Article
pubs.acs.org/jnp
Rotenoids, Flavonoids, and Chalcones from the Root Bark of Millettia
usaramensis
Tsegaye Deyou,†,‡ Ivan Gumula,†,‡ Fangfang Pang,§ Amra Gruhonjic,‡,⊥ Michael Mumo,† John Holleran,∥
Sandra Duffy,∥ Paul A. Fitzpatrick,⊥ Matthias Heydenreich,∇ Göran Landberg,⊥ Solomon Derese,†
Vicky Avery,∥ Kari Rissanen,§ Máté Erdélyi,*,‡,# and Abiy Yenesew*,†
†
Department of Chemistry, University of Nairobi, P.O. Box 30197-00100, Nairobi, Kenya
Department of Chemistry and Molecular Biology, ⊥Sahlgrenska Cancer Centre, and #Swedish NMR Center, University of
Gothenburg, SE-40530, Gothenburg, Sweden
§
Department of Chemistry, Nanoscience Center, University of Jyvaskyla, P.O. Box. 35, FI-40014 Jyvaskyla, Finland
∥
Discovery Biology, Eskitis Institute for Drug Discovery, Griffith University, Nathan Qld 4111 Australia
∇
Institut für Chemie, Universität Potsdam, Karl-Liebknecht-Straße 24-25, D-1146, Potsdam, Germany
‡
S Supporting Information
*
ABSTRACT: Five new compounds, 4-O-geranylisoliquiritigenin (1), 12-dihydrousararotenoid B (2), 12-dihydrousararotenoid C (3), 4′-O-geranyl-7-hydroxyflavanone (4), and 4′O-geranyl-7-hydroxydihydroflavanol (5), along with 12 known
natural products (6−17) were isolated from the CH2Cl2/
MeOH (1:1) extract of the root bark of Millettia usaramensis
ssp. usaramensis by chromatographic separation. The purified
metabolites were identified by NMR spectroscopic and mass
spectrometric analyses, whereas their absolute configurations
were established on the basis of chiroptical data and in some
cases also by X-ray crystallography. The crude extract was
moderately active (IC50 = 11.63 μg/mL) against the ER-negative MDB-MB-231 human breast cancer cell line, and accordingly
compounds 6, 8, 9, 10, 12, and 16 also showed moderate to low cytotoxic activities (IC50 25.7−207.2 μM). The new natural
product 1 exhibited antiplasmodial activity with IC50 values of 3.7 and 5.3 μM against the chloroquine-sensitive 3D7 and the
chloroquine-resistant Dd2 Plasmodium falciparum strains, respectively, and was also cytotoxic to the HEK293 cell line.
T
he genus Millettia (Leguminoseae, subfamily: Papilionoideae) consists of more than 200 species that are native in
the tropical and subtropical regions of Africa, Asia, and
Australia.1,2 Of these, 139 species are endemic to Africa.
Millettia usaramensis ssp. usaramensis, a shrub or tree that can
grow up to 10 m high, is one of the six Millettia species that are
found in Kenya.3 In traditional medicine, its roots are used as
antidote against snake bite.4 Whereas the roots of this plant
have not yet been phytochemically analyzed, previous
investigations of its stem bark yielded unique 12a-hydroxyrotenoids with the unusual trans-B/C ring junction as well as
chalcones and isoflavones.5,6 These compound groups have
lately been recognized as emerging leads for antimalarial7,8 and
anticancer9,10 therapy. Herein, the isolation and identification
of a new chalcone (1), two new 12-dihydrorotenoids (2, 3), a
new flavanone (4), a new dihydroflavonol (5), and 12 known
secondary metabolites (6−17) are reported. The antiplasmodial activity of compound 1 and the cytotoxic activities of some
of the compounds are also presented.
© 2015 American Chemical Society and
American Society of Pharmacognosy
■
RESULTS AND DISCUSSION
Column chromatographic separation of the CH2Cl2/MeOH
(1:1) extract of the dried and ground root bark of M.
usaramensis ssp. usaramensis, followed by gel filtration over
Sephadex LH-20, and further purification by MPLC and RPHPLC afforded five new secondary metabolites (1−5) and the
12 known compounds usararotenoid A (6),5 12-dihydrousararotenoid A (7),5 millettosin (8),11 12a-epimillettosin (9),5
usararotenoid C (10),6 jamaicin (11),12 4′-O-geranylisoliquiritigenin (12),13 7-O-geranyl-5-hydroxyflavanone (13),14 tephrosin (14),15 maximaisoflavone H (15),16 colenemol (16),17
and 7-hydroxy-8,3′,4′-trimethoxyisoflavone (17).18 Compounds 6−17 were previously reported from the stem bark
of the plant,5,6 from the seeds of Millettia dura,11 and from the
roots of Tephrosia villosa.14 The identities of 6−17 were
confirmed by comparison of their spectroscopic and physical
data to those previously published.
Received: July 1, 2015
Published: December 14, 2015
2932
DOI: 10.1021/acs.jnatprod.5b00581
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Journal of Natural Products
Article
Chart 1
As part of the structural work, the X-ray structures of
usararotenoid A5 (6, Figure 1), 12-dihydrousararotenoid A11
Figure 2. X-ray crystal structure of 12-dihydrousararotenoid A (7).
Figure 1. X-ray crystal structure of usararotenoid A (6).
(7, Figure 2), and 12a-epimillettosin5 (9, Figure 3) were
obtained. The latter solid-state structure was accomplished for
the first time, confirming the 6aR,12aS absolute configuration
of the B/C ring junction of 9, which was previously proposed
based on an [α]20
D value of +230.4 and the positive and negative
Cotton effects at 348 and 324 nm, respectively, in the electronic
circular dischroism (ECD) spectrum.
Figure 3. X-ray crystal structure of 12a-epimillettosin (9).
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Compound 1 was isolated as a yellow solid. Its HREIMS
molecular ion at m/z 392.1968 and 13C NMR data are
consistent with the molecular formula C25H28O4 (calcd
392.1988). Its UV absorbance at λmax 299 and 370 nm along
with the characteristic trans-olefinic doublets H-α (δH 7.39) and
H-β (δH 7.80) with 3J = 15.4 Hz, carbonyl (δC 192.2), C-α (δC
118.9), and C-β (δC 144.7) observed by NMR spectroscopy
(Table 1) is indicative of a chalcone core skeleton.19,20 A broad
This conclusion is further corroborated by the NOE between
H-1″ and CH3-10″. The NOE between the oxymethylene H-1″
(δH 4.60) of the geranyloxy side chain and the H-3/5 (δH 6.91)
of ring A reveals the position of geranyloxy substitution at C-4,
which was further supported by the HMBC correlation of H-1″
(δH 4.60) and C-4 (δC 161.3). The E-configuration of the 2″double bond was confirmed by the observation of an NOE
between H-2″ (δH 5.50) and H-4″ (δH 2.10−2.17) and the
absence of an NOE between H-2″ (δH 5.50) and CH3-10″ (δH
1.77, Supporting Information). On the basis of the above
spectroscopic data 1 was characterized as (E)-1-(2,4-dihydroxyphenyl)-3-(4-[{(E)-3,7-dimethylocta-2,6-dien-1-yl}oxy]phenyl)prop-2-en-1-one and was assigned the trivial name 4-Ogeranylisoliquiritigenin.
Compound 2 was obtained as a colorless, amorphous solid
and was assigned the molecular formula C19H18O8 based on
HREIMS analysis (M+ obs m/z 374.0999, calcd 374.1002) and
13
C NMR data (Table 2). Its 1H and 13C NMR data are
compatible with a 12-dihydro-12a-hydroxyrotenoid derivative.5
The ABC spin system of H-6a (δH 4.28, Table 2), H-6α (δH
4.34), and H-6β (δH 4.39) supports that 2 is a 12ahydroxyrotenoid derivative.5 In contrast to common rotenoids
possessing a C-12 carbonyl group, C-12 of 2 is an oxymethine
functionality, as revealed by its chemical shift of δC 70.6. The C12 hydroxylation is indicated by the coupling (d, J = 11.4 Hz)
of H-12 (δH 4.89) and OH-12 (δH 2.78); upon addition of D2O
to the solution, the signal of OH-12 disappeared and the
doublet of H-12 collapsed into a singlet. In ring D, the locations
of CH3O-8 (δH 3.79) and CH3O-9 (δH 3.84) are revealed by
their HMBC cross-peaks to C-8 (δC 136.5) and C-9 (δH 153.3),
respectively, and by the HMBC cross-peaks of the orthocoupled (J = 9.0 Hz) H-10 (δC 6.68) to C-8 and H-11 (δC
7.27) to C-9, indicating that ring D of 2 is ortho-dioxygenated.
The proposed di-ortho-substitution is corroborated by the
deshielding of OCH3-8 (δC 60.9).22 Ring A possesses two
isolated aromatic protons, i.e., H-1 (δH 7.77) and H-4 (δH
6.39), and a methylenedioxy substituent (δH 5.92, δC 101.9),
whose substitution pattern has been previously reported for the
rotenoids of this plant.5 The trans-orientation of the B/C ring
junction is indicated by a deshielding of H-1.6,23−25 Moreover,
the NOE of H-6a and H-12 indicates their 1,3-diaxial
relationship and, hence, their β-orientations. H-6a and H-6α
are in a 1,2-trans-diaxial orientation, as revealed by their large
scalar coupling (J = 10.8 Hz). The similar chemical shifts of
rings A−C and the coupling constants of the H-6α, H-6β, and
H-6a ABX spin system of 2 (Table 2) to those of 12dihydrousararotenoid A (7, JH6α,6a = 9.9 Hz, JH6β,6a = 4.6 Hz),
whose configuration5 was previously established by X-ray
crystallography6 and confirmed in this investigation (Figure 2),
further support its proposed relative configuration. Compound
2 gave a high positive specific rotation, [α]20
D +134, and a
positive ECD Cotton effect at ca. 295 nm, similar to 7,
indicating that their absolute configurations should be the same.
On the basis of the above spectroscopic evidence, compound 2
was characterized as (6aR,12R,12aR)-8,9-dimethoxy-6,6adihydrochromeno[2,3-c][1,3]dioxolo[4,5-g]chromene-12,12a(12H)-diol and was given the trivial name 12-dihydrousararotenoid B.
Compound 3 was isolated as a colorless, amorphous solid
and was assigned the molecular formula C23H24O7 based on
HREIMS analysis ([M]+ obs m/z 412.1520, calcd 412.1522)
and 13C NMR data (Table 2). Its NMR spectra showed
similarities to those of 2, suggesting a close structural
Table 1. 1H and 13C NMR Spectroscopic Data for 4-OGeranylisoliquiritigenin (1) Acquired in CD2Cl2 (δH,
Multiplicity (J in Hz))
position
1
2/6
3/5
4
CO
C-α
C-β
1′
2′
OH-2′
3′
4′
5′
6′
1″
2″
3″
4″
5″
6″
7″
8″
9″
10″
δC, type
127.4,
130.5,
115.3,
161.3,
192.2,
118.9,
144.7,
114.2,
163.6,
C
CH
CH
C
C
CH
CH
C
C
103.7,
166.2,
108.3,
132.0,
65.2,
117.6,
142.0,
39.6,
25.8,
123.8,
131.9,
26.3,
17.8,
16.8,
CH
C
CH
CH
CH2
CH
C
CH2
CH2
CH
C
CH3
CH3
CH3
δH, m (J in Hz)
HMBC (H→C)
7.54, AA′
6.91, XX′
β, 4, 6
1, 4
7.39, d (15.4)
7.80, d (15.4)
1, β
2, 6
13.64, br s
6.45, m
1′, 3′, 4′
1′, 2′, 4′, 5′
6.47,
7.82,
4.60,
5.50,
1′, 3′
2′, 4′, CO
4, 2″, 3″
m
d (8.0)
d (7.2)
t (7.2)
2.10−2.17, m
2.10−2.17, m
5.12, t (7.2)
2″, 3″
3″, 6″
5″, 8″, 9″
1.71, s
1.63, s
1.77, s
6″, 7″, 9″
6″, 7″, 9″
2″, 3″, 4″
singlet at δH 13.64 suggests a hydrogen-bonded hydroxy group
(OH-2′), whereas three mutually coupled aromatic protons, H3′ (δH 6.45), H-5′ (δH 6.47), and H-6′ (δH 7.82), indicate a
trisubstituted, dioxygenated B ring. The exclusive orthocoupling (J = 8.0 Hz) of H-6′ (δH 7.82) is compatible with a
2′,4′-dioxygenation of this trisubstituted ring, which is
corroborated by the NOESY and the HMBC cross-peak
pattern of 1 (Table 1, Figures S5 and S7, Supporting
Information). The AA′XX′ spin system (δH 6.91, 7.54) of the
A ring indicates 1,4-disubstitution, whereas the chemical shift of
C-4 (δC 161.3) reveals oxygenation at this position. Connection
of ring A to C-β of the olefinic moiety is revealed by the
HMBC cross-peaks between H-2/6 (δH 7.54) and C-β (δC
144.7) as well as between H-β (δH 7.80) and C-2/6 (δC 130.5).
Three methyl (δH 1.63, 1.71, and 1.77), one oxymethylene (δH
4.60, d, J = 7.2 Hz), two methylene (δH 2.10−2.17, m), and two
methine olefinic protons (δH 5.12, t, J = 7.2 Hz and δH 5.50, t, J
= 7.2 Hz) connected by COSY and TOCSY cross-peaks
(Figures S3 and S4, Supporting Information) suggest the
presence of a geranyloxy or a neryloxy substituent. The
chemical shifts of C-4″ (δC 39.6) and C-10″ (δC 16.8) are in
better agreement with a geranyloxy rather than a neryloxy
group, whose corresponding carbons would be expected to give
rise to signals at approximately δC 32 and δC 23, respectively.21
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Table 2. 1H and 13C NMR Spectroscopic Data for 12-Dihydrousararotenoid B (2) and 12-Dihydrousararotenoid C (3) Acquired
in CD2Cl2 and Acetone-d6, Respectively (δH, Multiplicity (J in Hz))
2
position
1
2
3
4
4a
6α
6β
6a
7a
8
9
10
11
11a
12
12a
12b
1′
2′
3′
4′
5′
OMe-8
OMe-9
−OCH2O-10/11
OH-12
OH-12a
δC, type
107.2,
142.7,
147.4,
98.4,
149.4,
62.6,
CH
C
C
CH
C
CH2
73.4,
149.9,
136.5,
153.3,
106.6,
123.6,
119.7,
70.6,
64.6,
115.7,
CH
C
C
C
CH
CH
C
CH
C
C
60.9, CH3
56.3, CH3
101.9, CH2
δH, m (J in Hz)
3
HMBC(H→C)
7.77, s
2, 3, 4a, 12a, 12b
6.39, s
2, 4a, 12b
4.34, dd (10.8, 9.6)
4.39, dd (9.6, 4.8)
4.28, dd (10.8, 4.8)
12, 12a
6.68, d (9.0)
7.27, d (9.0)
8, 9, 11a
9, 12
4.89, d (11.4)
11, 11a, 12b
3.79,
3.84,
5.92,
5.93,
2.78,
2.50,
s
s
d (1.2)
d (1.2)
d (11.4)
s
12a
8
9
2, 3
δC, type
107.2,
142.5,
149.1,
98.3,
149.6,
62.3,
CH
C
C
CH
C
CH2
70.7,
151.2,
118.0,
157.8,
105.3,
126.9,
117.0,
73.0,
64.4,
115.1,
22.3,
122.2,
131.5,
17.8,
25.8,
CH
C
C
C
CH
CH
C
CH
CH
C
CH2
CH
C
CH3
CH3
55.8, CH3
101.4, CH2
12, 12a
6a, 12, 12a, 12b
δH, m (J in Hz)
HMBC (H→C)
7.81, s
2, 3, 4, 4a, 12b
6.45, s
1, 2, 3, 4a
4.34, dd (13.8, 10.2)
4.37, dd (10.2, 3.6)
4.26, dd (10.8, 4.8)
4a, 12, 12a
6.67, d (9.0)
7.42, d (9.0)
7a, 8, 9, 11a
7a, 9, 10, 11a
4.91, d (10.8)
2.78, d (10.8)
7a, 11a, 12b
3.36, m
5.25, t (7.2)
7a, 8, 9, 2′, 3′
1′, 4′, 5′
1.68, s
1.78, s
2′, 3′
2′, 3′
3.85,
5.93,
5.95,
2.78,
9
2′, 3′
s
d (1.2)
d (1.2
d (10.8)
4a, 12a
the NOEs and J-couplings corresponding to that explained
above for 2. Moreover, its comparable chemical shifts and
coupling constants to those of 7, whose configuration was
confirmed by X-ray crystallography,6 support the proposed
relative configuration at C-6a, C-12a, and C-12. Once again the
high positive specific rotation [α]20
D +96 and positive ECD
Cotton effect at ca. 295 nm are consistent with the identical
configurations of the 12-dihydrorotenoids (2 and 7) of this
plant. On the basis of the above spectroscopic evidence, this
new compound (3) was characterized as (6aR,12R,12aR)-9methoxy-8-(3-methylbut-2-en-1-yl)-6,6a-dihydrochromeno[2,3-c][1,3]dioxolo[4,5-g]chromene-12,12a(12H)-diol and was
given the trivial name 12-dihydrousararotenoid C.
Compound 4 was obtained as a white solid. Its molecular
formula was determined to be C25H28O4 on the basis of HRMS
(EI: [M]+ obs m/z 392.1968, calcd 392.1987; ESI: [M + H]+
obs m/z 393.2068, calcd 393.2066) and 13C NMR (Table 3).
Analysis of its NMR data identified four 1H/1H spin systems:
the aromatic ABX system of ring A with H-5 (δH 7.79, Table 3),
H-6 (δH 6.57), and H-8 (δH 6.48), the aliphatic ABX spin
system of ring C with H-3a (δH 2.78), H-3b (δH 3.06), and H-2
(δH 5.40), the AA′XX′ system of ring B [(H-2′ (δH 7.37) and
H-3′ (δH 6.93)], and the spin system of the H-1″−H-10″
geranyloxy moiety13 (Table 3). The above data along with the
observation of the 13C NMR resonances 4-CO (δC 191.9),
C-2 (δC 79.9), and C-3 (δC 44.1) indicated a flavanone core
skeleton. Ring A is expected to be oxygenated at C-7 (δC
164.2), based on biogenetic considerations,23 while oxygenation
at C-4′ of ring B is revealed by its high chemical shift (δC
resemblance. Hence, for ring A of 3, two aromatic singlets H-1
(δH 7.81) and H-4 (δH 6.45) and a C-2/C-3 methylenedioxy
group (δH 5.94, δC 101.4) were evident. Its B ring possesses an
ABC spin system, H-6a (δH 4.26), H-6α (δH 4.34), and H-6β
(δH 4.37), typical for 12a-hydroxyrotenoids.5 Similar to 2,
compound 3 is also a 12-dihydrorotenoid derivative, evidenced
by the presence of the signals of a C-12 (δC 73.0) oxymethine
carrying a hydroxy group (δH 2.78). The latter proton couples
(J = 10.8 Hz) to H-12 (δH 4.91), and its signal along with its
coupling to H-12 disappears upon addition of D2O, confirming
its exchangeable nature. Further similarities of 2 and 3 are
confirmed by observation of a methylenedioxy at C-2/C-3 on
ring A, which has a pair of ortho-coupled aromatic protons, H10 (δH 6.67, d, J = 9.0 Hz) and H-11 (δH 7.42, d, J = 9.0 Hz),
and a methoxy functionality (δH 3.85, δC 55.8) at C-9 (δC
157.8) of ring A, whose placement is confirmed by the H-10
(δH 6.67) to CH3O-9 (δH 3.85) NOE. However, the C-8
methoxy of 2 is replaced with a C-prenyl unit in 3, as revealed
by the signals H-1′ (δH 3.36, m), H-2′ (δH 5.25, t, J = 7.2 Hz),
H-4′ (δ H 1.68, s), and H-5′ (1.78, s), showing the
corresponding COSY, NOESY, and HMBC cross-peak patterns
(Table 2, Figures S19−S22, Supporting Information). The
placement of the 3,3-dimethylallyl group at C-8 is confirmed by
the NOEs observed between CH3O-9 (δH 3.85) and H-1′ (δH
3.36) as well as H-2′ (δH 5.25, Figure S20, Supporting
Information) and by the HMBC cross-peaks of H-1′ (δH 3.36)
to C-7a (δC 151.2), C-8 (δC 118.0), and C-9 (δC 157.8, Table
2). Similar to 2, the trans-geometry of the B/C ring junction of
3 was derived from the strong deshielding of H-1 (δH 7.81) and
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Table 3. 1H and 13C NMR Spectroscopic Data for (2S)-4′-O-Geranyl-7-hydroxyflavanone (4) and (2R,3R)-4′-O-Geranyl-7hydroxyflavonol (5) Acquired in CD2Cl2 and DMSO-d6, Respectively (δH, Multiplicity (J in Hz))
4
position
2
3
3-OH
4
4a
5
6
7
8
8a
1′
2′
3′
4′
1″
2″
3″
4″
5″
6″
7″
8″
9″
10″
5
δC, type
δH, m (J in Hz)
HMBC (H→C)
δC, type
δH, m (J in Hz)
HMBC (H→C)
79.9, CH
44.1, CH2
5.40, dd (2.4, 13.2)
2.78, dd (2.4, 16.8)
3.06, dd (13.2, 16.8)
3, 1′, 2′, 6′
2, 1′
83.1, CH
72.5, CH
5.09, d (11.6)
4.50, dd (2.5, 11.6)
3, 4, 8a, 2′/6′
2, 4, 1′
5.52, d (2.5)
191.9,
114.9,
129.4,
110.9,
164.2,
103.6,
164.0,
130.9,
128.0,
115.7,
159.5,
65.3,
119.6,
141.6,
39.8,
26.5,
124.0,
132.0,
17.7,
25.7,
16.6,
CO
C
CH
CH
C
CH
C
C
CH
CH
C
CH2
CH
C
CH2
CH2
CH
C
CH3
CH3
CH3
7.79, d (8.4)
6.57, dd (2.4, 8.4)
7, 8a
7, 8, 4a
6.48, d (2.4)
6, 7, 4a, 8a
7.37, m
6.93, m
2, 3′, 4′, 6′
1′, 4′, 5′
4.56, d (6.6)
5.46, t (6.0)
4′, 2″, 3″
4″, 10″
2.11, m
2.11, m
5.10, m
2″, 6″
4″, 6″, 7″
8″, 9″
1.60, s
1.67, s
1.73, s
192.3,
111.9,
128.6,
111.0,
165.2,
102.4,
162.8,
129.5,
129.3,
114.2,
158.6,
64.4,
119.7,
140.2,
39.0,
25.8,
123.8,
131.0,
25.5,
17.6,
16.3,
CO
C
CH
CH
C
CH
C
C
CH
CH
C
CH2
CH
CH
CH2
CH2
CH
C
CH3
CH3
CH3
7.63, d (8.7)
6.52, dd (2.2, 8.7)
4, 7, 8a
4a, 8
6.28, d (2.2)
4a, 6, 7, 8a
7.42, d (8.6)
6.95, d (8.6)
2, 1′, 4′
1′, 4′, 3′/5′
4.56, d (6.6)
5.43, t (6.6)
4′, 2″, 3″
4″, 10″
2.06, m
2.08, m
5.08, m
2″, 3″, 5″, 10″
3″, 4″, 6″, 7″
8″, 9″
1.64, s
1.57, s
1.71, s
6″, 7″, C9″
6″, 7″, 8″
2″, 3″, 4″
sequential positive and negative Cotton effects at 334 and
300 nm are consistent with the 2R,3R absolute configuration.25
On the basis of the above data, the structure of the new
compound was characterized as (2R,3R)-2-(4-[{(E)-3,7-dimethylocta-2,6-dien-1-yl}oxy]phenyl)-3,7-dihydroxychroman-4one and was given the trivial name (2R,3R)-4′-O-geranyl-7hydroxydihydroflavonol.
The crude extract of the root bark of M. usaramensis ssp.
usaramensis and some of its constituents were tested for
cytotoxicity against the MDB-MB-231 human breast cancer and
against the HEK293 human embryonic kidney cell lines (Table
4). The cytotoxicity of the crude extract (IC50 11.63 μg/mL)
on MDB-MB-231 cells is comparable to that of 10, whereas all
other tested constituents show lower toxicities. The antiplasmodial activity of 4′-O-geranylisoliquiritigenin (12) against
chloroquine-sensitive (D6) and chloroquine-resistant (W2)
Plasmodium falciparum has been previously reported (IC50 10.6
and 8.7 μM, respectively).6 Its isomer, the new geranylated
chalcone 1, shows moderate antiplasmodial activity against the
chloroquine-sensitive 3D7 (IC50 3.7 μM) and the chloroquineresistant Dd2 (IC50 5.3 μM) P. falciparum strains. Compound 1
also shows toxicity against HEK-293 cells (100% inhibition at
40 μM; see the Experimental Section for details), demonstrating no selectivity for the malaria parasite and limiting its
development as an antimalarial lead compound. The major
rotenoids of this plant (Table 4) were also tested for
antiplasmodial activities against the two strains but were only
moderately active.
In conclusion, a new chalcone (1), two new 12dihydrorotenoids (2 and 3), a new flavanone (4), a new
dihydroflavonol (5), and 12 known natural products (6−17)
were isolated from the root bark of M. usaramensis ssp.
159.5). The HMBC correlation of CH2-1″ to C-4′ indicates the
connection of the geranyloxy group to C-4′ of ring B through
an ether linkage, which conclusion is corroborated by the NOE
correlation observed between CH2-1″ and H-3′/H-5′. The Econfiguration of the 2″-double bond was confirmed by the
observation of an NOE between H-2″ (δH 5.46) and H-4″ (δH
2.11) and the absence of an NOE between H-2″ (δH 5.46) and
CH3-10″ (δH 1.73, Supporting Information). The ECD
spectrum of 4 displayed positive and negative Cotton effects
at 332 and 302 nm, respectively, consistent with a 2Sconfiguration.25 This new compound was identified as (S)-E2-(4-[{3,7-dimethylocta-2,6-dien-1-yl}oxy]phenyl)-7-hydroxychroman-4-one and was given the semisystematic name (S)-4′O-geranyl-7-hydroxyflavanone.
The molecular formula of compound 5, isolated as a white,
amorphous solid, was determined to be C25H28O5 on the basis
of HRESIMS ([M + H]+ obs m/z 409.2020, calcd 409.2015)
and 13C NMR. Its NMR spectroscopic features were similar to
those of compound 4 except that those of 5 were typical of a
dihydroflavonol. Thus, ring A of 5 exhibits an AMX spin system
(Table 3), i.e., H-5 (δH 7.63, d, J = 8.7 Hz), H-6 (δH 6.52, dd, J
= 8.7, 2.2 Hz), and H-8 (δH 6.28, d, J = 2.2 Hz), compatible
with C-7 (δC 165.2) oxygenation, similar to 4. The large scalar
coupling constant (J = 11.6 Hz) of H-2 (δH 5.09) and H-3 (δH
4.50) of ring C, which is hydroxylated (δH 5.52) at C-3 (δC
72.5), indicates the diaxial orientation of these protons. The 4′O-geranyl substitution of ring B of 5 corresponds to that of 4,
as revealed by their similar NMR data. Accordingly, the Econfiguration of the 2″-double bond was confirmed by the
observation of an NOE between H-2″ (δH 5.43) and H-4″ (δH
2.06), whereas no NOE was observed between H-2″ (δH 5.43)
and CH3-10″ (δH 1.71, Supporting Information). The
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Article
February 2008. The plant material was identified by Mr. S. G.
Mathenge of the Herbarium, School of Biological Sciences, University
of Nairobi, where the voucher specimen (Mathenge 2008/374) was
deposited.
Extraction and Isolation. The dried and ground root bark of M.
usaramensis ssp. usaramensis (1 kg) was extracted using 3 × 3 L of
CH2Cl2/MeOH (1:1), for 24 h in each case, yielding 110 g of a
brown-orange crude extract following concentration using a rotary
evaporator. Approximately 100 g of the crude extract was subjected to
column chromatography on silica gel (500 g) eluting with n-hexane
containing increasing percentages of EtOAc. The fractions eluting with
2% EtOAc in n-hexane gave colenemol (16, 110 mg) and millettosin
(8, 12.1 mg). The fractions eluted with 3% EtOAc in n-hexane were
purified by crystallization from MeOH to yield 12a-epimillettosin (9,
140.4 mg). The mother liquid of the crystallization was subjected to
column chromatography on silica gel eluting with n-hexane and
increasing amounts of EtOAc and subsequently on Sephadex LH-20
using CH2Cl2/MeOH (1:1) eluent, yielding 12-dihydrousararotenoid
C (3, 19 mg). The fractions eluting with 4% EtOAc in n-hexane were
further purified by crystallization from MeOH, giving usararotenoid A
(6, 86.0 mg). The mother liquid of this fraction was further separated
by MPLC with n-hexane and increasing amounts of CH2Cl2 to give 4′O-geranylisoliquiritigenin (12, 95.0 mg), 4-O-geranylisoliquiritigenin
(1, 94.7 mg), and an additional mixture of two compounds. The latter
mixture was separated by preparative HPLC, using the MeOH/H2O
gradient elution in decreasing polarity to give usararotenoid C (10, 9.4
mg) and 12a-epimillettosin (9, 6.0 mg). Another portion of this
fraction (4% EtOAc in n-hexane) was purified on Sephadex, using
CH2Cl2/MeOH (1:1) as an eluent, and gave maximaisoflavone H (15,
4.0 mg) and an additional 7 mg of usararotenoid A (6). The fractions
eluted by 5% EtOAc in n-hexane were combined and subjected to
column chromatography, using CH2Cl2/n-hexane (8:2), to afford
three subfractions. One of these was purified by preparative HPLC to
give 7-O-geranyl-5-hydroxyflavanone (13, 2.6 mg). Tephrosin (14, 2.5
mg) was obtained from the fractions eluted with 6% EtOAc in nhexane, by purification on Sephadex with CH2Cl2/MeOH (1:1) and
subsequently by preparative RP-HPLC, using MeOH/H2O gradient
elution. The fraction eluted with 7% EtOAc in n-hexane was subjected
to column chromatography over silica gel using a 0−70% CH2Cl2 in nhexane eluent mixture to afford jamaicin (11, 27.6 mg), 4′-O-geranyl7-hydroxydihydroflavonol (5, 2.8 mg), and 4′-O-geranyl-7-hydroxyflavanone (4, 207.4 mg). 12-Dihydrousararotenoid B (2, 15.0 mg) was
isolated from the fraction eluted with 8% EtOAc in n-hexane by
purification on Sephadex with a CH2Cl2/MeOH (1:1) eluent mixture.
The fractions eluted with 11−15% EtOAc in n-hexane were combined,
concentrated, and crystallized from MeOH to give 12-dihydrousararotenoid A (7, 157.8 mg). Following crystallization, the mother liquid
was subjected to preparative TLC (eluted with 4% acetone in
CH2Cl2), yielding 7-hydroxy-8,3′,4′-trimethoxyisoflavone (17, 2.1
mg).
4-O-Geranylisoliquiritigenin (1): yellow solid; mp 95−97 °C; UV
(MeOH) λmax (log ε) 299 (4.2), 370 (3.5) nm; 1H and 13C NMR data,
see Table 1 and Figures S1−S7; HREIMS m/z 392.1968 (calcd for
C25H28O4 392.1988).
12-Dihydrousararotenoid B (2): colorless, amorphous solid; [α]20
D
+134 (c 0.0001, CH2Cl2); UV (MeOH) λmax (log ε) 301 (4.2) nm;
1
13
ECD (MeOH) 295 (+2.1); H and C NMR data, see Table 2 and
Figures S9−S14; HREIMS m/z 374.0999 (calcd for C19H18O8
374.1002).
12-Dihydrousararotenoid C (3): colorless, amorphous solid; [α]20
D
+96 (c 0.0001, CH2Cl2); UV (MeOH) λmax (log ε) 299 (4.3) nm;
ECD (MeOH) 295 (+2.4); 1H and 13C NMR data, see Table 2 and
Figures S17−S22; EIMS m/z 412 [M]+ (18), 389 (34), 256 (40), 192
(26), 163 (25), 150 (27), 137 (42), 107 (43), 81 (37), 69 (100);
HREIMS m/z 412.1520 (calcd for C23H24O7 412.1522).
(S)-4′-O-Geranyl-7-hydroxyflavanone (4): colorless, sticky oil;
[α]20
D −28 (c 0.0002, MeOH); UV (MeOH) λmax (log ε) 275 (4.2),
310 (3.7) nm; ECD (MeOH) 332 (+4.3), 302 (−8.7); 1H and 13C
NMR data, see Table 3 and Figures S25−S29; HREIMS m/z 392.1968
(calcd for C25H28O4 392.1988).
Table 4. Cytotoxic Activities of the M. usaramensis ssp.
usaramensis Crude Root Extract and of Some of Its Isolated
Constituents against MDB-MB-231 Cells
IC50a,b
sample
MDB-MB-231
M. usaramensis crude root extracta
4-O-geranylisoliquiritigenin (1)
usararotenoid A (6)
12-dihydrousararotenoid A (7)
millettosin (8)
12a-epimillettosin (9)
usararotenoid C (10)
jamaicin (11)
4′-O-geranylisoliquiritigenin (12)
tephrosin (14)
colenemol (16)
11.63
87.3
>279.3
61.7
100.7
25.7
3D7
Dd2
3.67
99%c
6.97
90%c
28%c
29%c
94%c
81%c
13.27
12.05
125.5
207.2
a
IC50 is given in μg/mL for crude and in μM for pure compounds;
95% confidence intervals are given in the Supporting Information
(S74). bAs positive controls pyrmethamine (IC50 = 6.1 ± 5.1 nM
(3D7), 62% at 40 μM (Dd2), 75% at 40 μM (HEK293)), chloroquine
(IC50 = 4.3 ± 0.3 nM (3D7), IC50 = 69.9 ± 34.5 nM (Dd2), 51% at 40
μM (HEK293)), pyronaridine (IC50 = 10.7 ± 10.0 nM (3D7), IC50 =
12.6 ± 7.2 nM (Dd2), IC50 = 2.71 ± 1.3 μM (HEK293)), puromycin
(IC50 = 43.7 ± 29.7 nM (3D7), IC50 = 54.3 ± 12.8 nM (Dd2), IC50 =
0.46 ± 1.41 μM (HEK293)), artesunate (IC50 = 1.6 ± 1.5 nM (3D7),
IC50 = 0.8 ± 0.5 nM (Dd2), 73% at 20 μM (HEK293)), and
dihydroartemisinin (IC50 = 0.4 ± 0.5 nM (3D7), IC50 = 0.4 ± 0.3 nM
(Dd2), 54% at 40 μM (HEK293)) were used. Details are given in the
Supporting Information. cThe largest percentage inhibition observed
at 100 μM concentration is given, where IC50 could not be accurately
determined. The inhibitory activities are given as the mean value of at
least two independent measurements.
usaramensis. Among the compounds tested for cytotoxicity,
usararotenoid C (10) showed the highest activity. 4-OGeranylisoliquiritigenin (1) showed moderate antiplasmodial
activity with marginal selectivity index.
■
EXPERIMENTAL SECTION
General Experimental Procedures. Melting points were
obtained on a B-545 Switzerland Büchi melting point apparatus,
optical rotations were measured on a PerkinElmer 341-LC polarimeter, whereas ECD experiments were run on a Jasco J-715
spectropolarimeter. UV spectra were recorded on a Specord S600
(Analytik Jena AG) spectrophotometer. NMR spectra were acquired at
400, 500, 600, and 800 MHz (1H NMR) on Varian MR-400, Varian
VNMR-S 500, Bruker Avance 600, and Bruker Avance III HD 800
spectrometers, using the residual solvent peaks as reference. The
spectra were processed using MestReNova 10.0. EIMS spectra were
obtained on a Micromass GC-TOFmicro mass spectrometer (Micromass, Wythenshawe, Waters Inc., UK), using direct inlet and 70 eV
ionization voltage. LC-ESIMS were acquired on a PerkinElmer PE
SCIEX API 150EX instrument equipped with a Turbolon spray ion
source connected to a Gemini 5 mm RPC18 110 Å column and
applying a H2O/MeCN (80:20−20:80) gradient with a separation
time of 8 min. TLC was carried out on Merck precoated silica gel 60
F254 plates. Column chromatography and MPLC were run on silica
gel 60 (70−230 mesh). Gel filtration was done on Sephadex LH-20.
Preparative HPLC was carried out on a Waters 600E instrument using
the Chromulan (Pikron Ltd.) software and an RP C8 Kromasil (250
mm × 55 mm) column with a H2O/MeOH solvent system. X-ray data
were obtained using an Agilent SuperNova Dual diffractometer with
Atlas detector at T = 123.0(1) K using mirror-monochromatized Cu
Kα radiation (λ = 1.541 84 Å).
Plant Material. The root bark of M. usaramensis ssp. usaramensis
was collected from the Jadini forest, at the Coast Province, Kenya, in
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(2R,3R)-4′-O-Geranyl-7-hydroxydihydroflavonol (5): colorless,
amorphous solid; UV (MeOH) λmax 279, 305 nm; ECD (MeOH)
334 (+0.8), 300 (−3.3); 1H and 13C NMR data, see Table 3 and
Figures S31−S36; HRESIMS m/z 409.2020 [M + H]+ (calcd for
C25H28O5 409.1937).
12a-Epimillettosin (9): needles (MeOH); mp 256−258 °C; [α]20
D
+230 (c 0.0013, CH2Cl2); ECD (MeOH): 324 (−10.2), 348 (+31.9);
UV (MeOH) λmax (log ε) 235 (4.54), 240 (2.38), 276 (4.38), 312
(3.99); 1H and 13C NMR data, see Figures S67−S75; ESIMS m/z
395.0 [M + H − H2O]+, 377.3 [M]+.
Cytotoxicity Assays. MDB-MB-231 human breast cancer cells
were cultured in Dulbecco’s modified Eagle’s medium (DMEM),
supplemented with 10% (v/v) fetal bovine serum, 2 mM L-glutamine,
100 units/mL penicillin, and 100 μg/mL streptomycin at 37 °C in
humidified 5% CO2. For cytotoxicity assays, cells were seeded in 96well plates at optimal cell density (10 000 cells per well) to ensure
exponential growth for the duration of the assay. After a 24 h
preincubation growth, the medium was replaced with experimental
medium containing the appropriate drug concentrations or vehicle
controls (0.1% or 1.0% v/v DMSO). After 72 h incubation, cell
viability was measured using Alamar Blue reagent (Invitrogen Ab,
Lidingö, Sweden) according to the manufacturer’s instructions.
Absorbance was measured at 570 nm with 600 nm as a reference
wavelength. Results were expressed as the mean ± standard error for
six replicates as a percentage of vehicle control (taken as 100%).
Experiments were performed independently at least six times.
Statistical analyses were performed using a two-tailed Student’s t
test. P < 0.05 was considered to be statistically significant (Supporting
Information, S74).
To assess the cytotoxicity of compounds on HEK-293 cells in dose
response, a resazurin-based viability assay was used. In brief, HEK293
cells were grown in DMEM medium (Life Technologies), containing
10% fetal calf serum (FCS; Gibco), trypsinised, counted, and seeded at
2000 cells per well in 45 μL of media into TC-treated 384-well plates
(Greiner) and left to adhere overnight at 37 °C, 5% CO2, and 60%
humidity. Test compounds were prepared by diluting compounds 1 in
25 in sterile H2O and then another 1 in 10 dilution, to give a final test
concentration of 40 μM, 0.4% DMSO. Plates were incubated for 72 h
at 37 °C, 5% CO2, and 95% humidity, and then the media was
removed and replaced by 35 μL of 44 μM resazurin in DMEM without
FCS. The plates were incubated for another 4−6 h at 37 °C, 5% CO2,
and 95% humidity, before reading on an EnVision plate reader
(PerkinElmer) using fluorescence excitation/emission settings of 530
nm/595 nm. The percent growth was standardized to controls (5 μM
puromycin as positive and 0.4% DMSO as negative control) using
Microsoft Excel 2013. A statistical analysis including IC50 determination and graphical output was performed in GraphPad Prism 6 using
nonlinear regression variable slope curve fitting.
P. falciparum Culture. In vitro parasite cultures of the P.
falciparum strains 3D7 and Dd2 were maintained in RPMI with 10
mM Hepes (Life Technologies), 50 μg/mL hypoxanthine (Sigma),
and 5% human serum from male AB plasma and 2.5 mg/mL
AlbuMAX II (Life Technologies). Human 0+ erythrocytes were
obtained from the Australian Red Cross Blood Service (Agreement
No. 13-04QLD-09). The parasites were maintained at 2−8%
parasitemia (% P) at 5% hematocrit (% H) and incubated at 37 °C,
5% CO2, 5% O2, 90% N2, and 95% humidity.
P. falciparum Growth Inhibition Assay. A well-established
asexual P. falciparum imaging assay was used to determine parasite
growth inhibition.26 In brief, sorbitol (5% w/v) synchronization was
performed twice, approximately 8 h apart, on each synchronization day
for two consecutive ring cycles, i.e., on days 1 and 3 of assay
preparation. On day 2, the culture was split to approximately 2%
trophozoite parasitemia. On day 4, the culture was split to 1−1.5%
trophozoite parasitemia, which yielded approximately an 8% ring
parasitemia after 48 h on day 5, the day of the assay. Compound stocks
(10 mM in 100% DMSO) were diluted 1 in 25 in H2O, just prior to
use. An additional 1 in 10 dilution was performed, resulting in a 1:250
overall compound dilution and a final DMSO concentration of 0.4%.
For dose−response curves, a three-step logarithmic serial dilution was
prepared at 40 μM top concentration for test compounds for the
asexual assay and 2 μM for the positive control artemisinin. A 5 μL
portion of the diluted test compound or control solutions (2 μM
artemisinin as positive and 0.4% DMSO as negative control) was
added to 384-well CellCarrier imaging plates (PerkinElmer). Parasite
cultures were added to a final concentration of 2% parasitemia and
0.3% hematocrit. Plates were incubated for 72 h at 37 °C, 5% CO2,
and 95% humidity. On day 8, the permeabilization and nuclear staining
buffer was prepared in PBS containing 10 μg/mL saponin, 0.01%
Triton X, 5 mM EDTA (all Sigma), and 0.5 μg/mL 4′,6-diamidino-2phenylindole (DAPI; Life Technologies).26
Prior to imaging on an Opera Confocal Imager (PerkinElmer) at
405 nm excitation with a 20× water, objective plates were incubated
overnight at room temperature. An automated primary image analysis
was performed concurrent with image acquisition, utilizing an Acapella
software (PerkinElmer) script to determine the number of parasites
based on object size and fluorescence intensity.26 Determination of the
percent growth compared to controls (2 μM artemisinin as positive
and 0.4% DMSO as negative control) was performed in Microsoft
Excel 2013. Statistical analysis including IC50 determination and
graphical output was performed in GraphPad Prism 6 using nonlinear
regression variable slope curve fitting. As positive controls pyrimethamine (3D7: 2.5 nM, Dd2: 50% at 40 μM, HEK270: 4.22 nM),
chloroquine (3D7: 4.5 nM, Dd2: 45.5 nM, HEK270: 60% at 40 μM),
pyrionaridine (3D7: 3.6 nM, Dd2: 7.51 nM, HEK270: 1.79 nM),
puromycin (3D7: 22.7 nM, Dd2: 45.2 nM, HEK270: 361 nM),
artesunate (3D7: 0.5 nM, Dd2: 0.443 nM, HEK270: 75% at 20 μM),
and DHA (3D7: 0.1 nM, Dd2: 0.146 nM, HEK270: 50% at 20 μM)
were used. Statistical data are given in the Supporting Information.
■
ASSOCIATED CONTENT
* Supporting Information
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jnatprod.5b00581.
1D and 2D NMR, MS, UV, and CD spectra and data for
X-ray crystallography, cytotoxicity, and antiplasmodial
assays (PDF)
■
AUTHOR INFORMATION
Corresponding Authors
*Tel: +46 31 786 9033. E-mail: mate@chem.gu.se (M.
Erdélyi).
*Tel: +254 733 832 576. Fax: +254 20 444 6138. E-mail:
ayenesew@uonbi.ac.ke (A. Yenesew)
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to Mr. S. G. Mathenge of the Herbarium,
Botany Department, University of Nairobi, for the identification
of the plant species. T.D. thanks DAAD-NAPRECA for a Ph.D.
scholarship; I.G. thanks the Swedish Institute for a KenyanSwedish sandwich Ph.D. scholarship. The International Science
Program (ISP Sweden, grant KEN-02), the Swedish Research
Council (2012-6124), and the Australian Research Council
(grant LP120200557 to V.M.A.) are thankfully acknowledged
for funding support. We thank the Australian Red Cross Blood
Service for the provision of human blood.
■
■
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