Two new polyacetylenes (1 and 2), along with two known C-30 steroids (3 and 4) were identified from the Red Sea sponge, Xestospongia sp. The chemical structures were determined based on extensive spectroscopic measurements 1D (1H, 13C and DEPT) and 2D (COSY, HSQC and HMBC) NMR, UV, IR and MS. The new compounds 1 and 2 were evaluated for their antimicrobial and antitumor activities. 1 and 2 were active against multidrug- resistant bacteria with MICs ranged from 2.2 to 4.5 μM. No toxicity was recorded for the two tested compounds up to 5 μM using Artemia salina as a test organism. Compound 2 showed excellent antifungal activity against some pathogenic fungi such as Aspergillus niger and Candida albicans (MIC 2.2–2.5 μM) and antitumor activity against both Ehrlich ascites carcinoma and lymphocytic leukemia (LD50 5.0 μM).
Barrel sponges, or the species of the genus Xestospongia (class Demospongia, order Haplosclerida, family Petrosiidae) might be considered as exclusive producers of some interesting secondary metabolites. These include the halogenated polyacetylenic lipids, which have been shown to possess a broad spectrum of biological activities such as antimicrobial, HIV-integrase inhibition, and Na+/K+ ATPase inhibition [1–4]. The genus Xestospongia contains long-chain polyacetylenic alcohols with chemotaxonomic markers , and polyacetylenes with antimicrobial, cytotoxic, antitumor, antiviral, and immunosuppressant bioactivities, and also as enzyme inhibitors [6, 7]. Search for bioactive metabolites from Xestospongia led to the identification of two new polyacetylenes, viz. acylglycerolipid and xestospongiamide. Similarly, the extract from Xestospongia sp., collected from Pohnpei, Federated States of Micronesia, exhibited antimicrobial activity against Pseudomonas aeruginosa and Mycobacterium intracellulare with IC50 values of 1.03–2.07 μg/mL . Here we present the isolation and identification of two further new polyacetylenes from this sponge.
2 Materials and methods
Silica gel GF 254 (Merck, Darmstadt, Germany) was used for analytical TLC. PTLC was performed on aluminum oxide plates (20 × 20 cm) of 250 μm thickness. Electron impact mass spectra were determined at 70 ev on a Kratos MS-25 instrument (Manchester, UK). 1D and 2D NMR spectra were recorded on Bruker AVANCE III WM 600 MHz spectrometers (Karlsruhe, Germany) and 13C NMR at 150 MHz. Tetramethylsilane (TMS) was used as internal standard. Plates were sprayed with 50%-sulphuric acid in methanol and heated at 100 °C for 1–2 min.
2.2 Sponge sample
The sponge Xestospongia sp. was collected from deep water (15 m depth) of Sharm Obhur (21°29′31″N 39°11′24″E), Jeddah, Saudi Arabia, and was identified by Biologist Kamal Al-Dahoody (Faculty of Maritime Studies, King Abdulaziz University). A Voucher sample (JAD 02013) has been deposited at the Chemistry Department, Faculty of Science, King Abdulaziz University. The sponge was barrel shaped (25 cm length and 15 cm diameter) with a large cavity in the center. It is characterized by the presence of finger-like structures on the outside surface. The color is maroon to pink. Out of the water, it appears brown or light pink. The skeleton is composed of a tangential layer of single spicules in the ectosomal skeleton. The choanosomal skeleton shows multispicular reticulation forming regular tracts.
2.3 Extraction and isolation of compounds
The freeze-dried sponge (90 g) was extracted two times with 6 L of a mixture of CH2Cl2-MeOH (1:1, v/v) for 24 h at 22°, and a viscous dark reddish oil was obtained (7.8 g). This extract was fractionated on NP-Silica (2.5 × 75 cm, 500 g, Merck 7739), employing a gradient technique with four solvent systems: Pet. ether: diethyl ether (9:1, v/v); Pet. ether: CHCl3 (9:1); Pet. ether: EtOAC (7:3) and CHCl3: MeOH (9:1), each of 500 mL, giving four fractions F-A, F-B, F-C and F-D of 90, 14, 88 and 30 mg, respectively. The fraction (F-A) that was purified by RP-18 HPLC (eluent, MeOH:H2O, 65:35), and re-purified on PTLC-silica gel using CHCl3:MeOH (9.5:0.5), yielded compound 4. The fraction (F-B) which was purified by PTLC-silica gel using CHCl3: ETOAc (6:4), yielded compounds 3, 2 and 1, respectively.
2.4 Acid Hydrolysis of 1.
A solution of 1 (2 mg in 2 mL of MeOH) and 10 mL of 7% HCl/MeOH (3:7) was left for 10 h (23 °C), monitored by TLC. At the end of this period, the mixture was extracted with EtOAc, and the aqueous layer neutralized with 7% KOH. After evaporation of the solvent, the residue was extracted with C6H5N. HPLC purification of the C6H5N extract (Lichrosorb100 NH; Knauer, Berlin, Germany) using MeCN/H2O (8:2) as eluent yielded glucose (0.5 mg).
Compound 1 was isolated as a white amorphous powder, Rf = 0.29 CHCl3: EtOAc (6:4, 2.5 mg). IR (film) cm–1: 3371, 2921, 2851, 2206, 1736, 1459; 1H and 13C NMR spectral data are shown in Table 1. Compound 2 was isolated as pale yellow oil, with Rf = 0.25 (CHCl3: EtOAc (6:4, 1.5 mg). IR (film) cm–1: 3434, 2852, 2215, 1741, 1459, 1200; 1H and 13C NMR spectral data are shown in Table 1.
|1||104.7 (d)||4.28 (d, 7.8)||13″||29.6 (t)||1.50 (m)||1||173.1 (s)||–|
|2||75.0 (d)||3.17 (t, 8.4)||14″||19.8 (t)||2.27 (td, 6.6, 1.8)||2||33.9 (t)||2.33 (t, 7.2)|
|3||77.9 (d)||3.33 (t, 9.0)||15″||93.7 (s)||–||3||24.4 (t)||1.63 (p, 7.2)|
|4||71.9 (d)||3.32 (t, 9.0)||16″||78.3 (s)||–||4||28.3 (t)||1.41 (p, 7.2)|
|5||78.0 (d)||3.27 (m)||17″||118.0 (d)||6.23 (dt, 13.8, 1.8)||5||27.9 (t)||1.52 (p, 7.2)|
|6||62.7 (t)||3.67 (dd, 12.0, 5.4)||18″||119.1 (d)||6.69 (d, 13.8)||6||19.1 (t)||2.29 (2H, td, 7.2, 1.2)|
|3.87 (dd, 12.0, 2.0)||1″′||174.7 (s)||–||7||88.5 (s)||–|
|1′||68.7 (t)||3.75 (dd, 10.8, 6.0)||2″′||34.8 (t)||2.34 (td, 7.2, 1.8)||8||79.2 (s)||–|
|3.99 (dd, 10.8, 6.0)||3″′||25.5 (t)||1.62 (m)||9||110.2 (d)||5.47 (1H, dd, 15.6, 1.2)|
|2′||71.6 (d)||5.27 (m)||4″′||28.9 (t)||1.43 (m)||10||142.6 (d)||6.02 (1H, dt, 15.6, 6.6)|
|3′||63.9 (t)||4.22 (dd, 12.0, 6.0)||5″′||29.1 (t)||1.51(m)||11||32.3 (t)||2.10 (2H, q, 6.6)|
|4.44 (dd, 12.0, 6.0)||6″′||19.8 (t)||2.27 (td, 6.6, 1.8)||12||27.7 (t)||1.48 (2H, m)|
|1″||175.0 (s)||–||7″′||88.5 (s)||–||13||28.4 (t)||1.50 (2H, m)|
|2″||34.0 (t)||2.45 (t, 7.2)||8″′||80.8 (s)||–||14||19.2 (t)||2.25 (2H, td, 6.6, 18)|
|3″||25.2 (t)||1.80 (quin, 7.2)||9″′||112.0 (d)||5.46 (dt, 15.6, 1.8)||15||92.7 (s)||–|
|4″||19.3 (t)||2.34 (td, 7.2, 1.8)||10″′||143.5 (d)||5.98 (dt, 15.6., 6.6)||16||77.5 (s)||–|
|5″||89.1 (s)||11″′||33.0 (t)||2.20 (q, 6.6)||17||118.0 (d)||6.17 (1H, dt, 13.8, 1.8)|
|6″||80.3 (s)||12″′||33.3 (t)||2.20 (q, 6.6)||18||117.1 (d)||6.57 (1H, d, 13.8)|
|7″||111.6 (d)||5.46 (dt, 15.6, 1.8)||13″′||145.7 (d)||6.13 (dt, 15.6, 6.6)||1′||68.9 (d)||5.26 (1H, dd, 6.6, 4.2)|
|8″||142.8 (d)||5.98 (dt, 15.6, 6.6)||14″′||111.1 (d)||5.62 (dd, 15, 1.8)||2′||62.1 (t)||4.28 (1H, dd, 12.6, 4.2)|
|9″||33.5 (t)||2.10 (q, 6.6)||15″′||91.3 (s)||–||4.13 (1H, dd, 12.6, 6.6)|
|10″||28.9 (t)||1.43 (m)||16″′||85.6 (s)||–|
|11″||28.9 (t)||1.43 (m)||17″′||118.7 (d)||6.37 (dd, 13.8, 1.8)|
|12″||29.4 (t)||1.50 (m)||18″′||118.9 (d)||6.79 (d, 13.8)|
aAll assignments are based on 1D and 2D measurements (HMBC, HSQC, COESY). bImplied multiplicities were determined by DEPT (C = s, CH = d, CH2 = t). cJ in Hz. Compound 2 was measured in CDCl3, while compound 1 was measured in MeOH-d.
2.5 Antimicrobial activity and toxicity
2.5.1 Tested microbes:
The tested bacteria Acinetobacter baumannii, Escherichia coli, Klebsiella pneumoniae, P. aeruginosa, Staphylococcus aureus, and Streptococcus pneumoniaewere obtained from King Fahd General Hospital. The tested fungi were Aspergillus niger, Candida albicans ATCC10231, Candida tropicalis, Cryptococcus neoformans, Epidermophytonsp., Microsporum gypseum, Trichophyton mentagrophytes and Trichophyton rubrum were from the culture collection of Dr. Roger Bonaly, Biochimie Microbienne, Faculté de Pharmacie, Nancy, France.
The sensitivity of the pathogenic bacteria and fungi to compounds 1 and 2 was tested using the agar well diffusion method . The two compounds were dissolved in DMSO, which was also employed as negative control, and 100 μL were used to fill each well. Minimal inhibitory concentration (MIC) was determined using the microdilution method . Toxicity of 1 and 2 was determined using the brine shrimp lethality test . The antitumor activity against Ehrlich carcinoma and lymphoma cell lines was determined and the percentage of cell viability (LD50) was calculated . Each reading is the mean value of three replicates ± SD.
3 Results and conclusion
In continuation of our research program on compounds from marine organisms, the sponge material was collected from Saudi territorial waters. Extensive fractionation of the organic extract employing NP-Silica gel, preparative thin layer chromatography (TLC) and high performance liquid chromatography (HPLC) yielded four metabolites (1–4). Two were new acetylenic derivatives: an acylglycerolipid,2′-O-(7E,17E-18-bromo-octaoctadeca-7,17-dien-5,15-diynoyl)-3′-O-(9E,13E,17E)-18-bromo-octaoctadeca-9,13,17-dien-7,15-diynoyl-1′-O-β-D-galactopyranosyl glycerol (1), and xestospongiamide (2). The other two metabolites had been previously identified: known 26,27-Dimethylergosta-5,24(28)-dien-3β-ol (xestosterol) (3) and the xestosterol ester of 18-bromooctadeca-(9E,17E)-diene- 7,15-diynoic acid (4; Figure 1).
Compound 1 was isolated as a white powder. The molecular formula, C45H58O10Br2, was determined by HRESIMS at m/z 916.2384 [M]+. Liquid chromatography mass spectrometry (LCMS; positive-ion-mode) at m/z 916 [M]+ indicated 16 degrees of unsaturation. The 1:2:1 isotopic distribution at m/z 916, 918 and 920 indicated the presence of two bromine atoms. Moreover, the peak at m/z 179 could be assigned to a hexose sugar unit (Figure 2). The 13C NMR spectrum indicated the presence of 45 signals, categorized by an DEPT NMR experiment into two carbonyl δC (174.3 and 175.0 ppm), eight quaternary (93.7, 91.3, 89.1, 88.5, 85.6, 80.8, 80.3, and 78.3), 10 olefinic (145.7, 143.5, 142.8, 112.0, 119.1, 118.9, 118.7, 118.0, 111.6 and 111.1), 16 methylene (34.8, 34.0, 33.5, 33.3, 33.0, 29.7, 29.4, 29.1, 28.9, 28.9, 28.9, 25.5, 25.2, 19.8, 19.8 and 19.3), an acetal (104.7) and three oxygenated methylene (68.7, 63.9 and 62.7) carbons, respectively. The 1H NMR spectrum comprised signals due to a sugar structure, such as oxymethines (δH 4.28, 3.33, 3.32, 3.27 and 3.17), and 13C NMR revealed the signal of a typical acetal carbon δC 104.7. 13C, 1H and H-H COESY NMR spectral data indicated the presence of a glycerol moiety, based on the assignment of the three vicinal oxygenated carbons δC 68.7, 71.6 and 63.9. The downfield chemical shift indicated a substituted glycerol. Two esterified carbonyl functions (δC 174.3 and 175.0) and the IR absorption band at 1736 cm-1 assigned a diglyceride skeleton. The stereochemistry of the sugar moiety was revealed to be that of a glucose derivative based on the large coupling constants (9 Hz) between H2-H3, H3-H4, H4-H5. The β-anomeric structure was assigned from the coupling constant (J1–2 = 7.8 Hz) and NOESY correlations of H1/H3 and H1/H5. Hence, 1 is a glucopyranosyl diacylglycerol. The acid hydrolysis and methanolysis method of Hirsh et al.  yielded two UV254 absorbing substances after purification employing column and PTL chromatography (cf. exp.). The two substances were assigned as R1 and R2 methyl esters (R1 and R2, respectively).
R1 was obtained as yellow oil with a molecular formula of C19H25O2Br, as established by high resolution fast-atom bombardment mass spectrometry (HRFABMS).
Gas chromatography-mass spectrometry (GCMS) revealed the presence of one bromide atom (two mass unit isotopic peaks in 1:1 ratio). Combination of 13C NMR spectrometry with a heteronuclear single quantum correlation (HSQC) experiment revealed the presence of four singlet acetylenic carbon atoms in R1, four protonated olefinic carbon atoms, along with signals for nine paraffinic methylene carbon atoms (Table 1).
The 1H NMR spectrum featured the following: a brominated vinyl group [δH 6.23 (dt, 13.8, 1.8 Hz, H-17″ and 6.69 (d, 13.8, Hz, H-18″)] and a trans-disubstituted double bond [δH 5.46 (dt, 15.6, 1.8 Hz, H-7″ and 5.98 (dt, 15.6, 6.6 Hz, H-8″)], i.e. H-7″. The COESY and HSQC experiments allowed the assignment of the subunits C-2″/ C-4″, C-7″/C-8″ and C-17″/C-18″ within R1, which were substantiated by heteronuclear multiple bond correlation (HMBC) data. HMBC correlations of H2-4″/C-5″, H-7″/C-5″, and H-17″/C-16 established the unambiguous placement of the conjugated diyne moieties at C-8″/C-5″ and C-15″/ C-18″. The structure elucidation of R1 based on NMR data was further confirmed by mass spectral analysis. Electron impact mass spectrometry (IMS) data of R1 exhibited several conspicuous fragments containing linear alkyl chains . Fragments observed at m/z 263, 199, 129, 101, and 64 supported the structure assigned for R1 (Figure 2). Accordingly, the moiety R1 was assigned the structure 7E, 17E-18-bromo-octaoctadeca-7,17-dien-5,15-diynoyl.
R2 was obtained as yellow oil with a molecular formula of C19H23O2Br, as established by HRFABMS. GCMS revealed the presence of one bromide atom (two mass units’ isotopic peaks in 1:1 ratio). By a similar treatment of both NMR and EIMS data, the connectivities, the geometry, and positions of double and triple bonds were indicated (Table 1 and Figure 2). Accordingly, the moiety R2 was assigned the structure 9E,13E,17E-18-bromo-octaoctadeca-9,13,17-dien-7,15-diynoyl. Taking into consideration that R1 and R2 may be interchangeable, compound 1 is a new acylglycerolipid; 2′-O-(7E,17E-18-bromo-octaoctadeca-7,17-dien-5,15-diynoyl)-3′-O-(9E,13E,17E)-18-bromo-octaoctadeca-9,13,17-dien-7,15-diynoyl-1′-O-β-D-glucopyranosyl glycerol (Figure 2).
Compound 2 was isolated as pale yellow oil. The molecular formula, C20H28BrClN2O, was deduced by LCMS at m/z 426:428:430 (2:3:1) [M]+, indicating that 2 has two halogen atoms (one Br and one Cl), with seven degrees of unsaturation. The infrared (IR) spectrum showed the presence of a carbon–carbon triple bond (2215 cm–1), a carbonyl ester group C=O (1741 cm-1), amide and amine function bands (3120–3434 cm–1). The 13C and DEPT NMR measurements indicated 20 signals, categorized into 1) a quaternary carbonyl δC (173.1 ppm), 2) four quaternary acetylenic carbons (92.7, 88.5, 79.2 and 77.5), 3) four olefinic methine, (142.6, 118.0, 117.1 and 110.2), 4) a methine (68.9), 5) a halogenated methylene (62.1), and 6) nine methylene δc (33.9, 32.3, 28.4, 28.3, 27.9, 27.7, 24.4, 19.2 and 19.1) carbons, respectivekly (Table 1). So, compound 2 must be acyclic.
The 1H and 13C NMR together with 1H-1H COSY spectral data indicated a disubstituted carbon-carbon double bond [δH 6.17 ppm (dt, 13.8, 1.8 Hz, H-17; δC 118.0, C-17) and 6.57 (d, 13.8 Hz, H-18; δC 117.1, C-18)], which was coupled on one side only to a two proton signal at δH 2.25 (td, 6.6, 1.8 Hz, H-14). The small coupling (1.8 Hz) originated from a long range effect between H-17 and H-14 through the acetylenic group. The large coupling constants between H-17 and H-18 (13.8 Hz) indicated the E geometry of the olefinic moiety. Investigation of the HMBC spectral data indicated correlations between H-18 and C-17, C-16 and C-15 and between H-17 and C-18 and C-15 (Figure 3). The presence of an ion peak at m/z 129[C4H2Br]+ in the LCMS supported this fragment (Figure 4), led to establishing fragment 1 (Figure 4).
The 1H-1H COSY spectrum showed that chlorinated methylene protons (H2-2′) at δH 4.28 (dd, 12.6, 4.2 Hz) and 4.13 (dd, 12.6, 6.6 Hz), are coupled to a low field methine proton H-1′ at δH 5.26 (dd, 6.6, 4.2 Hz) of an ABX system. The attachment of a primary amine and amide to C-1′ shifted the absorption of H-1′ to a more down field value (δH 5.26). Further investigation of the HMBC spectrum indicated a correlation between H-1′ and C-2′ with the amidic carbonyl between H-2′ and C-1′ and C-1 (Figure 3) establishing fragment 3 (Figure 4).
With the aid of the data in Table 1 and Figure 4, the structures of fragments 2, 4, and 5 can be deduced. The connections between the fragments were performed through interpretation of 1H-1H COSY and HMBC spectral data. A computer survey of different data bases, including SciFinder, indicates that 2 is a new xestospongia amide. Compounds 3 and 4 were identified as xestosterol and xestosterol (9E,17E)-18-bromooctadeca-9,17-dien-7,15-diynoic acid, respectively, by comparing their spectroscopic data with those in the literature [15, 16]. The antimicrobial activities of 1 and 2 were determined against some multidrug-resistant bacteria using an agar well diffusion assay. DMSO was used as negative control while ampicillin and amphotericin B were used as positive antibacterial and antifungal agents, respectively. The mean diameter of the inhibition zones was in the range of 12–23 mm (Table 2). The antifungal activity was recorded against the dermatophytes A. niger,C. albicans ATCC1023, C. tropicalis, C. neoformans. Product 1 showed no antifungal activity, while compound 2 exhibited excellent antifungal activity against A. niger, C. albicans, C. tropicalis, C. neoformans, Epidermophyton sp., M. gypseum, T. mentagrophytes, and T. rubrum (diameters of inhibition zones ranged from 14–22 mm). MICs were calculated for each product and ranged from 2.2 to 4.5 μM for A. baumannii, K. pneumonia and P. aeruginosa; those for C. albicans, A. niger, and Epidermophyton sp. were in the range of 2.2–2.5 μM (Table 3). Compounds 1 and 2 were not cytotoxic against Artemia salina, while compound 2 exhibited antitumor activity against Ehrlich ascites carcinoma and lymphocytic leukemia at LD50 5 μM (Table 4).
|Tested bacteria||Diameter of inhibition zonea±SD (mm)||Tested fungi||Diameter of inhibition zone±SD (mm)|
|Acinetobacter baumannii||14±3.2||15±3.2||NF||Candida albicans||NF||24±1.2||14±1.2|
|Escherichia coli||23±1. 7||12±3.2||9±1.2||Candida tropicalis||NF||17.4±1.7||NF|
|Klebsiella pneumoniae||15±1.2||19±3.2||7±2.2||Cryptococcus neoformans||NF||15.8±0.7||9±2.1|
|Pseudomonas aeruginosa||24±2.2||19±3.2||9±3.0||Aspergillus niger||NF||13±2.1||13±1.1|
|Staphylococcus aureus||12±1.4||13±1.8||19±1.4||Epidermophyton sp.||NF||14.0±1.8||10±1.2|
|S. epidermidis||16±1.4||14±3.2||NF||Trichophyton rubrum||NF||16.0±1.0||9±1.7|
|Streptococcus pneumoniae||13±1.5||14±3.2||10±1.2||Trichophyton mentagrophytes||NF||19.4±1. 4||11±2.7|
aMean diameter of inhibition ± standard deviation; NF, no inhibition zone; Amph. B, amphotericin B; MRSA, methicillin-resistant Staphylococcus aureus.
|Tested bacteria||MIC (μM)||Tested fungi||MIC (μM)|
|A. baumannii||4.2||2.5||≥2.6||C. albicans||2.2||≥5||0.9|
|K. pneumoniae||4.5||2.5||2.6||A. niger||2.5||≥5||1.1|
|P. aeruginosa||≥5||≥5||2.6||Epidermophyton sp.||2.5||≥5||0.9|
|Toxicity against Artemia salina (LD50, μM)||≥5||≥5||≥5|
|Antitumor (LD50, μM)|
|Ehrlich Ascites carcinoma||≥5||5.0||0.02|
In conclusion, the results of this study confirm that product 2, obtained from the Xestospongia extract, can be considered a potential material for designing effective antibacterial, antifungal and antitumor agents for treatment of multidrug resistant bacteria, tumors, dermatophytosis, and other fungal infections with no cell toxicity.
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