Stellettapeptins A and B, HIV-inhibitory cyclic depsipeptides from the marine sponge Stelletta sp.

Article abstract of DOI:10.1016/j.tetlet.2015.05.058

Two new HIV-inhibitory depsipeptides, stellettapeptins A (1) and B (2), were isolated from an extract of the marine sponge Stelletta sp., collected from northwestern Australia. Structures of these cyclic nonribosomal peptides were elucidated on the basis of extensive NMR data analysis, and chemical degradation and derivatization studies. Stellettapeptins contain numerous nonproteinogenic amino acid residues and they are the first peptides reported to contain a 3-hydroxy-6,8-dimethylnon-4-(Z)-enoic acid moiety. Compounds 1 and 2 potently inhibit infection of human T-lymphoblastoid cells by HIV-1_RF with EC₅₀ values of 23 and 27 nM, respectively.

Full text of DOI:10.1016/j.tetlet.2015.05.058

Tetrahedron Letters 56 (2015) 4215–4219
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Stellettapeptins A and B, HIV-inhibitory cyclic depsipeptides
from the marine sponge Stelletta sp.
Hee Jae Shin a, , Mohammad A. Rashid a,à, Laura K. Cartner a,b, Heidi R. Bokesch a,b, Jennifer A. Wilson a,
James B. McMahon a, Kirk R. Gustafson a,
⇑
a Molecular Targets Laboratory, Center for Cancer Research, National Cancer Institute, Frederick National Laboratory for Cancer Research, Building 562, Room 201, Frederick,
MD 21702-1201, USA
b Basic Science Program, Leidos Biomedical Research, Inc., Frederick National Laboratory for Cancer Research, Frederick, MD 21702-1201, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new HIV-inhibitory depsipeptides, stellettapeptins A (1) and B (2), were isolated from an extract of
the marine sponge Stelletta sp., collected from northwestern Australia. Structures of these cyclic nonribo-
somal peptides were elucidated on the basis of extensive NMR data analysis, and chemical degradation
and derivatization studies. Stellettapeptins contain numerous nonproteinogenic amino acid residues and
they are the first peptides reported to contain a 3-hydroxy-6,8-dimethylnon-4-(Z)-enoic acid moiety.
Compounds 1 and 2 potently inhibit infection of human T-lymphoblastoid cells by HIV-1RF with EC50 val-
ues of 23 and 27 nM, respectively.
Received 13 March 2015
Revised 12 May 2015
Accepted 18 May 2015
Available online 22 May 2015
Keywords:
Depsipeptide
Sponge peptide
Stelletta
Published by Elsevier Ltd.
Anti-HIV
Marine sponges in the genus Stelletta have proven to be an
extremely rich source of structurally diverse and biologically active
natural products.1–3 Marine sponges frequently contain nonriboso-
mal peptides that have unusual amino acid residues and aliphatic
moieties reminiscent of the mixed nonribosomal peptide syn-
thetase (NRPS)–polyketide synthase (PKS) pathways of microor-
ganisms, which suggests the involvement of symbiotic microbes
in their production.4 In the course of our search for bioactive
metabolites from marine organisms, we obtained two new cyclic
depsipeptides from a sponge of the genus Stelletta. We report
herein the isolation, structure determination, and HIV-inhibitory
properties of stellettapeptins A (1) and B (2).
characteristic of a peptide derivative. Detailed analysis of 2D
NMR data enabled us to assign 11 amino acid residues:
N-methylalanine (NMeAla), b-methoxytyrosine (b-OMeTyr),
N-methylglutamine (NMeGln), leucine (Leu), glycine (Gly),
3-methoxyalanine (3-OMeAla), threonine (Thr), 3,4-dimethylglu-
tamine (3,4-DiMeGln), 2,3-diaminobutanoic acid (Dab), 3-hydrox-
yglutamine (3-OHGln), and 3-hydroxyasparagine (3-OHAsn)
(Table 1). Additionally, a 3-hydroxy-6,8-dimethylnon-4-enoic acid
(Hdna) moiety was identified (Fig. 1).
The sequence of amino acid residues in stellettapeptin A (1) was
deduced from inter-residue NH/CHa ROE interactions, acquired in
CD3OH, and HMBC correlations as shown in Figure 2. The Hdna
unit was defined by 1H–1H COSY and heteronuclear correlation
NMR data, and its double bond assigned Z geometry based on the
10.5 Hz vicinal coupling between the H-4 (dH 5.37) and H-5 (dH
5.21) olefin protons. The Hdna moiety was linked to the N-termi-
nus of the 3-OHAsn residue by an HMBC correlation from the
3-OHAsn NH (dH 8.25) to the carbonyl (dC 175.2) of Hdna. This
linkage was also supported by ROESY correlations between the
3-OHAsn amide proton and the Hdna H2-2 (dH 2.36, 2.61) methy-
lene protons. The secondary amide NH signal (dH 7.88) of
3-OHGln correlated with the C-1 carbonyl (dC 171.8) of 3-OHAsn
in the HMBC spectrum, which connected these residues, while
Stellettapeptin A (1) was isolated as an amorphous solid, which
had
a molecular formula of C67H108N16O23 from analysis of
HRESIMS data coupled with 1H and 13C NMR spectral data
(Table 1). The presence of a large number of exchangeable
amide NH protons (dH 6.80–9.98 ppm) and carbonyl resonances
(dC 170.0–182.4 ppm) in the 1H and 13C NMR spectra of 1 was
⇑
Corresponding author. Tel.: +1 301 846 5197; fax: +1 301 846 6851.
E-mail addresses: shinhj@kiost.ac (H.J. Shin), rashidma@univdhaka.edu
(M.A. Rashid), gustafki@mail.nih.gov (K.R. Gustafson).
Permanent address: Marine Natural Products Laboratory, Korea Institute of Ocean
Science & Technology, 1270 Sadong, Ansan 426-744, Republic of Korea.
the 3-OHGln
a-methine (dH 4.52) showed a ROESY correlation with
à
Permanent address: Department of Pharmaceutical Chemistry, University of
the amide NH (dH 8.75) of Dab. The amino acid sequence was
Dhaka, Dhaka 1000, Bangladesh.
http://dx.doi.org/10.1016/j.tetlet.2015.05.058
0040-4039/Published by Elsevier Ltd.

4216
H. J. Shin et al. / Tetrahedron Letters 56 (2015) 4215–4219
Table 1
NMR spectral data for 1 (600 MHz, CD3OH)
Position
dC
dH (J in Hz)
HMBC
NMeAla
1
2
3
171.4
51.9
13.3
29.5
5.42 q (7.0)
1.28 d (7.0)
2.68 s
1, 3, N-Me, 13-OMeTyr
1, 2
2, 13-OMeTyr
N-Me
3-OMeTyr
1
2
3
170.0
53.8
84.4
4.92b dd (10.5, 9.8)
4.53a d (9.8)
3.10 s
1, 3
1, 2, 4, 5, 9
3
3-OMe
4
5, 9
6, 8
7
56.9
129.5
131.5
116.2
158.8
7.17 d (8.3)
6.80 d (8.3)
3, 6, 7, 8
4, 7
NH
8.23 d (10.5)
2, 1NMeGln
NMeGln
1
2
3
170.8
55.8
25.1
4.76a
m
1, 4, N-Me, 1Leu
5
2, 4, 5
2, 3, 5
2, 3, 5
1.29 m
1.54 m
1.61 m
1.69 m
4
32.1
5
177.9
5-NH2
6.80 br s
7.05 br s
2.93 s
4, 5
5
2, 1Leu
N-Me
30.4
Leu
1
2
174.0
49.4
40.7
4.72 m
3, 1Gly
3
1.23 m
1.61 m
5
4
5
6
NH
26.2
21.5
23.6
1.66 m
3
0.90a d (6.5)
0.95 d (6.5)
7.20 d (9.2)
3, 4, 6
3, 4, 5
1Gly
Gly
1
2
172.3
44.2
3.51 dd (17.0, 5.2)
3.95 dd (17.0, 6.1)
9.08 dd (6.1, 5.2)
1, 13-OMeAla
1, 13-OMeAla
2, 13-OMeAla
NH
3-OMeAla
1
2
3
172.8a
55.4
71.6
4.48 q (7.2)
3.74 m
3.79 m
1, 3, 1Thr
1, 2, 3-OMe
1, 2, 3-OMe
3
3-OMe
NH
59.5
3.39 s
8.47 d (7.2)
2, 3, 1Thr
Thr
1
2
172.9a
57.4
5.20 dd (10.2, 2.8)
5.60 dq (6.6, 2.6)
1.18 d (6.3)
1, 3, 4
4, 1NMeAla
2, 3
3
4
71.6
14.8
NH
8.93 d (10.2)
2, 13,4-DiMeGln
3,4-DiMeGln
1
2
3
174.1
59.3
36.8
17.3
44.9
14.0
182.4
4.09 dd (9.3, 2.9)
2.47 m
1.28 d (6.9)
2.72 m
1, 3, 3-Me, 4
2, 3-Me, 4-Me, 5
2, 3, 4
2, 3, 3-Me, 4-Me, 5
3, 4, 5
3-Me
4
4-Me
5
1.32 d (7.1)
5-NH2
7.14 br s
7.87a br s
9.98 br s
4, 5
5
1Dab
NH
Dab
1
2
3
171.6
56.4
49.5
4.56 t (6.1)
3.95 m
1, 3, 4, 13-OHGln
3-NH2
4
NH
7.73 2H, br s
1.43 d (6.8)
8.75 d (6.9)
16.8
2, 3
2, 3, 13-OHGln

H. J. Shin et al. / Tetrahedron Letters 56 (2015) 4215–4219
4217
Table 1 (continued)
Position
dC
dH (J in Hz)
HMBC
1, 3
3-OHGln
1
2
3
4
172.9a
58.7
68.7
4.52a dd (8.5, 1.3)
4.75a
m
40.6
2.37 m
2.45 m
5
2, 3, 5
5
176.0
5-NH2
6.88 br s
4, 5
7.54 br s
5
NH
7.88a d (8.5)
2, 3, 13-OHAsn
3-OHAsn
1
2
171.8
59.5
4.73 dd (6.9, 3.9)
4.61 br d (3.9)
1, 3, 4, 1Hdna
4
3
72.7
4
176.6
4-NH2
7.36 br s
7.80 br s
3, 4
4
NH
8.25 d (6.9)
1Hdna
Hdna
1
2
175.2
45.4
2.36 m
2.61 m
1, 3, 4
1, 3, 4
3
4
5
6
66.6
131.0
139.3
31.3
22.1
48.2
26.9
23.7
23.0
4.90b
m
5.37 dd (10.5, 9.1)
5.21 dd (10.5, 5.0)
2.63 m
2, 6
3, 4, 6, 6-Me, 7
4, 5, 6-Me, 7
5, 6, 7
5, 6, 6-Me, 8
6, 7, 8-Me
7, 8
6-Me
7
8
8-Me
9
0.92a d (6.7)
1.13 2H, m
1.57 m
0.87a d (6.7)
0.88a d (6.7)
7, 8
a
Signals overlapped.
Buried under CD3OH signal.
b
further extended by a ROESY between the Dab
a
-methine (dH 4.56)
carbonyl (dC 174.0), and between the 3-OMeTyr NH (dH 8.23) and
C-1 (dC 170.8) of NMeGln linked these residues. An HMBC correla-
tion from the methyl group (dH 2.68) of NMeAla to the 3-OMeTyr
carbonyl (dC 170.0) completed the amino acid sequence. An ester
bond linking NMeAla and the Thr residue was evident from the
deshielded chemical shift of the Thr oxymethine (dH 5.60) and an
HMBC correlation from this proton to the NMeAla carbonyl (dC
171.4). Thus the planar structure of 1 was elucidated.
and the secondary amide NH (dH 9.98) of 3,4-DiMeGln. An HMBC
correlation from the Thr NH (dH 8.93) to C-1 (dC 174.1) of 3,4-
DiMeGln linked these two residues, while an HMBC from 3-
OMeAla NH (dH 8.47) to the Thr carbonyl (dC 172.9) joined these
subunits. ROESY interactions observed between the
a-methine
(dH 4.48) of 3-OMeAla and the Gly NH (dH 9.08), and between the
Gly methylene resonances (dH 3.51, 3.95) and the Leu NH (dH
7.20) helped define neighboring residues. HMBC correlations
between the methyl group (dH 2.93) of NMeGln and the Leu
The absolute configurations of
L-NMeAla, L-NMeGln, L-Leu,
D-allo-Thr,
D
-3-OMeAla, (2R,3R)-3-OHGln, and (2R,3S)-3-OHAsn
NH2
D
-3(S)-OHAsn
O
O
OH
OH
Gly2
OH
O
OH
OH
O
H
N
H
N
L
-Thr2
N
H
NH2
N
H
Hdna
D
-3(R)-OHGln
Hdna
O
O
HN
O
O
HN
O
O
H2N
-3(S)-Dab
H2N
-3(S)-Dab
O
O
L
-3(S),4(R)-DiMeGln
L
-3(S),4(R)-DiMeGln
L
L
HN
HN
HN
NH2
NH2
1
2
L
-NMeAla
D
-allo-Thr
HN
O
O
L
-NMeAla
O
O
D
-allo-Thr1
O
O
N
O
N
O
D
-3(R)-OMeTyr
D
-3(R)-OMeTyr
D
-3-OMeAla
D
-3-OMeAla
NH
O
NH
O
O
O
O
O
O
NH
O
O
NH
OH
O
NH
O
OH
O
NH
OH
H
N
H
N
NH2
-NMeGln
O
Gly
Gly1
N
N
L
-NMeThr
L
O
O
L
-Leu
L
-Leu
Figure 1. Structures and composition of stellettapeptins A (1) and B (2).

4218
H. J. Shin et al. / Tetrahedron Letters 56 (2015) 4215–4219
NH2
O
OH
OH
O
OH
O
H
N
N
H
NH2
O
HN
O
O
H2N
O
HN
HN
NH2
O
O
O
N
O
NH
O
O
O
O
O
NH
O
NH
OH
H
N
NH2
N
O
O
HMBC
ROESY
COSY
Figure 2. Key HMBC, ROESY, and COSY correlations for 1.
were determined by LC–MS analysis of the acid hydrolysate deriva-
tized with Marfey’s reagent and comparison with appropriate
amino acid standards. Both the L- and D-FDLA derivatives were pre-
pared and analyzed for amino acids with multiple chiral centers for
which a complete set of standards was not available.5 Due to
decomposition of b-OMeTyr during acid hydrolysis of 1, this resi-
due was converted to b-OMeAsp via ozonolysis prior to acid
hydrolysis and derivatization, as described by Zampella et al.6
Samples of 1, callipeltin A,7 and papuamide B8 were ozonized
and hydrolyzed, and then subjected to Marfey’s analysis. Both
Stellettapeptin B (2) analyzed for a molecular formula of
C63H103N13O20 by HRESIMS coupled with 1H and 13C NMR spectro-
scopic data (Supplementary data). Its 1H and 13C NMR spectra were
also characteristic of a peptide and suggested that 2 was similar in
structure to 1. Detailed analysis of 2D NMR data of 2 led to the
identification of 12 residues: NMeAla, b-OMeTyr, NMeThr, Leu,
Gly (2), 3-OMeAla, Thr (2), 3,4-DiMeGln, Dab, and Hdna (Fig. 1).
The sequence of these residues was deduced from analysis of
ROESY and HMBC correlations, as in 1. The structure of 2 differed
from 1 by the presence of NMeThr, Thr, and Gly in place of
NMeGln, 3-OHGln, and 3-OHAsn residues, respectively. Similar to
compound 1, macrocyclic ring closure in 2 was established by an
HMBC correlation from the Thr1 oxymethine (dH 5.65) and the car-
bonyl (dC 171.7) of the C-terminal NMeAla residue. The absolute
configurations of the amino acid residues of 2 were also deter-
mined by Marfey’s analysis of the acid hydrolysate using a similar
strategy employed with 1. Compound 2 was shown to have the
same absolute configuration for each amino acid residue it has in
the L- and D-FDLA derivatives were prepared and ion selective
monitoring for b-OMeAsp [m/z 458, (M+H)+] of the hydrolysates
from all three peptides showed the same retention times
(Supplementary data) corresponding to (2R,3S)-3-OMeAsp. Thus
the configuration of the b-OMeTyr residue in 1 was assigned as
2R,3R.6 The coupling constant (9.8 Hz) between H-2 (dH 4.92) and
H-3 (dH 4.53) of this residue was in good agreement with those
of callipeltin A and papuamide B, which also have a (2R,3R)-3-
OMeTyr, while neamphamide9 contains (2S,3R)-b-OMeTyr and its
oxymethine proton (H-3, dH 5.03) appears as a broadened singlet.
These data supported the (2R,3R)-configuration of b-OMeTyr in 1.
common with 1. It was also analyzed for
L-NMeThr and both a D-
allo-Thr and a -Thr residue. Based on the close structural similarity
L
between compounds 1 and 2, and the fact that all cyclic depsipep-
tides in this structural class that form a macrocycle via esterifica-
tion of the C-terminus with the hydroxyl group of a threonine
utilize a D-allo-Thr residue, the L-Thr was assigned to be adjacent
to the N-terminal Gly residue in 2.
Comparison by LC–MS of both the L- and D-FDLA derivatives of
3,4-DiMeGln and Dab of 1 with similar derivatives from the hydro-
lysates of authentic samples of callipeltin A and/or papuamide B
indicated these residues have the same configuration in all three
peptides. Thus, their configurations were assigned as (2S,3S,4R)-
DiMeGln and (2S,3S)-Dab. As (2S,3S)-Dab is a common fragment
of papuamides and callipeltin A, considerable effort has been made
toward its synthesis.10–13 In these synthetic studies, the diastere-
omers of Dab showed distinct differences in the coupling con-
stants, with the H-2/H-3 coupling constant in the (2R,3S)-isomer
being ꢀ3 Hz, while in the (2S,3S)-diastereoisomer it is 6.1–7 Hz,
supporting the presence of (2S,3S)-Dab in 1. Thus the absolute con-
figurations of all 17 stereogenic carbons in the amino acid portion
of stellettapeptin A (1) were successfully established. The polyke-
tide moiety was unstable and it decomposed during repeated
cleavage attempts, so the configuration of this portion of 1 could
not be assigned.
The anti-HIV activity of 1 and 2 was evaluated in an XTT based
cell viability assay using the human T-cell line CEM-SS infected
14
with HIV-1RF
.
After a six day incubation period, compounds 1
and 2 effectively inhibited the cytopathic effect of HIV-1 infection
with EC50 values (concentration at which 50% of the target cells are
protected from death by the virus) of 23 and 27 nM, respectively
(Fig. 3 and Supplementary data). Direct cytotoxicity of 1 and 2
against the host cells was observed with IC50 values (concentration
at which 50% of cells are killed by the test sample) of 367 and
373 nM, respectively.
In conclusion, stellettapeptins A (1) and B (2) are new depsipep-
tides with structural features characteristic of the family of anti-
HIV peptides which includes the callipeltins,6,7,15–17 papuamides,8

H. J. Shin et al. / Tetrahedron Letters 56 (2015) 4215–4219
4219
Research,
National
Institutes
of
Health
(contract
HHSN261200800001E). This research was supported in part by
the Intramural Research Program of NIH, National Laboratory for
Cancer Research, Center for Cancer Research. The content of this
publication does not necessarily reflect the views or policies of
the Department of Health and Human Services, nor does mention
of trade names, commercial products, or organizations imply
endorsement by the U.S. Government. This research was supported
in part by the Korea Institute of Ocean Science & Technology (Grant
PE98816 to H.J.S.).
Supplementary data
Supplementary data (1D and 2D NMR spectral data, ozonolysis,
amino acid analysis, anti-HIV data, and isolation of 1 and 2) asso-
ciated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.2015.05.058. These data include
MOL files and InChiKeys of the most important compounds
described in this article.
(2)
(1)
Figure 3. Anti-HIV activity of compounds 1 and 2. Uninfected cell cultures reveal
direct cytotoxicity of the test samples. HIV infected cultures show cytoprotective
effects of 1 and 2 against viral infection at lower concentrations, and cytotoxic
effects at higher concentrations.
References and notes
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a
previously undescribed polyketide subunit, 3-hydroxy-6,8-
4. Piel, J. Nat. Prod. Rep. 2004, 21, 519.
dimethylnon-4-enoic acid, and the 3-OHGln and 3-OHAsn residues
in 1 are rarely found in peptides. Compounds 1 and 2 potently
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tional evidence that this class of peptides may hold promise as
anti-HIV agents. Isolation of the stellettapeptins from Stelletta sp.,
which is phylogenetically distinct from sponges the callipeltins
and papuamides were isolated from, and the fact that 1 and 2 have
characteristic structural features of nonribosomal peptides, sug-
gest a possible microbial origin for the stellettapeptins.
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Acknowledgments
We thank professors Apurba Dutta, University of Kansas, and
Anders Broberg, Swedish University of Agricultural Sciences, for
synthetic samples of (2S,3R)- and (2R,3S)-3-OH-aspartic acid, and
threo- and erythro-3-OH-glutamic acid, respectively. We thank
the late Prof. Luigi Minale for a sample of callipeltin A, Catherine
Hixson (SAIC) for the hydrolysis of 1 and 2, and S. Tarasov and
M. Dyba, Biophysics Resource, Structural Biophysics Laboratory
for mass spectral analysis. This project was funded in part with
federal funds from the Frederick National Laboratory for Cancer
13. Burke, A. J.; Davies, S. G.; Hedgecock, C. J. R. Synlett 1996, 621.
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Virol. Methods 1991, 33, 87.
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Roussakis, C.; Le Bert, V. Tetrahedron 1996, 52, 9589.
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D’Auria, M. V. Tetrahedron Lett. 2002, 43, 6163.
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