Zelkovamycins B-E, Cyclic Octapeptides Containing Rare Amino Acid Residues from an Endophytic Kitasatospora sp

Acid Residues from an Endophytic Kitasatospora sp

Letter

Zelkovamycins B−E, Cyclic Octapeptides Containing Rare Amino

Xiaomeng Hao, Jiaqing Yu, Yujia Wang, Jack A. Connolly, Yufeng Liu, Yuqin Zhang, Liyan Yu,

Shan Cen,* Rebecca J. M. Goss,* and Maoluo Gan *

Cite This: https://dx.doi.org/10.1021/acs.orglett.0c03565

Read Online

ACCESS

Metrics & More

Article Recommendations

sı Supporting Information

ABSTRACT: Four unusual cyclopeptides, zelkovamycins B−E

1−4), were isolated from an endophytic Kitasatospora sp.

(

Zelkovamycin B was featured by an unprecedented 3-methyl-5-

hydroxypyrrolidine-2,4-dione ring system linked to the cyclo-

peptide skeleton. Their structures and full conﬁgurations were

established by spectroscopic analysis, Marfey’s method, and NMR

calculations. A plausible biosynthetic pathway for zelkovamycins

was proposed based on gene cluster analysis. Zelkovamycin E

displayed potent inhibitory activity against H1N1 inﬂuenza A virus.

ndophytic actinobacteria, which inhabit the internal

the solid cultures led to identiﬁcation of four novel

cyclopeptides, named zelkovamycins B−E (1−4), together

healthy tissues of living plants, are a little explored

group of microorganisms. The inner tissues of the host plants

constitute a niche ecological environment. In such mutualisms

the host plant confers the endophytic bacteria with a sheltered

growth environment and provides nutrition and in return the

endophytes produce a variety of bioactive secondary

metabolites that protect the plant. Such metabolites have

previously been indicated to enhance the host’s ability of

tolerance for abiotic and biotic stresses, enabling resistance

with the known zelkovamycin (Figure 1). Herein, we report

the isolation, structural determination, bioactivities of com-

pounds 1−4, and the revised structure and the full absolute

conﬁguration of zelkovamycin.

Compound 1 was obtained as a white amorphous powder.

The molecular formula was determined as C H N O S by

HRESIMS and NMR data (Tables S1 and S2 ). The H NMR

spectrum in DMSO-d displayed signals characteristic of a

against insects, pests, and pathogens. It is believed that

peptide including six exchangeable NH protons between δ_H

endophytic actinobacteria have evolved the ability to produce

specialized series of secondary metabolites through their long-

.20 and 10.87, seven α-protons of amino acids (δ_H2.99−

.35). Additionally, the H NMR spectrum showed signals of

term association with the host plants. Many studies have

shown that endophytic actinobacteria, especially those from

two oleﬁnic protons at δ 5.69 and 8.21 and four aromatic

protons corresponding to an indole unit of a monosubstituted

medical plants, appear to be a prospective source of novel

tryptophan (δ 6.28−6.91). The C NMR and DEPT spectra

metabolites with diverse biological activities, such as

showed signals for eight carbonyl (δ_C206.0, 172.3, 171.4,

antimicrobial, antiviral, antitumor, and anti-inﬂammatory

170.2, 167.4, 166.8, 166.7, 159.2), six aliphatic α-amino

,2,4

properties.

carbons at δ 43.0−56.6 (two methylenes, three methines, and

During our ongoing search for new bioactive microbial

metabolites, we focused our attention on microorganisms from

ecological niches such as oceans, extreme environments,

one nonprotonated), suggesting that 1 contained at least seven

amino acid residues.

Extensive analysis of 2D NMR spectra indicated the

presence of glycine (Gly), 2-aminobutyric acid (Abu),

dehydrobutyrine (Dhb), N-methylglycine (sarcosine, Sar),

alanine (Ala), a cysteine residue that had been transformed

to a thiazole (Tzl), and a methoxy-substituted tryptophan

lichens, and inner tissues of plants. Through screening

experiments, the extract of the actinomycete Kitasatospora sp.

CPCC 204717, an endophyte from the Chinese medicinal

plant Huperzia serrata (Shezu Shishan), was found to show

signiﬁcant antibacterial activities. Preliminary investigations of

the liquid cultures of the strain revealed zelkovamycin as the

main bioactive metabolite. To gain further insight into the

strain’s ability to produce diﬀerent secondary metabolites, the

strain was cultured on a variety of media. LC-MS analysis

revealed that the metabolites from the solid rice cultures

produced considerably greater chemical diversity than liquid

cultures (Figure S1 ). Subsequent chromatographic isolation of

Received: October 26, 2020

XXXX American Chemical Society

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

structure of zelkovamycin (6 ).

Letter

correlation from H -30 (δ_H3.51) to C-25 (δ_C153.8),

suggesting that the methoxy group was substituted at C-4 of

Trp in 1, rather than C-7 of Trp as previously assigned for

zelkovamycin. The ROESY correlations of NH-22/H-28 and

H -30/H-26 further conﬁrmed the presence of the 4-

methoxytryptophan (4-OMe-Trp) residue in 1. The HMBC

correlations of the α-protons with the amide carbonyls of

adjacent amino acids and ROESY correlations of NH with α-

protons established the amino acid sequence of Gly-Abu-Dhb-

Sar-Ala-Tzl-4-OMe-Trp. HMBC correlations of H -33 with C-

2 (δ 56.6), C-34 (δ 206.0), C-36 (δ 171.4), and H-35 (δ

C C C H

.76) with C-34 revealed the presence of an altered 2-methyl-

-oxobutyrine (MeObu) fragment. The HMBC correlations

of NH-32 with C-31 and H -1 with C-36 established the

connectivity of 4-OMe-Trp, the altered MeObu, and Gly.

Additionally, the HMBC correlation of H-35 with C-36 and

H -1 with C-35 and C-36 together with the downﬁeld shift of

Figure 1. Structures of compounds 1−5 and the originally proposed

C-35 (δ_C80.9) indicated the presence of a hemiaminal

functionality, which enabled us to determine the planar

structure for 1 as depicted, possessing a unique 3-methyl-5-

hydroxypyrrolidine-2,4-dione ﬁve-membered heterocyclic scaf-

fold. Analysis of MS/MS fragmentation patterns provided

additional data to support this assignment (Table S6). The

double bond of Dhb was assigned as Z geometry based on the

ROESY correlations of H-8/H -11 and H -9/NH-7. The

ROESY spectra recorded in both DMSO-d and CD OD did

not give correlations between H-35 and H -33, indicating that

they were likely on the opposite side of the pyrrolidine-2,4-

dione ring. Therefore, the planar structure of 1 was determined

and given the trivial name zelkovamyicn B.

Figure 2. Key COSY, HMBC, and ROESY correlations of 1.

Zelkovamycin C (2) was assigned the molecular formula

C H N O S. Comparison of NMR data of 1 and 2 (Tables

S1 and S2 ) suggested that the pyrrolidine-2,4-dione ring was

opened and the methine C-35 in 1 was replaced by a methyl in

, leading to establishment of the MeObu unit. In addition, the

loss of an aromatic proton double−doublet, the presence of the

exchange proton signal (δ 5.86 in CDCl , 8.47 in DMSO-d ),

and the shielded shift of C-25 (from δ 153.8 to δ 137.3)

indicated that the 4-OMe-Trp residue in 2 was substituted by

an extra hydroxy group. Analysis of 2D NMR data conclusively

revealed that the additional OH was attached at C-26 and

established the structure of 2.

Figure 3. Corrected absolute errors of ¹³C chemical shifts in the 3-

methyl-5-hydroxypyrrolidine-2,4-dione unit calculated for 1a−1d.

Zelkovamycin D (3) was identiﬁed as an isomer of 2 based

on HRESIMS and NMR data. The deshielded signals of H -9

(

δ 4.37, dd; 4.33, dd) and C-9 (δ 58.6) indicated that the

H C

methyl C-9 was hydroxylated to form a 4-hydroxy-

dehydrobutyrine (4-OH-Dhb) residue in 3 while the aromatic

protons and carbons data suggested the 4-OMe-Trp unit

remained in 3.

The molecular formula of zelkovamycin E (4) had one fewer

oxygen atom and one fewer CH₂unit than that of 3.

Comparison of NMR data of 4 and 3 clearly revealed that the

4-OH-Dhb and Abu residues in 3 were replaced by Dhb and

Ala unit (δ_H1.48, d; 4.33, qd; δ 14.1, 48.6, 169.7) in 4,

respectively.

The MS and NMR data of 5 were identical to those reported

for zelkovamycin (6) (Table S3), suggesting that they were

Figure 4. Experimental ECD spectrum of 1 in MeOH and calculated

ECD spectra for 1a−1d.

the same compound. However, analysis of the HMBC and

ROESY correlations indicated that the methoxy group was

located at C-4 of Trp in 5 whereas it was originally assigned at

C-7 of Trp in zelkovamycin (6). Accordingly, the structure 5

with 4-OMe-Trp revised that of zelkovamycin.

residue (OMeTrp) (Figure 2). The NMR data ascribed to

these residues in 1 were similar to those of zelkovamycin (6)

Table S3 ). However, compound 1 showed an HMBC

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

Letter

Figure 5. Proposed model of zelkovamycins biosynthesis. (A) Organization of the putative zelkovamycins biosynthetic gene cluster (Zel BGC) in

Kitasatospora sp. CPCC204717. (B) Proposed biosynthetic pathway for 1−5.

The absolute conﬁguration of zelkovamycin had not been

determined, as its FDLA derivatives were not detected in

Marfey’s analysis.

determined by Omura et al. We investigated the absolute

conﬁgurations of the amino acid residues in 2 −5 using a

combination of the advanced Marfey’s analysis (Figures S3 −S7

Marfey’s analysis on 1 was not performed due to the scarcity

of the isolated material. The chiralities of Abu, Tzl-Ala, and 4-

OMe-Trp in 1 were deduced to be the same as those in 2−5

based on biosynthetic consideration. To determine the

conﬁguration of C-32 and C-35 in 1, we employed the

and Table S7 ) and NMR calculation. The Marfey analysis was

performed twice. The ﬁrst one included acid hydrolysis of 2−5,

to liberate 4-OMe-Trp, 4-OMe-5-OH-Trp (for 2), Ala-Tzl,

Abu, and Ala (for 4), followed by derivatization with L/D-

FDLA (5-ﬂuoro-2,4-dinitrophenyl-L/D-leucinamide). Analysis

of the retention times of the L/D-FDLA derivatives of the

hydrolysates revealed that 4-OMe-Trp and 4-OMe-5-OH-Trp

were of the L conﬁguration and that Abu and Ala were of the D

conﬁguration. However, the absolute conﬁguration of Ala-Tzl

could not be determined by this analysis since the thiazole-

based amino acid in peptides tends to epimerize easily during

quantum-mechanical NMR calculation method, which was

successfully used in the conﬁguration assignment of natural

products including the ﬂexible cyclic peptides and cyclo-

lipopeptides. The H and C NMR chemical shifts were

calculated for four possible diastereoisomers,

R,14R,20S,32S,35S-1 (1a), 3R,14R,20S,32R,35R-1 (1b),

R ,14 R ,20 S ,32 S ,35R-1 (1c), and 3R,14R,20S,32R,3S-1 (1d)

(

Tables S8 and S9 ). The carbon data computed for 1a showed

the highest similarity to the experimental values with the

smallest corrected mean absolute error (CMAE) value (1a:

acid hydrolysis. To prevent racemization of Ala-Tzl, the

second procedure was carried out by an initial step of

ozonolysis of 2−5 prior to acid hydrolysis to cleave the

thiazole ring to liberate Ala, which ﬁnally led to unambiguous

assignment of D-Ala-Tzl. The chirality of MeObu was not

.11 ppm vs 1b−1d: 1.29, 1.29, and 1.91 ppm, respectively),

particularly for the 3-methyl-5-hydroxypyrrolidine-2,4-dione

unit (Figure 3). DP4+ analysis based on all C and H data

gave 99.94% probability for structure 1a (Table S10 ),

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

The Supporting Information is available free of charge at

Letter

indicating 32S,35S conﬁguration for 1. Furthermore, the

produce a derivative containing the 2-methyl-3,4-dioxobutyr-

ine (MeDob) residue, which ﬁnally forms 1 via an intra-

molecular nucleophilic addition by NH of Gly to the C-4

ketone group of MeDob. Another feature of this family of

peptides was the ﬁrst occurrence of the rare amino acids 4-

methoxy-5-hydroxytryptophan and 4-hydroxy-dehydrobutyrine

2S,35S-conﬁguration was supported by the calculated ECD

spectra for 1a (Figure 4), which matched well with the

experimental ECD curve. Therefore, the absolute conﬁguration

of 1 was concluded to be 3R,14R,20S,32S,35S. This result

suggested that the relative conﬁguration of H -33 and H-35 in

the pyrrolidine-2,4-dione ring were trans, in agreement with

the absence of ROESY correlations between the two groups

described above.

To determine the absolute conﬁguration of MeObu in 2−5,

we used two possible diastereoisomers of 4 [3R,14R,20S,32S-4

in natural peptide products. These residues are presumably

derived from 4-OMe-Trp and Dhb, respectively, by hydrox-

ylation catalyzed by a kind of oxygenases such as cytochrome

P450 enzymes, which were widely distributed in the genome

sequence of this strain as identiﬁed by antiSMASH 5.0. The

biosynthetic origin of compounds 1−5 is currently under

investigation.

(4a) and 3R,14R,20S,32R-4 (4b)] as the model structures of

compounds 2−5 for NMR calculation. The isomer 4a showed

smaller C and H CMAE values than 4b ( H: 0.22 vs 0.33

ppm; C: 1.59 vs 2.30 ppm, Table S11). Indeed, DP4+

analysis also predicted 4a as the correct structure with the

probability of 100% for all ¹³C and H data. These results

suggested the chirality of the MeObu residue in 2−5 as L-

ASSOCIATED CONTENT

Supporting Information

■

sı

https://pubs.acs.org/doi/10.1021/acs.orglett.0c03565.

conﬁguration.

In the antibacterial assay (Table S4), compound 5 displayed

the most potent activity against Staphylococcus aureus (MIC =

Experimental details, NMR data, UV, ECD, IR,

HRESIMS, and NMR spectra of compounds, and

calculation details (PDF )

.1 μM) while 3 and 5 were the most potent antibiotics against

S. epidermidis (MICs = 1.6 μM). Compound 1 was inactive at

the concentration as high as 200 μM, suggesting that the

MeObu residue is crucial for the antibiotic activity. In addition,

compound 5 was 8−64 times more potent than 2 and 3 against

S. aureus, indicating that hydroxylation at the Trp and Dhb

residues decreases the activity. Compound 4, which only

diﬀers from 5 by the substitution of Ala instead of Abu, showed

a 4−32-fold reduction in antibacterial activity.

Compounds 1−5 were also assessed for antiviral activities

against H1N1 inﬂuenza A virus (Table S5). Interestingly,

although compound 4 showed much weaker antibacterial

potency than 5, it exhibited potent inhibitory activity against

the H1N1 virus (EC₅₀0.3 μM, selective index (SI) = 149),

AUTHOR INFORMATION

Corresponding Authors

■

Shan Cen − Institute of Medicinal Biotechnology, Chinese

Academy of Medical Sciences and Peking Union Medical

College, Beijing 100050, P. R. China; Email: shancen@

imb.pumc.edu.cn

Rebecca J. M. Goss − School of Chemistry, University of St

Andrews, St Andrews, Fife KY16 9ST, U.K.; orcid.org/

0000-0001-9869-8205 ; Email: rjmg@st-andrews.ac.uk

Maoluo Gan − Institute of Medicinal Biotechnology, Chinese

Academy of Medical Sciences and Peking Union Medical

College, Beijing 100050, P. R. China; orcid.org/0000-

50-fold and 50-fold stronger than 5 and the positive control

0002-3089-8654 ; Email: ganml@imb.pumc.edu.cn

ribavirin, respectively.

From a biosynthetic point of view, compounds 1−5 are

nonribosomally derived peptide containing several unusual

amino acids. The amino acid sequences in 1−5 are very similar

Authors

Xiaomeng Hao − Institute of Medicinal Biotechnology, Chinese

Academy of Medical Sciences and Peking Union Medical

College, Beijing 100050, P. R. China

to those of argyrins, the major diﬀerence being the

replacement of the Dhb and MeObu residues by dehydroa-

lanin (Dha) and 4-OMe-Trp in argyrins, respectively. Analysis

Jiaqing Yu − Institute of Medicinal Biotechnology, Chinese

Academy of Medical Sciences and Peking Union Medical

College, Beijing 100050, P. R. China; School of Pharmacy,

Jining Medical College, Jining, Shandong 276800, P. R.

China

Yujia Wang − Institute of Medicinal Biotechnology, Chinese

Academy of Medical Sciences and Peking Union Medical

College, Beijing 100050, P. R. China

Jack A. Connolly − School of Chemistry, University of St

Andrews, St Andrews, Fife KY16 9ST, U.K.

Yufeng Liu − School of Pharmacy, Jining Medical College,

Jining, Shandong 276800, P. R. China

Yuqin Zhang − Institute of Medicinal Biotechnology, Chinese

Academy of Medical Sciences and Peking Union Medical

College, Beijing 100050, P. R. China

of the genome sequence using AntiSMASH 5.0 identiﬁed a

biosynthetic gene cluster (BGC) encoding an expected

octamodular nonribosomal peptide synthetase (NRPS)

assembly line, in which the NRPS module architecture and

modifying enzymes were highly homologous to those of

argyrins (Arg BGC) in Cystobacter sp. SBCb004 (Table S13

and Figure S8 ). This allowed us to conclude that this BGC

was responsible for the production of compounds 1−5. On the

basis of the NRPS domain arrangement and predicted

substrate speciﬁcities of the adenylation (A) domain, a

biosythetic pathway was proposed for compounds 1−5

(Table S14 and Figures 5 and S9). The most structurally

intriguing part of 1 was the 3-methyl-5-hydroxypyrrolidine-2,4-

dione ring system, which has never been found in natural

products. This unprecedent heterocyclic structure is assumed

to originate from threonine (Thr). Dehydrogenation of Thr by

oxygenase (Orf 11 or 13) generates 3-oxobutyrine (Obu),

which is subsequently methylated by C-methyltransferases

Liyan Yu − Institute of Medicinal Biotechnology, Chinese

Academy of Medical Sciences and Peking Union Medical

College, Beijing 100050, P. R. China; orcid.org/0000-

0002-8861-9806

(

Orf 2) to give MeObu. A subsequent two-step postoxygena-

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.orglett.0c03565

tion on the MeObu residue of the precursor peptide 5 would

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

Absolute Configuration of Constituent Amino Acids in a Peptide:

Letter

Notes

Elucidation of Limitations of Marfey ’s Method and of Its Separation

Mechanism. Anal. Chem. 1997, 69, 3346 −3352.

The authors declare no competing ﬁnancial interest.

8) Yonetani, K.; Hirotsu, Y.; Shiba, T. Racemization of Amino Acid

ACKNOWLEDGMENTS

Residues Fused in Thiazoline, Oxazoline, and Imidazoline Rings. Bull.

Chem. Soc. Jpn. 1975, 48, 3302−3305.

■

We would like to thank the HPCC of Peking Union Medical

College for computational facilities. This work was ﬁnancially

supported by the National Natural Science Foundation of

China (Grant No. 81872781), CAMS Innovation Fund for

Medical Sciences (CIFMS, 2016-I2M-2-002), and the Drug

Innovation Major Project of China (Grant No. 2018ZX-

(9) McDonald, L. A.; Ireland, C. M. Patellamide E: A New Cyclic

Peptide from the Ascidian Lissoclinum Patella. J. Nat. Prod. 1992, 55,

76−379.

10) Willoughby, P. H.; Jansma, M. J.; Hoye, T. R. A Guide to

Small-Molecule Structure Assignment through Computation of ( H

and C) NMR Chemical Shifts. Nat. Protoc. 2014, 9, 643−660.

(11) (a) Cooper, J. K.; Li, K.; Aube, J.; Coppage, D. A.; Konopelski,

9711001-007-002).

J. P. Application of the DP4 Probability Method to Flexible Cyclic

Peptides with Multiple Independent Stereocenters: The True

Structure of Cyclocinamide A. Org. Lett. 2018, 20, 4314−4317.

REFERENCES

■

(

1) (a) Bernardi, D. I.; das Chagas, F. O.; Monteiro, A. F.; dos

(b) Zhang, F.; Barns, K.; Hoffmann, F. M.; Braun, D. R.; Andes, D.

Santos, G. F.; de Souza Berlinck, R. G. Secondary Metabolites of

Endophytic Actinomycetes: Isolation, Synthesis, Biosynthesis, and

Biological Activities. In Progress in the Chemistry of Organic Natural

Products 108; Kinghorn, A. D., Falk, H., Gibbons, S., Kobayashi, J. i.,

Asakawa, Y., Liu, J.-K., Eds.; Springer International Publishing: Cham,

R.; Bugni, T. S. Thalassosamide, a Siderophore Discovered from the

Marine-Derived Bacterium Thalassospira Profundimaris . J. Nat. Prod.

017, 80, 2551−2555. (c) Dardic, D.; Lauro, G.; Bifulco, G.;

Laboudie, P.; Sakhaii, P.; Bauer, A.; Vilcinskas, A.; Hammann, P. E.;

Plaza, A. Svetamycins A-G, Unusual Piperazic Acid-Containing

Peptides from Streptomyces sp. J. Org. Chem. 2017, 82, 6032−6043.

019; pp 207 −296. (b) Qin, S.; Xing, K.; Jiang, J. H.; Xu, L. H.; Li, W.

J. Biodiversity, Bioactive Natural Products and Biotechnological

Potential of Plant-Associated Endophytic Actinobacteria. Appl.

Microbiol. Biotechnol. 2011, 89, 457−473.

(d) Saito, S.; Atsumi, K.; Zhou, T.; Fukaya, K.; Urabe, D.; Oku, N.;

Karim, M. R. U.; Komaki, H.; Igarashi, Y. A Cyclopeptide and Three

Oligomycin-Class Polyketides Produced by an Underexplored

Actinomycete of the Genus Pseudosporangium . Beilstein J. Org.

Chem. 2020, 16, 1100−1110.

(

2) (a) Dinesh, R.; Srinivasan, V.; T. E, S.; Anandaraj, M.;

Srambikkal, H. Endophytic Actinobacteria: Diversity, Secondary

Metabolism and Mechanisms to Unsilence Biosynthetic Gene

Clusters. Crit. Rev. Microbiol. 2017 , 43 , 546 −566. (b) Singh, R.;

Dubey, A. K. Diversity and Applications of Endophytic Actinobacteria

of Plants in Special and Other Ecological Niches. Front. Microbiol.

018, 9, 1767−1796.

3) Golinska, P.; Wypij, M.; Agarkar, G.; Rathod, D.; Dahm, H.; Rai,

M. Endophytic Actinobacteria of Medicinal Plants: Diversity and

Bioactivity. Antonie van Leeuwenhoek 2015, 108, 267−289.

4) Zhao, H.; Yang, A.; Liu, J.; Bao, S.; Peng, R.; Hu, Y.; Yuan, T.;

(12) Xin, D.; Jones, P. J.; Gonnella, N. C. D Ice: Diastereomeric in

Silico Chiral Elucidation, Expanded DP4 Probability Theory Method

for Diastereomer and Structural Assignment. J. Org. Chem. 2018, 83,

035−5043.

13) Pescitelli, G.; Bruhn, T. Good Computational Practice in the

Assignment of Absolute Configurations by TDDFT Calculations of

ECD Spectra. Chirality 2016, 28, 466−474.

(

14) During the course of our manuscript preparation, an

(

independent paper reported the structural revision and full

conﬁguration of zelkovamycin via total synthesis, which was in

agreement with our research results. Krahn, D.; Heilmann, G.; Vogel,

F. C. E.; Papadopoulos, C.; Zweerink, S.; Kaschani, F.; Meyer, H.;

Roesch, A.; Kaiser, M. Zelkovamycin is an Oxphos Inhibitory

Member of the Argyrin Natural Product Family. Chem. - Eur. J.

Hou, S.; Xie, T.; Zhang, Q.; Chen, X.; Wang, X.; Hu, L.

Chartspiroton, a Tetracyclic Spiro-Naphthoquinone Derivative from

a Medicinal Plant Endophytic Streptomyces . Org. Lett. 2020, 22,

5) (a) Li, J.; Chen, M.; Hao, X.; Li, S.; Li, F.; Yu, L.; Xiao, C.; Gan,

739−3743.

M. Structural Revision and Absolute Configuration of Burnettramic

Acid A. Org. Lett. 2020, 22, 98−101. (b) Li, J.; Hu, Y.; Hao, X.; Tan,

J.; Li, F.; Qiao, X.; Chen, S.; Xiao, C.; Chen, M.; Peng, Z.; Gan, M.

Raistrickindole A, an Anti-HCV Oxazinoindole Alkaloid from

Penicillium Raistrickii IMB17 −034. J. Nat. Prod. 2019, 82, 1391−

(

020, 26, 8524−8531.

15) Vollbrecht, L.; Steinmetz, H.; Hofle, G.; Oberer, L.; Rihs, G.;

Bovermann, G.; von Matt, P. Argyrins, Immunosuppressive Cyclic

Peptides from Myxobacteria. Ii. Structure Elucidation and Stereo-

chemistry. J. Antibiot. 2002, 55, 715−721.

395. (c) Chen, M.; Wang, R.; Zhao, W.; Yu, L.; Zhang, C.; Chang,

(16) Blin, K.; Shaw, S.; Steinke, K.; Villebro, R.; Ziemert, N.; Lee, S.

S.; Li, Y.; Zhang, T.; Xing, J.; Gan, M.; Feng, F.; Si, S.

Isocoumarindole A, a Chlorinated Isocoumarin and Indole Alkaloid

Hybrid Metabolite from an Endolichenic Fungus Aspergillus sp. Org.

Lett. 2019, 21, 1530−1533. (d) Hu, Y.; Wang, M.; Wu, C.; Tan, Y.;

Li, J.; Hao, X.; Duan, Y.; Guan, Y.; Shang, X.; Wang, Y.; Xiao, C.; Gan,

M. Identification and Proposed Relative and Absolute Configurations

of Niphimycins C −E from the Marine-Derived Streptomyces sp.

IMB7 −145 by Genomic Analysis. J. Nat. Prod. 2018, 81, 178−187.

Y.; Medema, M. H.; Weber, T. Antismash 5.0: Updates to the

Secondary Metabolite Genome Mining Pipeline. Nucleic Acids Res.

(

019, 47, W81−W87.

17) Pogorevc, D.; Tang, Y.; Hoffmann, M.; Zipf, G.; Bernauer, H.

S.; Popoff, A.; Steinmetz, H.; Wenzel, S. C. Biosynthesis and

Heterologous Production of Argyrins. ACS Synth. Biol. 2019, 8,

(

121−1133.

18) Yoganathan, S.; Vederas, J. C. 5.02 - Nonprotein L-Amino

6) (a) Tabata, N.; Tomoda, H.; Zhang, H.; Uchida, R.; Omura, S.

Acids. In Comprehensive Natural Products Ii; Liu, H.-W., Mander, L.,

Eds.; Elsevier: Oxford, 2010; pp 5−70.

Zelkovamycin, a New Cyclic Peptide Antibiotic from Streptomyces sp.

K96 −0670. Ii. Structure Elucidation. J. Antibiot. 1999, 52, 34−39.

b) Zhang, H.; Tomoda, H.; Tabata, N.; Oohori, M.; Shinose, M.;

(19) (a) Rudolf, J. D.; Chang, C.-Y.; Ma, M.; Shen, B. Cytochromes

(

P450 for Natural Product Biosynthesis in Streptomyces: Sequence,

Takahashi, Y.; Omura, S. Zelkovamycin, a New Cyclic Peptide

Antibiotic from Streptomyces sp. K96 −0670. I. Production, Isolation

and Biological Properties. J. Antibiot. 1999, 52, 29−33.

Structure, and Function. Nat. Prod. Rep. 2017, 34, 1141 −1172.

b) Greule, A.; Stok, J. E.; De Voss, J. J.; Cryle, M. J. Unrivalled

Diversity: the Many Roles and Reactions of Bacterial Cytochromes

7) (a) Fujii, K.; Ikai, Y.; Oka, H.; Suzuki, M.; Harada, K.-i. A

P450 in Secondary Metabolism. Nat. Prod. Rep. 2018, 35, 757−791.

Nonempirical Method Using LC/MS for Determination of the

Absolute Configuration of Constituent Amino Acids in a Peptide:

Combination of Marfey ’s Method with Mass Spectrometry and Its

Practical Application. Anal. Chem. 1997, 69, 5146−5151. (b) Fujii, K.;

Ikai, Y.; Mayumi, T.; Oka, H.; Suzuki, M.; Harada, K.-i. A

Nonempirical Method Using LC/MS for Determination of the