ¼ 6.6, 5.4 Hz, H-20), 5.23 (1H, t,
J ¼ 5.4 Hz, 50-OH), 4.29 (1H, dt, J ¼ 5.4, 3.0 Hz, H-30), 3.91 (1H, q, J ¼ 3.0 Hz, H-40), 3.64
(3H, s, H-900), 3.57–3.67 (2H, m, H2-50), 2.15 (3H, s, H-800). 13C NMR (DMSO-d6, 150 MHz,
d, ppm): 166.8 (C, C-700), 163.6 (C, C-4), 160.3 (C, C-400), 159.0 (C, C-200), 151.0 (C, C-2),
141.2 (CH, C-6), 138.7 (C, C-600), 113.8 (C, C-100), 109.5 (CH, C-500), 102.8 (CH, C-5), 97.4
(CH, C-300), 86.4 (CH, C-10), 85.7 (CH, C-40), 75.4 (CH, C-20), 69.4 (CH, C-30), 61.4 (CH2, C-
1
50), 56.1 (CH3, C-900), 20.0 (CH3, C-800). Kipukasin I (2): H NMR (DMSO-d6, 600 MHz, d,
ppm): 11.36 (1H, brs, 3-NH), 9.85 (1H, brs, 400-OH), 7.88 (1H, d, J ¼ 7.8 Hz, H-6), 6.29

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(1H, d, J ¼ 2.4 Hz, H-300), 6.24 (1H, d, J ¼ 2.4 Hz, H-500), 5.83 (1H, d, J ¼ 6.6 Hz, H-10), 5.73
(1H, brs, 50-OH), 5.71 (1H, d, J ¼ 7.8 Hz, H-5), 5.55 (1H, brs, 20-OH), 5.25 (1H, dd, J ¼ 5.4,
2.4 Hz, H-30), 4.36 (1H, dd, J ¼ 6.6, 5.4 Hz, H-20), 4.07 (1H, q, J ¼ 2.4 Hz, H-40), 3.70 (3H, s,
H-900), 3.64 (2H, m, H-50), 2.19 (3H, s, H-800). 13C NMR (DMSO-d6, 150 MHz, d, ppm): 166.4
(C, C-700), 162.8 (C, C-4), 159.5 (C, C-200), 159.5 (C, C-400), 150.6 (C, C-2), 140.2 (CH, C-6),
137.9 (C, C-600), 113.4 (C, C-100), 108.8 (CH, C-500), 102.1 (CH, C-5), 96.7 (CH, C-300), 85.7
(CH, C-10), 82.7 (CH, C-40), 73.2 (CH, C-30), 71.4 (CH, C-20), 60.9 (CH2, C-50), 55.4 (CH3, C-
900), 19.4 (CH3, C-800).
3.4. Preparation of acetylated derivatives of 3 and 4
To a solution of 3/4 (20 mg) in dry acetone (2.0 mL), Ac2O (100 mL) and K2CO3 (10 mg) were
added at RT, and the reaction mixture was stirred for 6 h. The solvent was evaporated in vacuo
and the residue was purified by semi-preparative HPLC eluting with 80% MeOH–H2O to give
3a (6.9 mg) and 4a (7.8 mg).
3.4.1. Diacetylkipukasin E (3a)
1
Colourless oil. H NMR (CDCl3, 600 MHz, d, ppm): 8.82 (1H, brs, 3-NH), 7.41 (1H, d,
J ¼ 8.4 Hz, H-6), 6.32 (2H, d, J ¼ 2.4 Hz, H-300, H-500), 6.10 (1H, d, J ¼ 6.0 Hz, H-10), 5.79 (1H,
dd, J ¼ 8.4, 1.8 Hz, H-5), 5.52 (1H, dd, J ¼ 6.0, 3.6 Hz, H-30), 5.42 (1H, t, J ¼ 6.0 Hz, H-20),
4.43–4.46 (2H, m, H-40, Ha-50), 4.37 (1H, dd, J ¼ 13.8, 3.6 Hz, Hb-50), 3.81 (6H, s, H-900, H-
1000), 2.31 (3H, s, H-800), 2.16 (3H, s, H-1100), 2.06 (3H, s, H-1300). ESI-MS m/z 529.1 [M þ Na]þ.
3.4.2. Diacetylkipukasin D (4a)
1
Colourless oil. H NMR (CDCl3, 600 MHz, d, ppm): 8.53 (1H, brs, 3-NH), 7.42 (1H, d,
J ¼ 8.4 Hz, H-6), 6.31 (1H, d, J ¼ 1.8 Hz, H-300 or H-500), 6.29 (1H, d, J ¼ 1.8 Hz, H-500 or H-300)
6.07 (1H, d, J ¼ 5.4 Hz, H-10), 5.79 (1H, dd, J ¼ 8.4, 1.8 Hz, H-5), 5.56 (1H, t, J ¼ 5.4 Hz, H-30
or H-20), 5.43 (1H, t, J ¼ 5.4 Hz, H-20 or H-30), 4.34–4.39 (3H, m, H-40, H-50), 3.80 (3H, s, H-900
or H-1000), 3.76 (3H, s, H-1000 or H-900), 2.29 (3H, s, Hþ-800), 2.16 (3H, s, H-1100 or H-1300), 2.08
(3H, s, H-1300 or H-1100). ESI-MS m/z 529.1 [M þ Na] .
3.5. Biological assays
The antibacterial assay was carried out as described previously (Pierce et al. 2008). Seven
bacterial strains were used, including Bacillus cereus (ACCC 11077), Micrococcus luteus
(ATCC 49732), S. epidermidis (ATCC 12228), Staphylococcus aureus (ATCC 27154),
Escherichia coli (ATCC 25922), Vibrio parahemolyticus (ATCC 17802) and Pseudomonas
putida (ATCC 49128), and ciprofloxacin was used as a positive control. The brine shrimp
lethality assay was performed on A. salina according to the published protocols (Meyer et al.
1982; Solis et al. 1993). DMSO was used as a positive control. The cytotoxic activities against
human erythroleukaemia K562 and human promyelocytic leukaemia HL-60 cell lines were
determined by the MTT method (Mosmann 1983). Adriamycin was used as a positive control.
Supplementary material
1
The 1D and 2D NMR, and MS spectra of 1/2, the H NMR and MS spectra of 3a and 4a and
1
selected H–1H COSY and HMBC correlations for 1 are available online.

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M. Chen et al.
Acknowledgements
We thank Dr Chang-Lun Shao for collection of the gorgonian samples. This work was supported by the
National Natural Science Foundation of China (Nos 41130858; 81172977), the National High Technology
Research and Development Program of China (863 Program) (No. 2013AA093001) and the Fundamental
Research Funds for the Central Universities of China (No. 201122005).
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