The Journal of Antibiotics
https://doi.org/10.1038/s41429-017-0015-x
ARTICLE
Two new secondary metabolites from a fungus of the genus
Robillarda
Takeo Shimoyama1 Mizuki Miyoshi1 Tatsuo Nehira2 Atsuko Motojima
Yasuhiro Igarashi1
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●
●
3
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Tsutomu Oikawa3 Olivier Laurence4
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Received: 6 July 2017 / Revised: 8 November 2017 / Accepted: 17 November 2017
© Japan Antibiotics Research Association 2018
1234567890
Abstract
Two new compounds, robillafuran and (+)-robillapyrone, were isolated from the culture extract of a fungal strain of
Robillarda along with (+)-monascuspyrone. The absolute configuration of (+)-robillapyrone and (+)-monascuspyrone was
determined by ECD calculation. These three compounds showed preadipocyte differentiation activity at 10–40 μM.
Introduction
In fungal species, Penicillium and Aspergillus are the
leading producers of secondary metabolites, accounting for
more than 2000 known fungal bioactive compounds [1, 2].
The phylum Ascomycota to which these species belong
contains a number of potential producers such as Pestalotiopsis and Xylaria [3, 4]. More specifically, these genera
are classified under the order Xylariales. Species of this
order are taxonomically diverse but many of them are not
known or not evaluated as secondary metabolite producers.
The genus Robillarda is one of such unexplored fungi.
Species of Robillarda are found from live plants and
This article is dedicated to Prof. Hamao Umezawa on the occasion of
the 60th anniversary of worldwide marketing of kanamycin.
Electronic supplementary material The online version of this article
(https://doi.org/10.1038/s41429-017-0015-x) contains supplementary
material, which is available to authorized users.
* Yasuhiro Igarashi
yas@pu-toyama.ac.jp
1
Biotechnology Research Center and Department of
Biotechnology, Toyama Prefectural University, 5180 Kurokawa,
Imizu, Toyama 939-0398, Japan
2
Graduate School of Integrated Arts and Sciences, Hiroshima
University, 1-7−1 Kagamiyama, Higashi-Hiroshima 739−8521,
Japan
3
School of Nutrition and Dietetics, Kanagawa University of Human
Services, 1-10-1 Heisei-cho, Yokosuka, Kanagawa 238-8566,
Japan
4
Mycosphere, 20, Avenue Jean Jaurès, 47500 Fumel, France
decayed plant materials and also in soil and 38 species are
listed [5]. However, there was no report on secondary
metabolites from this genus to date. In our continuing
investigation of chemical diversity in unexplored/underexplored microbes [6–8], a strain of Robillarda was found
to produce new compounds robillafuran (1) and (+)-robillapyrone (2) along with (+)-monascuspyrone [9] (3)
(Fig. 1). We herein describe the isolation and characterization of these compounds and their biological activity.
Results and discussion
Robillarda sp. MS9788 was cultured on A3M soft-agar
medium that gave the most stable and the highest production results. This strain did not grow well in liquid medium
or secondary metabolite production was not stable in solid
medium composed of grains such as buckwheat and millet.
HPLC-UV analysis of the culture extract indicated the
presence of two major peaks displaying the UV spectra with
the absorption maxima around 300 nm. Some minor peaks,
one of which showed a UV spectrum similar to the major
peaks, were also detected at earlier retention times. Several
steps of chromatographic purification led to the isolation of
one pyran (1) and two α-pyrone derivatives (2 and 3).
Robillafuran (1) was obtained as a pale yellow amorphous that gave an [M+Na]+ peak at m/z 203.0678 in the
positive-ion mode HR-ESITOFMS appropriate for a molecular formula of C10H12O3 (Δ −0.1 mmu, calcd. for
C10H12O3Na 203.0679). The IR spectrum displayed the
absorption band at 1702 cm−1 for the ester carbonyl group.
13
C NMR and HSQC spectral data established 10 carbons
T. Shimoyama et al.
Fig. 2 Selected 2D NMR correlations for 1 and 2
Fig. 1 Structures of robillafuran (1), (+)-robillapyrone (2), and
(+)-monascuspyrone (3)
Table 1
1
H and
13
C NMR data for robillapyran (1) in CD3OD
Position
δH mult (J in Hz)a
1
1.88 d (7.15)
14.4
3
2
6.65 q (7.15)
131.2
4, 9
3
δCb
HMBCc
128.5
4
162.9
5
6.14 d (2.05)
99.0
3, 4
6
5.59 d (2.05)
88.8
7, 8
7
167.2
8
174.3
9
1.90 s
12.3
2, 3, 4
10
3.88 s
57.2
8
a
Recorded at 500 MHz
b
Recorded at 125 MHz
c
HMBC correlations are from proton(s) stated to the indicated carbon
that were assigned to two methyls, one methoxy, three
olefinic methines, one quaternary sp2 carbon, and three
oxygenated sp2 carbons (Table 1). COSY spectrum showed
correlations between the methyl and an olefinic proton
(H1/H2) and between olefinic protons (H5/H6) (Fig. 2). The
doublet methyl H1 showed HMBC correlations to two
olefinic carbons C2 and C3, and the singlet methyl
H9 showed correlations to three sp2 carbons C2, C3, and
C4. These correlation data established 1-methyl-1-propenyl
group connecting at C4. Protons H5 and H6 were HMBC-
correlated to C3 and C4, and C7 and C8, respectively,
which confirmed the carbon connectivity from C3 to C8.
The methoxycarbonyl group at C7 was confirmed by an
HMBC correlation from the methoxy protons H10 to the
carbonyl carbon C8. Finally, the oxygen atom was placed
between C4 and C7 in consideration of the chemical shifts
of these carbons as well as the molecular formula. The E
configuration of the double bond between C2 and C3 was
deduced on the basis of NOESY analysis in which correlations were not detected for H2/H9 or H1/H5 while the
correlation was observed between H5 and H9.
Robillapyrone (2) was obtained as a colorless oil. The
molecular formula of C19H30O6, established by the HRESITOFMS data (m/z 377.1940 for [M+Na]+, Δ −0.1
mmu, indicating two hydrogen loss and one oxygen addition compared to monascuspyrone (3). The UV spectrum
showed an absorption band at 304 nm, slightly red-shifted
comparing with 3 that showed λmax at 298 nm. Analysis of
the 1H and 13C spectra revealed the presence of one triplet
and one singlet methyls, one methoxy group, nine methylenes, one olefinic methine, and six quaternary carbons (one
oxygenated sp3, one non-oxygenated and four oxygenated
sp2). Six sp2 carbons and the methoxy group could be
assigned to constitute the α-pyrone moiety in comparison
with the NMR data of 3 and by analysis of HMBC correlation data. The only significant difference of NMR data
between 2 and 3 was the presence of an additional oxygenated sp2 carbon and the absence of the oxygenated
methylene in 2. There are four oxygenated sp2 carbons (δC:
162.1, 167.6, 175.1, and 176.1) in 2, whereas three (δC:
164.9, 167.0, and 170.6) in 3, suggestive of the replacement
of the hydroxymethyl group of 3 with a carboxyl group.
This was supported by the IR spectrum of 3 that displayed
an absorption band at 1729 cm−1 for the carboxylic acid.
HMBC correlations from H18 to C3 and H4 to C2 and C5
4.0
5.0
2.0
2.5
Δε
Δε
Secondary metabolites of Robillarda
0
-2.0
Experimental ECD of 2
0
-2.5
Experimental ECD of 3
Calculated ECD of (R)-2
-4.0
200
Calculated ECD of (R)-3
Calculated ECD of (S)-2
250
300
350
400
wavelength (nm)
-5.0
200
Calculated ECD of (S)-3
250
300
350
400
wavelength (nm)
Fig. 3 Calculated and experimental ECD spectra of (+)-2
Fig. 4 Calculated and experimental ECD spectra of (+)-3
established the connectivity of the carbons from C2 to C5.
The remaining two carbons at δC 162.1 and 167.6 were
assigned to the pyrone carbonyl carbon C1 and the carboxyl
carbon C17 connecting at C2. The 11-carbon alkyl chain
bearing a tertiary hydroxy group and a methyl group (C19)
was connected to C5 on the basis of the HMBC correlations
from H7 and H19 to C5 to complete the planar structure of
2 (Fig. 2).
Monascuspyrone (3) was first reported from Monascus
pilosus without stereochemical assignment, but its specific
rotation was zero, implying that it was a racemate [9].
Meanwhile, monascuspyrone (3) produced by Robillard sp.
MS9788 was chiral with the positive specific rotation ([α]D
+ 53). Robillapyrone (2) also showed the same positive
sign and the similar absolute value of specific rotation ([α]D
+ 31), suggestive of the same absolute configuration for
these compounds. In the electronic circular dichroism
(ECD) spectra, both (+)-2 and (+)-3 displayed a negative
and a positive Cotton effects around 210 and 300 nm,
respectively. As no empirical rule was applicable to the
compounds, we performed theoretical calculations [10–12]
to predict the experimental ECD spectra of (+)-2 and (+)-3.
A thorough evaluation of stable conformers was carried out,
since both (+)-2 and (+)-3 have the same flexible
11-carbon chain that inevitably raises the number of stable
conformers. Under a systematic alteration of the elevencarbon chain of (+)-2 and (+)-3, conformational searches
with molecular mechanics [13–15] and subsequent structure
optimizations with quantum mechanics [16] were performed. Relevant stable conformers were subjected to the
ECD calculations with quantum mechanics [16] at high
approximation. As a result, it was shown that all evaluated
conformers had the positive Cotton effect at 290–300 nm,
whereas the sign of the Cotton effect at 210 nm varied
between conformers. Consequently, the absolute configurations of (+)-2 and (+)-3 were represented dominantly
with the positive Cotton effect at 290–300 nm. The calculated ECD spectrum of the R-enantiomer of (+)-2 showed a
good accordance with the experimental ECD spectrum
(Fig. 3). The calculated ECD of (R)-(+)-3 was also in good
agreement with the experimental data of (+)-3 (Fig. 4).
Therefore, the absolute configuration at C6 of (+)-2 and
(+)-3 was determined to be R.
Adipocyte differentiation is the process of cell differentiation in which preadipocytes change into adipocytes
[17]. The key function of mature adipocytes is the secretion
of adiponectin, which enhances glucose uptake and
improves insulin sensitivity in type 2 diabetes patients [18].
Therefore, inducers of preadipocyte differentiation can be a
promising therapeutic agent for insulin resistance and type 2
diabetes [19]. Inducing potential of robillafuran (1),
(+)-robillapyrone (2), and (+)-monascuspyrone (3) for
murine ST-13 preadipocyte differentiation were evaluated at
concentrations of 10–40 μM by analyzing the accumulation
of lipid droplets [20]. 1 was weakly active (50% differentiation) at 40 μM, while (+)-2 induced ca. 90% differentiation at 40 μM. Both 1 and (+)-2 displayed no toxic or
growth-retarding effect on ST-13 cells at 40 μM. (+)-3 was
the most potent among the three compounds: 80–100% of
preadipocytes were differentiated at 10–20 μM (Fig. 5). At
40 μM, (+)-3 inhibited the growth of preadipocyte cells
although it was not toxic.
In conclusion, our chemical investigation of Robillarda
sp. MS9788 resulted in the isolation of two new compounds
robillafuran (1) and (+)-robillapyrone (2). This is the first
report on the secondary metabolites from the genus Robillarda. This finding supports the idea that consideration of
taxonomic position is essential to the discovery of new
chemical entities. Continuous screening efforts along this
line will further disclose the biosynthetic potential of
unexplored species.
T. Shimoyama et al.
$UELWUDU\XQLWV
was growing was inoculated onto twenty 500-mL K-1 flasks
each containing 100 mL of A3M soft-agar medium [0.5%
glucose, 2.0% glycerol, 2.0% soluble starch, 1.5% Pharmamedia (Trader’s protein), 0.3% yeast extract, and 0.4% agar].
The flasks were placed in dark at 25 °C for 30 days without
shaking. An aliquot of 100 mL of 1-butanol was added to each
flask and the flasks were agitated for 1 h on a rotary shaker at
30 °C. The organic layer was separated by centrifugation (4000
rpm, 10 min) and concentrated to give crude extract (37.4 g).
Isolation
1&
( µM) ( µM) ( µM) ( µM)
3&
(NC: vehicle, PC: 0.02 µM rosiglitazone)
Fig. 5 Induction of adipocyte differentiation by 1–3
Experimental section
General experimental procedures
UV spectra were recorded on a Hitachi U-3210 spectrophotometer, and IR spectra on a Perkin-Elmer Spectrum
100. Optical rotations were measured using a JASCO DIP3000 polarimeter, and CD spectra on a JASCO J-720W
spectropolarimeter. NMR spectra were obtained on a Bruker
AVANCE 500 spectrometer in CDCl3 using the signals of
the residual solvent protons (δH 7.26) and carbons (δC 77.0)
as an internal standard. HR-ESITOFMS were recorded on a
Bruker microTOF focus.
Fungal strain
The producing strain MS9788 was isolated from submerged
decaying stems of Phragmipes sp. by Olivier Laurence
(Mycosphere, France) during a fungal survey expedition
near the Kaw swamps (French Guyana) in 2009. The strain
was identified as Robillarda based on the gene sequence of
the D1/D2 domain of 28S rRNA gene. Strain MS9788 (562
nucleotides, DDBJ accession number LC309276) showed
98.3% similarity to Robillarda sessilis BCC13393 (accession number FJ825378) and Robillarda sessilis CBS
114312 (accession number KR873284).
Fermentation
A piece of MYA2 agar medium [2.0% malt extract (Difco
Laboratories), 0.1% yeast extract (Kyokuto Chemical Co.,
Japan), and 1.5% agar] on which Robillarda sp. MS9788
A portion (5.25 g) of the crude extract was subjected to
silica gel column chromatography with a step gradient of
CHCl3/MeOH (1:0, 20:1, 10:1, 4:1, 2:1, 1:1, and 0:1 v/v).
The fraction eluted with 4:1 CHCl3–MeOH was containing
the target peaks and concentrated to give 817 mg of brown
oil, which was further purified by reversed phase ODS
column chromatography with a gradient of MeCN/0.1%
HCO2H (2:8, 3:7, 4:6, 5:5, 6:4, 7:3, and 8:2 v/v). The 5:5
fraction (27 mg) was evaporated and the remaining aqueous
solution was extracted with EtOAc, and the organic layer
was concentrated to dryness. The residual amorphous solid
was purified by reverse-phase HPLC using a Cosmosil
5C18-AR-II column (Nacalai Tesque Inc., 10 × 250 mm)
with a linear gradient of MeCN/0.1% HCO2H
(40:60–50:50) over 30 min at a flow rate of 4 mL/min,
followed by evaporation and extraction with EtOAc,
yielding robillafuran (1, 8.6 mg, tR 12.7 min). Similarly, the
7:3 ODS fraction (37 mg) was purified by preparative
HPLC with isocratic elution (MeCN/0.1% HCO2H =
52:48) to give (+)-monascuspyrone (3, 9.6 mg, tR 22.8
min). HPLC purification of the 8:2 ODS fraction (38 mg)
gave (+)-robillapyrone (2, 16.6 mg, tR 26.6 min).
Robillafuran (1): Pale yellow amorphous; UV (MeOH)
λmax (log ε) 224 (4.77), 310 (4.29) nm; IR (ATR) νmax 1702,
1076 cm−1; 1H and 13C NMR data, see Table 1; HRESITOFMS [M+Na]+ 203.0678 (calcd for C10H12O3Na,
203.0679).
(+)-Robillapyrone (2): Colorless oil; [α]21D + 31 (c 0.16,
MeOH); UV (MeOH) λmax (log ε) 207 (4.18), 304 (3.82)
nm; CD (MeCN) Δε216 − 1.98, Δε303 + 3.11; IR (ATR)
νmax 3405, 1729, 1668, 1238 cm−1; 1H and 13C NMR data,
see Table 2; HR-ESITOFMS [M+Na]+ 377.1940 (calcd for
C19H30O6Na, 377.1941).
(+)-Monascuspyrone (3): Colorless oil; [α]21D + 53 (c
0.21, CHCl3) {lit. [α]22D ± 0 (c 0.08, CHCl3)}; UV (MeOH)
λmax (log ε) 206 (4.21), 298 (3.68) nm; CD (MeCN) Δε205
− 4.29, Δε298 + 4.12; IR (ATR) νmax 3366, 1665, 1258 cm
−1 1
; H and 13C NMR data, see Table 2; HR-ESITOFMS [M
+Na]+ 363.2148 (calcd for C19H32O5Na, 363.2147).
Secondary metabolites of Robillarda
Table 2 1H and 13C NMR data for robillapyrone (2) and monascuspyrone
(3) in CDCl3
Position Robillapyrone (2)
δH mult
(J in Hz)a
Monascuspyrone (3)
δC
c
b
HMBC δH mult
(J in Hz)
δCb
1
167.6
164.9
2
94.6
103.8
3
176.1
4
6.81 s
5
94.3
167.0
2, 5, 6
6.52 s
175.1
92.2
170.6
6
74.6
74.1
7
1.76 ddd
40.7
(13.8, 12.4, 4.6)
40.6
5, 6, 8, 1.71 ddd
(13.9, 12.2, 4.5)
19
1.91 ddd
(13.9, 12.2, 4.5)
1.93 ddd
(13.8, 12.4, 4.6)
8
1.11 m
23.6
1.13 m
1.40 m
9
10
23.5
1.34 m
1.25–1.31 m
29.5d
1.11–1.39 m
29.3e
1.25–1.31 m
d
1.11–1.39 m
29.47e
d
29.6
11
1.25–1.31 m
29.7
1.11–1.39 m
29.54e
12
1.25–1.31 m
29.7d
1.11–1.39 m
29.54e
13
1.25–1.31 m
d
29.9
1.11–1.39 m
29.6e
14
1.25 m
32.1
15
1.26 m
22.9
16
0.88 t (6.8)
14.3
17
14, 15
31.9
22.7
0.88 t (6.8)
14.1
4.56 s
54.7
18
4.18 s
58.7
3
3.95 s
56.6
19
1.58 s
27.5
5, 6, 7
1.52 s
27.3
a
162.1
1.25 m
1.25 m
Recorded at 500 MHz
b
Recorded at 125 MHz
c
HMBC correlations are from proton(s) stated to the indicated carbon
d,e
Interchangeable
on a strategic modification of the alkyl chain. The 11-carbon
alkyl chain of 2 was stepwise shortened to 7-carbon and
expanded to 13-carbon. All the structures of interest were
composed and subjected to conformational searches with
CONFLEX7 using MMFF94S (2010-12-04HG) as the force
field, where initial stable conformers were generated for up
to 50 kcal/mol. In cases there were conformers with >1% of
abundance (7-, 8-, and 9-carbon derivatives), all of those
stable conformers were subjected to further structure DFTbased optimizations with B3LYP/6-31G(d) using acetonitrile as the solvent with the polarizable continuum model
(PCM) method. In other cases (10-, 11-, 12-, and 13-carbon
derivatives), over 100,000 stable conformers were found
and no conformer showed >1% of abundance. Based on the
results from the above-mentioned shorter-chained derivatives, conformers used for the further calculations were
limited for only the most stable six conformers. The
selected stable conformers were then relayed to the timedependent density functional theory-based (TDDFT-based)
ECD calculations with B3LYP/cc-pVDZ using acetonitrile
as the solvent. As a result, all the considered stable conformers, whatever the chain lengths were, possessed the
positive ECD peak at 290 nm, for which the wavelengths of
the calculated UV spectra were corrected to reproduce the
absorption at 300 nm. For the genuine 2 with the flexible
11-carbon alkyl chain, the outstandingly most stable four
conformers were selected. The obtained rotational strengths
were converted into Gaussian curves (bandwidth sigma =
3000 cm−1). The resultant ECD spectra were summed based
on the Bolzmann distribution to give the ECD spectrum.
The same calculation strategy was applied to 3, except
that the 11-carbon chain modification was considered with
only 10-carbon and 12-carbon chain models. Derived was
the same conclusion that the ECD signs at 290 nm for all
conformers of the simulated models with the R configuration
at C6 were positive.
ECD calculation
Biological assay
Theoretical electron circular dichroism (ECD) spectra were
obtained by integrating a typical calculation procedure
[10–12] and a strategic structure modification. Conformational searches were performed with CONFLEX7 (Ver. 7.
A.0910 by CONFLEX, Tokyo) [13–15] using a commercially available PC (operating system: Windows7 Professional SP1 64-bit, CPU: QuadCore Xeon E3-1225
processor 3.10 GHz, RAM 8 GB) and density functional
theory (DFT) calculations were conducted with Gaussian 09
(Revivion D.01 by Gaussian, Wallingford, CT) [16] with a
PC (Operating System: CentOS 6.5 a Linux, CPU: 12 Intel
Xeon E5-2643 v3 processors 3.40 GHz, RAM 32 GB).
The numbers of relevant conformers for 2 and 3 that
possess a flexible 11-carbon alkyl chain were reduced based
Adipocyte differentiation assay was carried out according to
the procedure previously described [21–23]. Rosiglitazone,
an antidiabetic drug, was used as a positive control in the
adipocyte differentiation assay. It induced differentiation in
80% of murine ST-13 preadipocyte cells at 0.02 μM.
Acknowledgements This research was supported by JSPS KAKENHI
Grant No. JP16K07719 to YI and a Grant-in-Aid for Scientific
Research (A) (25252037) from JSPS to TO.
Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of
interests.
T. Shimoyama et al.
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