The ethyl acetate extract of the marine Streptomyces sp. B 1848 drew our atten-tion due to the presence of two dark red spots and one orange band during TLC.
Their negative colouration with dilute sodium hydroxide solution excluded peri-hydroxyquinones. The strain showed a high activity against Mucor miehei (Tü284) and Candida albicans, a moderate activity against Escherichia coli and the alga Chlorella vulgaris.
Well-grown agar cultures of B 1848 served to inoculate 12 of 1 liter-Erlenmeyer flasks, each containing 250 ml of M2+ medium. The flasks were incubated at 28 ºC while shaking with 110 rpm on a linear shaker for 3 days. The seed culture was in-oculated to 25-liter jar fermenter, and the fermentation was carried for additional 3 days at 28 °C. The pH was automatically maintained (pH 6.5± 1.25). The culture broth was filtered and extracted with ethyl acetate. The brown crude extract was de-fatted with cyclohexane, and the methanolic extract was subjected to silica gel flash column chromatography, eluting with gradient of dichloromethane-methanol.
Working up and purification of fractions II~V delivered the metabolites Nβ -acetyltryptamine (34), N(2-phenethyl)acetamide (43), 1-acetyl-β-carboline (81), an-thranilic acid (48), 2'-deoxyadenonsine (87), tyrosol (50), indolyl-3-acetic acid (46a), phenyl acetamide (49b), 2'-deoxythymidine (95), 2'-deoxyuridine (96), indolyl-3-carboxylic acid (45), N-acetyltyramine (142), p-hydroxybenzoic acid (105a) and 4-methylamino-7,8-dimethyl-isoquinoline-3,6-dione (97), in addition to the two new compounds, 6-hydroxy isatine (93) and 7-methylamino-3H-pyrrolo[2,3-c]isoquino-line-6,9-dione (102) (Figure 21).
B1848 (25l Fermenter)
Mycelium Filtrate
Crude extract (10.53 g)
filtration on celite by filterpress
3 x with EtOAc and 1x with
acetone 5 x with EtOAc
i.vac
dissolved in methanol and defatted with cyclohexane
Methanol phase (7.0 g)
Cyclohexane phase (2.5 g)
Fraction I Ffraction II Fraction III Fraction IV Fraction V SC. on silica gel, CHCl3-MeOH gradient
Sephadex LH 20 (MeOH)
Sephadex LH 20 (MeOH/CHCl3 4:6))
Sephadex LH 20 (MeOH/CHCl3 4:6))
Sephadex LH 20 (MeOH/CHCl3 4:6))
Sephadex LH 20 (MeOH)
4-methylamino-7,8-dimethyl-isoquinoline-3,6-dion 7-methylamino-3H-pyrrolo(2,3-c)isoquinoline-6,9-dion
PTLC (CHCl3/10%MeOH);
Sephadex LH 20 (MeOH)
Figure 21: Work-up procedure of marine Streptomyces sp. isolate B 1848 4.2.1 1-Acetyl-β-carboline
Compound 81 was isolated from the fast moving phase of fraction II as faint yellow solid (blue UV fluorescence at 366 nm), which was stained to faint yellow by anisaldehyde/sulphuric acid. The 1H NMR spectrum showed a strong solvent de-pendency. In CDCl3, it showed a broad singlet at δ 10.30 of 1 acidic H, two o-coupled doublets each of 1H at δ 8.54 and 8.16 which, may belong to a heteroaro-matic ring due to their small coupling constant (3J = 5.1 Hz). An additional 1,2-disubstituted aromatic ring was observed due to the presence of four multiplet pro-tons at δ 8.15 (5-H), 7.62 ( 7,8-H) and 7.31 (6-H). In the aliphatic region, it showed a
compound 81 was better resolved in acetone (Table 2).
Table 2: 1H NMR comparison of 81 in CDCl3 and [D6]acetone, (J in Hz)
Position CDCl3 [D6]acetone
1 - -
3 8.54 (d, 3J = 5.1) 8.52 (d, J = 5.1) 4 8.16 (d, 3J = 5.1) 8.36 (d, J = 5.1) 5 8.15 (m) 8.28 (dd, J = 8.1,1.3) 6 7.31 (m) 7.33 (td, J = 8.1, 1.3) 7 7.62 (m) 7.61 (td, J = 8.1, 1.3) 8 7.62 (m) 7.84 (dd, 8.1, 1.3) 9 10.30 (s br.) 11.25 (s br.) 11 2.90 (s) 2.75 (s)
The 13C/APT NMR spectrum revealed an acetyl carbonyl carbon at δ 202.5, five quaternary sp2 carbons at δ 142.7, 137.2 and 135.6, 132.1 and 121.4, of which the first three carbons could be attached to nitrogen or oxygen. In addition, six aromatic methine carbons at δ 138.6, 129.8, 122.5, 121.2, 119.8 and 113.5 were observed.
Furthermore, one sp3 carbon of a methyl group appeared at δ 25.7.
CI and EI mass spectra determined the molecular weight of 81 as 210 Dalton.
The obtained molecular ion m/z 210 of 81 in EI mass spectrum showed an expulsion of CO to give a peak at m/z 182. In addition, it showed a base peak at m/z 168 ([M - (COCH2)]+., which is most likely attributed to β-carboline moiety (83a), on the basis of comparison with a closely related moiety was previously investigated by our re-search group. A re-search in AntiBase led to elucidate the compound as 1-acetyl-β -carboline (81). Compound 81 was isolated at the first time from bacteria in our group[48].
According to 2J and 3J of hetero coupling (H→C) correlations (HMBC), the se-quence of carbon values in 81 were confirmed (Figure 22). It is worth to mention that the carbon values of compound 81 were previously reported with wrong assignment
[127], perhaps due to the missing 2D measurements. Although, the compound reported in literature was measured in a different solvent (dimethylsulfoxide), it showed no deviation in the shift of most carbons except for the wrongly assigned signals (Table 3).
N
Figure 22: HMBC correlations of 1-acetyl-β-carboline (81)
Table 3: Comparison of 13C NMR of 1-acetyl-β-carboline (81) using 2 D with those reported in the literature[127]
Pos. Exp. ([D6]acetone) Lit. ([D6]DMSO)
Compound 81 was isolated from several plant species. It is identical with the lycii Alkaloid I which was previously wrongly assigned as N-9-formylharman (82). Re-cently compound 82 was confirmed by Bracher et al.[127] to be 1-acetyl-β-carboline (81). Compound 81 was isolated also from the bark of Ailanthus malabarica (Sima-roubaceae), as well as the sponge Tedania ignis[24,128]. Harman (83b) is the simplest 1-alkyl-β-carboline-alkaloid, occurs in plants[129], fungi, microorganisms and marine animals[130,131]. Harman (83b) displayed a high pharmacological effect e.g. inhibition of the mononaminooxidase and the cAMP-phosphodiasterase[132] as well as antim-icrobial activity[133]. 1-Ethyl-β-carboline (84), a simple β-carboline derivative, exhib-ited a pronounced antimicrobial activity. It is used as an antagonist of catechola-mines[133,134]. The observed antifungal activity of the marine Streptomyces sp. B 1848
N N CH3 O H
N N
CH3 H
N N
H CH3
82 83a: R = H, 83b: R = CH3 84 4.2.2 2'-Deoxyadenosine
Compound 87 from fraction III was isolated as colourless solid exhibiting an UV absorbing band, which turned to green-blue with anisaldehyde/sulphuric acid.
The 1H NMR spectrum of 87 exhibited two singlets each of 1H at δ 8.31 and 8.12, and a broad singlet of 2 acidic protons at δ 7.23 (NH2). In the aliphatic and sugar regions, it showed one doublet of 1H at 6.34, two broad singlets (δ 5.25 and 5.15) of 2 OH groups, two oxygenated methines at δ 4.40 (dd) and 3.87 (ddd), and one ABX signal at δ 3.64~3.51 of an oxygenated methylene group. Furthermore, two methylene multiplets each with 1H at δ 2.68 and 2.24 of a cyclic sugar were ob-served.
The 13C NMR spectrum of compound 87 showed three sp2 quaternary carbon signals at δ 156.1, 148.9 and 119.3, and two sp2 methines (δ 152.4 and 139.5). In the sugar moiety, five sp3 carbons signals were observed, from which three were oxy-genated methines at δ 88.0, 84.0 and 71.0, and two at δ 61.9 and 40.3 were methyl-ene carbons, of which the first one was oxygenated.
The molecular weight of compound 87 was established as 251 Dalton by EI mass spectrum. A search in AntiBase led to four possible structures, namely cordy-cepin (85), oxetanocin, (86) 2'-deoxyadenosine (87) and 2'-deoxyinosine (88).
N
However, comparison of our spectral data with the literature of the above com-pounds, as well as with authentic spectra, led to identify the compound as 2'-deoxyadenosin (87)[48,135]. Cordycepin (85) is one of the first adenine-nucleoside analogues, which was isolated from cultures of the fungus Cordyceps militaris[136]. Cordycepin (85) is a strong inhibitor of RNA synthesis showing cytostatic activity.
In contrast, 2'-deoxyadenosin (87) is an essential base of DNA.
4.2.3 6-Hydroxy isatine
Compound 93 formed an orange middle polar band in both fractions III and IV.
It exhibited no colouration with NaOH and sulphuric acid confirming the absence of peri-hydroxyquinone moieties. It was isolated by PTLC and Sephadex LH-20 as an orange solid. The 1H NMR spectrum showed three different spectral resonances in the aromatic region each with intensity of 1 H, two ortho-coupled protons at δ 7.42 (d) and 6.37 (dd), and a doublet of a m-coupled proton at δ 6.26 (4J = 2.4 Hz) as of an 1,2,4-trisubstituted aromatic ring.
Figure 23: 1H NMR spectrum (CD3OD, 300 MHz) of 6-hydroxy isatine (93).
exhibited shift values, which were ∆δ 0.2-0.4 higher than those after complete purifi-cation. This could be due to the interaction with other compounds forming a com-plex. The molecular weight of compound 93 was found to be 163 Dalton in ESI MS.
HREI MS of 93 gave the molecular formula C8H5NO3.
The 13C NMR spectrum exhibited three sp2 down field carbon signals at δ 178.0, 166.8, and 155.9, where the first one could be the carbonyl of an ester, flanked by two sp2 carbon systems. The second one (166.8) could be an oxygenated sp2 carbon, carbonyl of acid or amid, while the third one could be of an oxygenated sp2 carbon or an imide group. The spectrum showed three additional sp2 methine carbons at δ 129.7, 116.8, and 102.8.
According to the above spectraldata as well as the molecular formula, the for-mula could be constructed by two rings, one as a six memberd aromatic ring with three protons, two in o- and the third in a m-position, and a second five memberdring containing two carbonyls in lactone or lactam form. This allowed suggesting five possible structures: 89, 90, 91, 92 and 93.
Compounds 89, 90 and 91 were plausible, but should be colourless and so do not match. In addition, the peri proton in 89 and 90 should be displayed at higher chemical shift (≈ 7.4) due to the adjacent carbonyl group. Compounds 89 and 90 are not known from nature, even not as constituents of natural compounds[23], while 91 is known as a constituent of several active compounds e.g. Staurosporin (94).
Compounds 92 and 93 are isatine derivatives, which are characterised by their orange-red colour, and one of them could match with our chemical data. The hy-droxyl group could be either at 5-position as in 92 or at 6-position as in 93. 1H and
13C NMR predictions with ACD pointed to isomer 93 as the more plausible one (Table 4).
Table 4: 1H NMR predicted values of both 5- and 6-hydroxy isatine compounds (92, 93) in comparison with the experimental data.
H no. Exp. Val. of 93 93 92
4 7.42 (d, J = 8.1) 7.69 (d, J = 8.1) 7.07 (d, J = 2) 5 6.37 (dd, J = 8.1, 1.5) 6.56 (dd, J = 8.1, 1.5) -
6 - - 7.31 (dd, J = 8.2, 2) 7 6.26 (d, J = 1.5) 7.11 (d, J = 1.5) 7.77 (d, J = 8.3 )
The UV spectrum of compound 93 was measured in comparison with isatine (65): compound 93 showed maxima at λmax 263, 280 (sh), 348.9 (br) and 420 nm (br). The observed UV values of compound 93 are very similar to isatine [λmax 241, 296.6 and 419 (br)]. The differences could be attributed to the bathochromic effect of the hydroxyl group (n-electrons) in 93.
N
A search in AntiBase for both isomers 92 and 93 resulted without any hit.
This referred to that 93 is a new natural product. However, by search in CA, both compounds were found as synthetic products, but no spectral data have been re-ported. Compound 93 was obtained after demethylation of the corresponding meth-oxy derivative by pyridinium hydrobromide perbromide (Py+ HBr3-)[137].
4.2.4 2'-Deoxythymidine, and 2'-Deoxyuridine
Compound 95 was obtained from fraction IV as colourless solid. It exhibited an UV absorbance, which turned to blue-green by anisaldehyde/sulphuric acid.
The 1H NMR spectrum showed three signals characteristic for the thymine moi-ety, a broad 1H singlet at δ 11.20 (NH), an aromatic 1H singlet at δ 7.68 (6-H), and an up-field singlet of a methyl group at δ 1.78, probably linked to an sp2 carbon in an α,ß-unsaturated system. Furthermore, the spectrum exhibited a triplet at δ 6.16 (1 H), signals of two oxygenated methine protons at δ 4.24 (td) and 3.76 (dd), two
multi-The 13C NMR spectrum exhibited 10 carbons, five of which are similar as in thymine. In addition, it contains five sp3 carbon atoms, three oxygenated methines and two methylene carbons.
The molecular weight was determined by EI mass spectrum as 242 Dalton. Fur-ther more, two characteristic peaks were observed at m/z 126 and 117, of which the first of due to a thymine moiety, and the second of deoxypentose, respectively. A search in AntiBase led to 2'-deoxythmidine (95), which was confirmed by the litera-ture data[138]. Figure 24: H,H COSY and HMBC correlations of 2'-deoxythymidine (95).
In addition to 2'-deoxy-thymidine (95), 2'-deoxyuridine (96) was isolated from fraction IV as colourless solid, which turned to blue by anisaldehyde/sulphuric acid.
Compound 96 was identified by comparing its spectral data with the literature[48]. 4.2.5 4-Methylamino-7,8-dimethyl-isoquinoline-3,6-dione
From fraction IV, the dark red middle polar compound 97 was isolated using PTLC and Sephadex LH-20. It showed no colour reaction with sodium hydroxide indicating the absence of peri-hydroxyquinones. The 1H NMR spectrum showed three 1H singlets at δ 9.02, 5.82 and 5.72. The broad singlet at δ 5.82 is an exchange-able NH proton of an amide. In addition, a methyl doublet was detected at δ 2.93 (J =
4.6 Hz), indicating a CH3NH- fragment. Two CH3 singlets at δ 2.76 and 2.70 for aromatic bound methyl groups were detected. The molecular weight of compound 97 was determined as 216 Dalton by EI and CI mass spectra. HREI MS led to the mo-lecular formula C12H12N2O2.
The 13C and APT NMR spectra indicated two aromatic methines at δ 145.1 and 103.3, and three aliphatic carbon signals were found at δ 29.0, 24.6 and 16.0, corre-sponding of three methyl groups. Furthermore signals of seven quaternary carbons, of which two at δ 185.0 and 181.9 representing carbonyl groups, were observed. A search in AntiBase led to 4-methylamino-7,8-dimethyl-isoquinoline-3,6-dione (97), which was further confirmed by direct comparison with the literature. The compound was isolated previously in our group by M. Speitling[114]. The unusual quinone exhib-ited four peaks at λmax 234, 273, 338 and 443 in the UV spectrum, which were iden-tical with the previous data[114].
Cribrostatin 1[139] (98) is a similar structural analogue. It was isolated recently together with some related compounds from the marine sponge Cribrochalina sp.
and is characterised by its cytotoxicity against lymphocyte P388 leukaemia cells. The biosynthetic origin of these compounds is still unknown. Additionally, four isoquino-line quinones as well as two isoquinoisoquino-line iminoquinones were obtained recently from the marine Bryozoan Caulibugula intermis. They are designated as novel cytotoxic compounds, and assigned as caulibugulones A-F (A, 99; F, 100)
N
Figure 25: 1H NMR spectrum (CDCl3, 300 MHz) of 4-methylamino-7,8-dimethyl-isoquinoline-3,6-dione (97).
4.2.6 7-Methylamino-3H-pyrrolo[2,3-c]isoquinoline-6,9-dione
Compound 102 was found in fraction IV as a second dark red middle polar component, which showed high similarity with compound 97, pointing to a related structure analogue. This was confirmed by the UV spectrum, which showed similar maxima as compound 97 (Figure 26).
Figure 26: UV Spectra (methanol) of compounds 97 (1) and 102 (2)
The 1H NMR spectrum revealed a broad H/D exchangeable singlet at δ 11.88 and two singlets at δ 8.98 and 5.61 confirming the protons at positions 2 and 5 in 97.
It showed also a doublet of an NHCH3 group at δ 2.83 (J = 5.0 Hz). The signal of the NH proton at 7-position appeared in DMSO as a broad quartet at δ 7.82 with higher shift than that in 97, measured in CDCl3. It displayed furthermore two 1H doublets at δ 7.92 and 6.69 with small coupling constants (< 4.0 Hz) characteristic for a hetero-cyclic five membered heterocycle. This also excluded the possibility of a benzene or pyridine derivative, which exhibits normally higher coupling constants (>5 Hz). The molecular weight (m/z 227 by EI and CI mass spectra) of 102 was odd referring to an odd number of nitrogens. HREI MS revealed the molecular formula C12H9N3O2.
Figure 27: 1H NMR spectrum ([D6]DMSO, 300 MHz) of 7-methylamino-3H-pyrrolo[2,3-c]isoquinoline-6,9-dione (102)
The 13C and APT NMR spectra indicated four aromatic methines, two of which were displayed at δ 140.7 and 99.0, corresponding to the two methine carbons C-2, C-5 in 97. The other two aromatic carbons at δ 137.6 and 102.7 are belonging to the two pyrrole methine carbons. Additionally, one aliphatic carbon signal was found at δ 28.9, fixing the NHCH3 fragment. Furthermore, signals of seven quaternary sp2 carbons were detected, of which two at δ 182.8 and 181.2 are representing the car-bonyl groups in positions 6 and 3, similar as in 97
The spectral data and molecular formula allowed three structural alternatives, 7-methylamino-1H-pyrrolo[3,2-c]isoquinoline-6,9-dione (101), 7-methylamino-3H-pyrrolo[2,3-c]isoquinoline-6,9-dione (102) and 7-methylamino-2H-pyrrolo[3,2-c]iso-quinoline-6,9-dione (103).
Figure 28: 13C NMR spectrum ([D6]DMSO, 150 MHz) of
The observed similarity between the signal patterns of pyrrole protons and those of the 2,3-protons in indole (106) confirmed their direct o-arrangement. Furthermore, this conclusion was established by the H,H COSY spectrum, which showed a 3J cor-relation between the two protons (δ 7.92, 6.60). The proton at δ 7.92 showed a 3J coupling to the pyrrol NH proton (11.88), and the proton at δ 6.60 exhibited a long range coupling (4J) with the same NH proton (Figure 29). Additionally, the 1J corre-lations (HMQC) confirmed the attachment of the two protons (7.92 and 6.60) to the corresponding carbons at δ 137.6 and 102, respectively. This agrees with the
assump-tion that the methine carbon at δ 137.6 is linked with the NH of the pyrrol ring, while the other one (δ 102.7) is attached to an sp2 carbon atom. This excluded compound 103.
Figure 29: Partial structures of 7-methylamino-3H-pyrrolo[2,3-c]isoquinoline-6,9-dione (102) delivered from H,H COSY and HMBC correlations
Based on the long-range correlations (HMBC), both structures 101 and 102 are possible. The observed long range coupling (3J) between the pyrrol proton (δ 7.92) and the quaternary carbon in o-position of the pyridine ring (δ 153.6) exhibited the same 3J coupling in both cases of compounds 101 and 102. However, the low δ value of the m-carbon in the pyridine ring (δ 122.4) directed its linkage to a carbon atom as in compound 102 rather than to nitrogen (101). This assumption was confirmed by comparison with similar fused pyrrolo-pyridine skeletons[140] (A and B), establishing the compound to be 102 but not 101. Furthermore, the residual main isoquinolone part of compound 102 was confirmed with the aide of 2 D correlations, presenting the same skeleton as in compound 97 (Figure 29).
The structurally related cribrostatin 6[141] (104) was recently obtained from the marine sponge Cribrochalina sp. It exhibited anticancer activity and was highly bio-active against bacteria and fungi.
N
Figure 30: H,H COSY and HMBC correlations of compounds 101 and 102
N
Compound 102 has not been described so far, pointing to a novel natural prod-uct from bacteria. It is worth to mention that the compound 102 was isolated in a very little amount from the strain using M2+ medium (two 25-liter shaker cultures).
However, after optimisation, it was found that the C medium + 50 % sea water (meat extract, 1 g; glucose, 10 g, peptone, 2 g; yeast extract, 1 g) is much better for cultiva-tion of the strain. A 50-liter fermentacultiva-tion delivered now sufficient material for all spectroscopic and biological measurements. Additionally, three further new isoqui-noline quinones were isolated, which are still under investigation.
4.2.7 p-Hydroxy benzoic acid
Working up of fraction V led to a colourless solid substance 105a. On the basis of chromatographic and spectral data, compound 105a was identified as p-hydroxy benzoic acid, which further confirmed by methylation giving p-methoxybenzoic acid methyl ester (105b) as a colourless solid. p-Hydroxy benzoic acid (105a) is
wide-spread in many plants, as well as in microorganisms free or in form of derivatives. It was first isolated from fruits of Catalpa bignonioides[24].
O OH RO
105a: R = H, 105b: R = CH3