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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
J Nat Prod. Author manuscript; available in PMC 2010 May 1.
Published in final edited form as:
PMCID: PMC2765494
NIHMSID: NIHMS111724

Antimicrobial Ambiguine Isonitriles from the Cyanobacterium Fischerella ambigua

Abstract

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Five new antibacterial ambiguine K-O isonitriles (1-5) and nine previously described indole alkaloids were isolated from the cultured cyanobacterium Fischerella ambigua (UTEX 1903) by bioassay-guided fractionation. The planar structures of the new compounds were determined by spectroscopic analysis including MS, 1D- and 2D-NMR. X-ray crystallography was used to determine the absolute stereo-configuration of ambiguine K isonitrile. The isolates were evaluated for their antibacterial activities against a set of bacterial targets, including Mycobacterium tuberculosis and Bacillus anthracis. Ambiguine K and M isonitriles showed the most potent activity against M. tuberculosis with MIC values of 6.6 μM and 7.5 μM, respectively. Ambiguine A isonitrile showed the most potent activity against B. anthracis with a MIC of 1.0 μM.

Cyanobacteria (blue-green algae) are known to be a rich source of secondary metabolites with diverse chemical structures and biological activities.1-6 To date, over 1000 natural products of cyanobacterial origin have been reported.1-6 These include many biologically active compounds with anticancer, antimicrobial, antiviral, and immunosuppressive activities.7,8 Branched filamentous cyanobacteria belonging to the order Stigonematales are known to produce antibacterial, antifungal and antialgal isonitrile containing indole alkaloids, such as hapalindoles,9-14 ambiguine isonitriles,9,15,16 fischerindoles,17 and wetwitindolinones.18,19 These alkaloids all have tetra- or pentacyclic carbon skeletons derived from tryptophan and geranyl diphosphate.14,18 The ambiguine isonitriles contain an additional 1,1-dimethylallyl moiety at the 2 position of the indole ring, which often undergoes further cyclization to form the seven-membered ring observed in many ambiguine isonitriles.20 Their promising biological activities and complex chemical structures have led to several synthetic efforts towards this class of alkaloids since Moore et al. originally discovered them from cyanobacteria.20-31

In our preliminary studies the crude extract of Fischerella ambigua (Nageli) Gomont (UTEX 1903) showed significant antibacterial activity against Mycobacterium tuberculosis and Bacillus anthracis. Earlier investigations of F. ambigua (UTEX 1903), by Moore et al. led to the isolation of several ambiguine isonitriles and hapalindoles.15 Initial LC-MS dereplication indicated the presence of a number of previously described ambiguine isonitriles in the extract.32 In addition, the dereplication indicated the presence of several potentially novel ambiguine isonitriles. Bioassay-guided fractionation led to the isolation of fourteen antibacterial alkaloids including five new ambiguine isonitrile derivatives as well as nine previously described indole alkaloids.

Results and discussion

F. ambigua was mass cultured in Allen medium.33 The freeze-dried biomass was extracted with CH2Cl2:MeOH (1:1) to yield a crude extract which showed significant antibacterial activity. Five new ambiguine isonitriles (1-5), and eight previously reported alkaloids, ambiguine A-C, E, F, and I isonitriles, hapalindoles G and H, were isolated using silica gel, Sephadex LH-20, and reversed phase chromatography. The chemical structures were identified by analysis of 1D and 2D NMR, MS, and X-ray data.

Ambiguine K isonitrile (1) was obtained as colorless needles (MeOH). The UV spectrum indicated the presence of an indole moiety [λmax (log ε) 230 (2.72), 275 (2.11)]. The presence of an isonitrile moiety was evident from both 13C NMR (δ 158.2) and IR (νmax 2125 cm-1) data. The ESI mass spectrum of 1 exhibited a 3:1 ion cluster at m/z 443/445 [M+Na]+, indicating the presence of one chlorine atom. The HRMS gave a quasi-molecular ion [M-H]- at m/z 419.18979 for the molecular formula of C26H29ClN2O. Various features of the 1H (Table 1) and 13C NMR (Table 2) chemical shift data suggested that 1 was an ambiguine isonitrile derivative.

Table 1
1H NMR data of compounds 1-5 (900 MHz) δH, mult., (J in Hz)
Table 2
13C NMR data of compounds1-5 (226 MHz) δC, mult.

Analysis of the 1H, COSY (Figure 1), and HSQC spectra demonstrated the presence of a 1,2,3-trisubstituted aromatic moiety with signals at δH 6.98 (H-5), δH 7.07 (H-6), δH 7.09 (H-7), vinyl group signals at δH 6.06 (H-20), δH 5.43 (H-21E), δH 5.36 (H-21Z), and double bond signals at δH 5.73 (H-25) and δH 5.69 (H-26) as well as an isolated spin system consisting of three carbons: a methine at δH 4.47 (H-13), a methylene at δH 2.53 (H-14ax) and δH 2.29 (H-14eq), and a methine at δH 2.44 (H-15). In addition five methyl groups could be observed in the 1H NMR spectrum (δH 1.57, H-28; 1.58, H-27; 1.62, H-19; 1.38, H-18; 1.51, H-17).

Figure 1
Key HMBC and COSY correlations of ambiguine K isonitrile (1)

The 13C NMR spectrum displayed all 26 carbon resonances implied by the molecular formula. Thirteen of these signals were observed in the region from 100-160 ppm, eight of which were assigned to the indole moiety (δC 138.8, 111.0, 140.6, 114.2, 123.2, 108.3, 135.3, and 126.0). Another four carbons were attributed to the vinyl group (δC 143.2 and 118.7) and the double bond (δC 143.4 and 125.6), while the final deshielded resonance (δC 158.2) was assigned to the isonitrile moiety.15 Of the thirteen degrees of unsaturation required by the molecular formula, ten were accounted for by the indole ring, two double bonds, and the isonitrile moiety. Hence ambiguine K isonitrile was deduced to possess 3 additional rings.

The planar structure of ambiguine K isonitrile was established by analysis of the correlations observed in the HMBC spectrum (Figure 1). The connectivity within the indole moiety was established by the correlations from H-5 to C-9, H-6 to C-4 and C-8, as well as H-7 to C-9. The gem-dimethyl group at C-16 was connected to C-4 based upon correlations observed from H3-17 and H3-18 to both C-4 and C-16. Further correlations from H3-17 and H3-18 to C-15 connected this portion to the H-15-H-13 fragment COSY coupled spin system. Three bond correlations from both H3-19 and H-20 to C-11 and C-13 as well as from H-20 to C-12 revealed the position of the vinyl and C-19 methyl groups on C-12 and connected this fragment to C-13. In addition, correlations from H-26 to C-10, C-11, and C-12 as well as from H-25 to C-11 established a linkage of the Δ25 double bond to C-11. This double bond was in turn connected to C-2 of the indole via a second gem-dimethyl group as deduced by the correlations from H-25, H3-27, and H3-28 to C-2 and C-24.

Substitution with chlorine at position 13 was deduced by comparison of the chemical shift of H-13 (δH 4.47) and C-13 (δC 66.4) with those published for other ambiguine isonitriles.15,16 Similarly, placement of the isonitrile group at C-11 (δC 71.7) and hydroxyl group at C-10 (δC 76.5) was based upon chemical shift comparison with published data.15,16 Although no correlations were observed to C-3 of the indole moiety in the HMBC spectrum, the connection between carbons C-10 and C-3 was established by considering the thus completed structure and the thirteen degrees of unsaturation required by the molecular formula, which necessitated a pentacyclic system.

The coupling constants between H-14ax and H-13 (J= 12.5 Hz), as well as H-14ax and H-15 (J= 13.1 Hz) suggested that both H-13 and H-15 were in the axial orientation. In addition, correlations observed in the 2D NOESY spectrum between H-15 and H-13, and H-13 and H-20 indicated all to be in the same plane. This was further confirmed by the correlation observed between H-14ax and H3-19. Together these observations established the relative configuration at positions 12, 13, and 15 as 12R*, 13R*, and 15R*. The configuration of the remaining two stereo-centers (positions 10 and 11) was determined by X-ray crystallography, which also confirmed the assigned structure. The absolute configuration of 1 was determined on the basis of the Flack parameter value,34 -.00(3), as 10R, 11S, 12R, 13R, and 15R. The ORTEP diagram is shown in Figure 2.

Figure 2
The ORTEP diagram of ambiguine K isonitrile (1)

Ambiguine L isonitrile (2) was obtained as a white amorphous powder. Again, the UV spectrum indicated the presence of an indole moiety [λmax (log ε) 223 (3.47), 271 (2.91)] and the presence of an isonitrile moiety was evident from both 13C NMR (δC 157.2) and IR (νmax 2127 cm-1) data. The HRMS peak at m/z 387.24347 [M+H]+ suggested a molecular formula of C26H30N2O for 2 and the absence of chlorine in the structure. Comparison of 1H and 13C NMR spectra for 2 with those of 1 (Table (Table11 and and2)2) showed that they both where closely related ambiguine isonitrile compounds. However, in the 1H NMR spectrum of 2, the resonance observed in 1 for the chlorinated methine group at δH 4.47 (H-13) was replaced by two signals at δH 1.52 (H-13eq) and δH 1.84 (H-13ax). The chlorinated C-13 methine resonance (δC 66.4) found in the spectrum for compound 1 was also absent in the DEPT spectrum for 2, while an additional methylene carbon at δC 35.4 (C-13) was observed. Together these observations indicated that 2 lacked the chloro-substitution at position 13 observed in 1. This conclusion was confirmed by analysis of COSY and HSQC data, which showed the presence of a CH2CH2CH spin system from C-13 to C-15. Based on all of the above information, as well as analysis of HMBC correlations, the planar structure of 2 was established as the deschloro derivative of ambiguine K isonitrile.

The relative configuration of 2 was determined by comparison of chemical shifts and coupling constants with those of 1, as well as correlations observed in the 2D NOESY spectrum. Similar to 1, the coupling constant between H-14ax and H-15 (J= 13.0 Hz) indicated that H-15 was in the axial orientation, while the 2D NOESY correlations observed between H-13ax,H-20 and H-15 suggested that all of these protons were in the same plane. This suggested a relative configuration of 12R* and 15R*. The relative configuration at C-10 (δC 75.8) and C-11 (δC 69.7) was assigned as 10R*, 11S* by comparison of chemical shifts with those of 1C-10 76.5 and δC-11 71.7). Thus, ambiguine L isonitrile has the relative configuration of 10R*, 11S*, 12R*, and 15R*. This represents the same configuration as observed in ambiguine K isonitrile (1) at these positions and suggests the identical absolute configuration for ambiguine L isonitrile.

Ambiguine M isonitrile (3) was obtained as a white amorphous solid. Again, the UV spectrum indicated the presence of an indole moiety [λmax (log ε) 221 (2.64), 278 (1.92)] and the presence of an isonitrile moiety was evident from both 13C NMR (δC 160.2) and IR (νmax 2130 cm-1) data. The ESI mass spectrum of 3 exhibited a 3:1 ion cluster at m/z 439/441 [M+H]+, indicating the presence of a chlorine. The HRMS peak at m/z 439.2143 [M+H]+ coupled with NMR information established the molecular formula as C26H31ClN2O2. 1H and 13C NMR spectra (Tables (Tables11 and and2)2) of compound 3 showed signals similar to those found in ambiguine K isonitrile (1). The major difference was the absence of the Δ25double bond, which had been replaced by a -CHCH2-spin system (CH-25 and CH2-26). The chemical shift of H-25 (δH 3.97) and C-25 (δC 74.2) and consideration of the molecular formula placed a hydroxyl group at C-25. The linkage of the C-25-C-26 fragment to C-11 was determined by HMBC correlations from H-25 as well as H-26ax and H-26eq to C-10, C-11, and C-12. Similarly, the correlations from H-25, H3-27, and H3-28 to C-2 and C-24 established the connection of this moiety to C-2 of the indole via a gem-dimethyl group.

The relative configuration of the molecule was determined analogously to 1. The coupling constant between H-14ax and H-13 (J= 12.6 Hz), as well as H-14ax and H-15 (J= 12.8 Hz) suggested that both H-13 and H-15 were in the axial orientation. This was confirmed by the 2D NOESY correlations observed between H-13 and both H-15 and H-20, indicating all to be in the same plane. This established the relative configuration as 12R*, 13R*, and 15R*. The chemical shift of C-10 (δC 75.4) and C-11 (δC 74.4) were similar to those observed in ambiguine K isonitrile (see Table 2), indicating both compounds to have identical configuration at these positions. In addition, correlations observed in the 2D NOESY spectrum between H-26ax, H3-27 and H3-19 suggested that they were in the same plane (the signals for H3-19 and H3-17 overlap at δH 1.54, but the observed correlation was assigned to H3-19 due to the considerable distance between H-26ax and H3-17). The coupling constant for H-26ax and H-25 (J=10.9 Hz) indicated that H-25 was anti to H-26ax, in agreement with the NOESY correlations observed between H-25 and H3-28. Therefore, the relative configuration of C-25 was assigned as R*. Thus, the relative configuration of ambiguine M isonitrile (3) was determined as 10R*, 11S*, 12R*, 13R*, 15R*, and 25R*. This is the same configuration as observed in ambiguine K isonitrile (1) at positions 10, 11, 12, 13, and 15. The shared biosynthetic pathway for 1 and 3 suggests the absolute configuration for ambiguine M isonitrile to be 10R, 11S, 12R, 13R, 15R, and 25R.

Ambiguine N isonitrile (4) was obtained as a white amorphous solid. The UV spectrum indicated the presence of an indole moiety [λmax (log ε) 229 (3.53), 270 (3.48)], while IR (νmax 2142 cm-1) and 13C NMR (δC 157.8) data showed the presence of an isonitrile moiety. The molecular formula was determined as C26H32N2O2 by HRMS (m/z 405.2534 [M+H]+) and lacked chlorine in the structure. The 1H and 13C spectra (see Tables Tables11 and and2)2) for 4 indicated that it was the deschloro derivative of ambiguine M isonitrile (3). Comparison of 1H NMR spectra showed that the signal at δH 4.51 (H-13) found in 3 had disappeared and was replaced by two signals at δH 1.55 (H-13eq) and δH 2.02 (H-13ax) in 4. Similarly in the 13C NMR spectra, the methine C-13 resonance for 3C 67.2) had been replaced by a methylene carbon at δC 37.6 for 4. The COSY and HSQC data also showed the presence of a CH2CH2CH spin system from C-13 to C-15.

The relative configuration of 4 was determined by comparison of chemical shifts and coupling constants to those of 1 and 3 (Tables (Tables11 and and2).2). The coupling constant between H-14ax and H-15 (J= 12.5 Hz) indicated H-15 was in the axial orientation. This was confirmed by inspection of the 2D NOESY experiment, where correlations were observed between H-15, H3-17 and H-13ax, as well as between H3-19, H-13eq and H-14ax, indicating H-15 and H3-19 to be in the opposite plane. The chemical shift of C-10 (δC 75.8) and C-11 (δC 73.5) were similar to those observed in ambiguine K isonitrile (see Table 2), indicating a 10R*, 11S* configuration. Similar to ambiguine M isonitrile, correlations were observed in the 2D NOESY spectrum between H-26ax, H3-19 and H3-27, suggesting that they were in the same plane. The coupling constant for H-26ax and H-25 (J=10.9 Hz) indicated that H-25 was anti to H-26ax. Thus, the relative configuration of ambiguine N isonitrile (4) was determined as 10R*, 11S*, 12R*, 15R*, and 25R*. Again, this is the same configuration as observed in ambiguine K isonitrile (1) at positions 10, 11, 12, and 15 and suggests the absolute configuration for ambiguine N isonitrile to be 10R, 11S, 12R, 15R, and 25R.

Ambiguine O isonitrile (5) exhibited a 3:1 ion cluster at m/z 451/453 [M-H]- in the ESI MS. HRMS (m/z 451.1802 [M-H]-) determined the molecular formula as C26H29ClN2O3. The UV spectrum indicated the presence of an indole moiety [λmax (log ε) 220 (3.44), 277 (2.83)]. 1H and 13C NMR spectra showed signals indicative of an ambiguine isonitrile derivative (see Tables Tables11 and and2).2). The major differences as compared to ambiguine K-N isonitriles were observed in the seven-membered ring portion of the molecule. In particular, a new signal at δH 4.94 (H-26) appeared in 1H spectrum for compound 5, and was ascribed to a hydroxylated methine (δC 85.5, C-26). In addition, the 13C NMR spectrum indicated the presence of a ketone moiety (δC 209.3, C-25). HMBC correlations from H-26 to C-24 and C-25, and from H3-27 and H3-28 to C-24 and C-25 suggested the presence of a 1-hydroxy-3,3-dimethyl-2-propanone moiety as part of the seven membered ring system (Figure 3). The HMBC correlations from H-26 to C-12, C-11, and C-10 established the C-26 to C-11 connection, while the correlations from H3-27 and H3-28 to C-2 determined the connection of C-24 to C-2 of the indole moiety. Although no signal could be observed for the C-23 isonitrile in the 13C NMR spectrum, this substituent at C-11 was determined by a characteristic IR peak (νmax 2123 cm-1), the molecular formula, and comparison of the chemical shift of C-11 (δ 69.3) with other ambiguine isonitrile compounds reported in this paper.

Figure 3
Structure and key HMBC correlations of the 1-hydroxy-3, 3-dimethyl-2-propanone moiety

Analysis of the coupling constants and correlations in the 2D NOESY spectrum established the relative configuration of 5. The coupling constant between H-14ax and H-13 (J= 12.9 Hz), as well as H-14ax and H-15 (J= 12.9 Hz) suggested that both H-13 and H-15 were axial. This was confirmed by the correlations observed in the 2D NOESY spectrum between H-13, H-15and H3-20 , which indicated that H-13, H-15and H-20 were all in the same plane. The chemical shifts of C-10 (δC 77.8) and C-11 (δC 69.3) were similar to those observed for ambiguine K isonitrile (see Table 2), indicating that both compounds have identical at these positions. In addition, correlations observed in the 2D NOESY spectrum between H-26 and H3-19, H-20 and H21Z indicated these protons to be spatially close (integrated volumes suggested that H-26 was almost equidistant to all three protons). This requires H-26 to be in pseudoaxial position of the cycloheptene ring and the configuration of C-26 was assigned as S*. In this configuration, OH group at C-26 is in pseudoequatorial orientation. minimizing steric interactions. In the opposite configuration (26R*), multiple 1,3-diaxial interactions would be present, which would result in a ring twist thus placing H-26 farther away from H3-19. The 26S*configuration is also supported by the lack of NOE correlation between H-26 and H3-27 (or H3-28). In the 26S*configuration, the interproton distance from H-26 to H3-27 (or H3-28) is increased due to the flattening of the C3-C24-C25 plane, similar as was observed for 3,4-benzocycloheptanone by Bodennes and St.-Jacques.37 Thus, the relative configuration of ambiguine O isonitrile was determined as 10R*, 11R*, 12R*, 13R*, 15R*, and 26S*.

All isolates were evaluated for inhibition of growth of Mycobacterium tuberculosis and Bacillus anthracis as well as Vero cell toxicity. In addition, the isolates were also evaluated against a set of microorganisms, Staphylococcus aureus, Mycobacterium smegmatis, and Candida albicans, to determine the spectrum of activity (Table 3). Several isolates were active against M. tuberculosis, the most active compound was ambiguine K isonitrile (MIC 6.6 μM), while ambiguine A isonitrile showed the most potent activity against B. anthracis with a MIC of 1.0 μM . Most ambiguine isonitriles showed moderate toxicity in the Vero cell assay (IC50 ranging from 26.0 to >128μM). Interestingly, ambiguine I isonitrile (MIC 11.3 μM) and hapalindole G (MIC 6.8 μM) displayed over twenty-and fifty-fold higher inhibition against M. tuberculosis than B. anthracis, respectively, with no detectable cytotoxicity in a Vero cell assay. Most of the isolates also possessed strong antifungal activities, similar to levels previously reported for other ambiguine isonitriles and hapalindoles.16,35

Table 3
MIC and IC50 values of compounds from Fischerella ambigua against test organisms in vitro

The co-occurrence of non-chlorinated and chlorinated ambiguine isonitriles in cyanobacteria of the order Stigonematales has been suggested to be due to some imperfection in the biosynthesis and the resulting arrays of compounds have been proposed to give an ecological advantage.16 In our study, we observed no distinct difference in the level of antimicrobial activity of non-chlorinated vs chlorinated ambiguine isonitriles, except for B. anthracis, where the chlorinated ambiguine isonitriles seem to possess somewhat lower MIC values than their non-chlorinated partner (i.e E vs I, K vs L, and M vs N).

Moore et al. have proposed that hapalindoles are biosynthesized by condensation of (Z)-β-ocimene and 3-((Z)-2′-isocyanoethenyl)indole.10 Ambiguine isonitriles have an additional isoprene unit attached to C-2 of the indole moiety. This isoprene moiety is often cyclized to form an additional seven-membered ring. In this study we observed substantial flexibility in the degree of oxidation of this seven-membered ring portion. Ambiguine K (1) and L (2) isonitriles, both possessing a C25-C26 double bond, can be considered the precursors for ambiguine M (3), N (4), and O (5) isonitriles as well as the previously reported ambiguine isonitriles containing a seven-membered ring. We only observed ambiguine D isonitrile in trace amounts (detected by LC-MS, data not shown) in this study. In contrast, ambiguine D isonitrile was found to be the major ambiguine isonitrile in this strain by Moore et al.15 We also did not observe ambiguine J isonitrile, the deschloro derivative of ambiguine D isonitrile recently found by Raveh and Carmeli 16 in a Fischerella sp. from Israel.

Experimental section

General experimental procedures

Optical rotations were determined on a Perkin-Elmer 241 polarimeter. UV spectra were obtained on an Agilent 1100 HPLC system with a diode array detector. IR spectra were obtained on a Jasco FTIR-410 Fourier transform infrared spectrometer. 1H and 13C NMR spectra were obtained on a Bruker Avance DRX600 MHz NMR spectrometer with 5mm CPTXI Z-gradient probe and a Bruker AVII900 MHz NMR spectrometer with 5mm ATM CPTCI Z-gradient probe, referenced to the corresponding solvent peaks. Low resolution ESI mass spectra were obtained on a ThermoFinnigan TSQuantum Triple Quadrupole Mass Spectrometer and an Agilent 1946A LC-MSD single quadrupole LC-mass spectrometer. High resolution ESI mass spectra were obtained on a Thermo Electron LTQ FT ICR mass spectrometer.

Biological Material

Fischerella ambigua was initially acquired from the Culture Collection of Algae at the University of Texas at Austin (UTEX 1903). The cyanobacterium was grown in a 2.8 L Fernbach flask containing 1 L of inorganic media (Allen media).33 Cultures were illuminated with fluorescent lamps at 1.93 klx with an 18/6 hour light/dark cycle. The temperature of culture room was maintained at 22° C. After 6-8 weeks, the biomass of cyanobacteria was harvested by centrifugation and then freeze-dried.

Extraction and isolation

The freeze-dried biomass (2.14 g) from a total of 9 L of culture was extracted by repeated maceration with CH2Cl2:MeOH (1:1) to yield 691.1 mg of crude extract. The crude extract showed potent inhibitory activity against the TB pathogen Mycobacterium tuberculosis (MIC 2.7 μg/mL). A portion of crude extract (682 mg) was fractionated on silica gel using a gradient with increasing amount of MeOH in CH2Cl2 to afford 18 fractions. The TB active fractions 2 (MIC 1.5 μg/mL, 247.7 mg), 7 (MIC 2.7 μg/mL, 11.5 mg), and 8 (MIC 8.3 μg/mL, 9.0 mg) were fractionated as follows. Fraction 2 was further separated using Sephadex LH-20 with MeOH as an eluent to obtain nine fractions. Fraction 2-8 (160.2 mg) was subjected to reversed-phase HPLC (Alltima C18, 10 m, 250×10 mm, 4 mL/min) with a solvent gradient of MeOH-H2O (85:15) to 100% MeOH over 20 minutes to afford ambiguine A isonitrile (tR = 12.8 min, 5.8 mg), ambiguine B isonitrile (tR = 9.8 min, 4.5 mg) and ambiguine E isonitrile (tR = 12.5 min, 13.5 mg), hapalindole H (tR = 11.1 min, 14.0 mg), and a peak containing a mixture (tR = 8.8 min). The mixture was further purified by RP-HPLC (Alltima C18, 10 μm, 250×10 mm, 4 mL/min) eluting with MeCN:H2O (75:25) to afford 1 (tR = 12.9 min, 2.8 mg) and 2 (tR = 13.7 min, 2.2 mg), as well as hapalindole G (tR = 11.2 min, 2.9 mg). Fractionation of Fr.2-7 using RP-HPLC (Alltima C18, 10 μm, 250×10 mm, 2 mL/min) and a solvent gradient of MeOH-H2O (60:40) to 100% MeOH over 20 minutes led to two subfractions. Further chromatography of the first subfraction using RP-HPLC (Alltima C8, 5 μm, 250×10 mm, 3 mL/min) and a solvent gradient of MeOH-H2O (80:20) to MeOH-H2O (90:10) over 15 minutes afforded ambiguine B isonitrile (tR = 13.2 min, 0.7 mg) and ambiguine C isonitrile (tR = 12.7 min, 0.8 mg). The second subfraction was further fractionated using RP-HPLC (Alltima C18, 10 μm, 250×10 mm, 3 mL/min) and a solvent gradient of MeOH-H2O (83:17) to MeOH-H2O (90:10) over 20 minutes to give ambiguine A isonitrile (tR = 18.6 min, 0.7 mg) and ambiguine I isonitrile (tR = 16.7 min, 0.9 mg). The biologically active silica gel fraction 7 was fractionated using RP-HPLC (Alltima C18, 10 μm, 250×10 mm, 4 mL/min) and a solvent gradient of MeOH-H2O (75:25) to 100% MeOH over 20 minutes to yield 3 (tR =14.1 min, 1.2 mg), 4 (tR = 13.8 min, 1.0 mg), and 5 (tR =16.8 min, 0.4 mg). Silica gel fraction 8 was fractionated using the identical protocol as for fraction 7 to give ambiguine F isonitrile (tR = 14.8 min, 0.8 mg).

Ambiguine K isonitrile (1)

colorless crystals (MeOH); [α]D -94 (c 0.14, MeOH); UV (MeOH) λmax (log ε) 230 (2.72), 275 (2.11) nm; IR (neat) νmax 3416, 2965, 2125, 1631, 1452, 1377, 1323, 1086, 971, 927, 828, 755 cm-1; 1H NMR (see Table 1); 13C NMR (see Table 2); HRESIMS m/z 419.18979 [M-H]- (calcd for C26H28ClN2O, 419.18956).

Ambiguine L isonitrile (2)

white amorphous powder; [α]D -97 (c 0.13, MeOH); UV (MeOH) λmax (log ε) 223 (3.47), 271 (2.91) nm; IR (neat) νmax 3734, 3410, 2961, 2127, 1568, 1454, 1320, 1082, 1022, 971, 929, 719 cm-1; 1H NMR (see Table 1); 13C NMR (see Table 2); HRESIMS m/z 387.24347 [M+H]+ (calcd for C26H31N2O, 387.24309).

Ambiguine M isonitrile (3)

white amorphous powder; [α]D -21 (c 0.07, MeOH); UV (MeOH) λmax (log ε) 221 (2.64), 278 (1.92) nm; IR (neat) νmax 3385, 2130, 1645, 1444, 1015, 951 cm-1; 1H NMR (see Table 1); 13C NMR (see Table 2); HRESIMS m/z 439.21430 [M+H]+ (calcd for C26H32ClN2O2, 439.21468).

Ambiguine N isonitrile (4)

white amorphous powder; [α]D -12 (c 0.05, MeOH); UV (MeOH) λmax (log ε) 229 (3.53), 270 (3.48) nm; IR (neat) νmax 3735, 3367, 2928, 2142, 1684, 1652, 1540, 1510, 1456, 1081, 978, 867, 755 cm-1; 1H NMR (see Table 1); 13C NMR (see Table 2); HRESIMS m/z 405.25340 [M+H]+ (calcd for C26H33N2O2, 405.25365).

Ambiguine O isonitrile (5)

white amorphous powder; [α]D -17 (c 0.03, MeOH); UV (MeOH) λmax (log ε) 220 (3.44), 277 (2.83) nm; IR (neat) νmax 3387, 2924, 2123, 1649, 1433, 1303, 1018, 950, 755 cm-1; 1H NMR (see Table 1); 13C NMR (see Table 2); HRESIMS m/z 451.1802 [M-H]- (calcd for C26H29ClN2O3, 452.18667).

Ambiguine A isonitrile15,16

colorless crystals (MeOH and MeCN); [α]D -22 (c 0.22, MeOH); 1H NMR (see Table 9 in Supporting Information).

Ambiguine B isonitrile15,16

white amorphous powder; [α]D -11 (c 0.08, MeOH); 1H NMR (see Table 9 in Supporting Information).

Ambiguine C isonitrile15

white amorphous powder; [α]D -6 (c 0.08, MeOH); 1H NMR (see Table 9 in Supporting Information).

Ambiguine E isonitrile15,16

white amorphous powder; [α]D -83 (c 0.31, MeOH); 1H NMR (see Table 9 in Supporting Information).

Ambiguine F isonitrile15,16

white amorphous powder; 1H NMR (see Table 9 in Supporting Information).

Ambiguine I isonitrile16

white amorphous powder; [α]D -106 (c 0.23, MeOH); 1H NMR (see Table 9 in supporting information).

Hapalindole G15,36

white amorphous powder; [α]D -41 (c 0.24, MeOH); 1H NMR (see Table 9 in Supporting Information).

Hapalindole H15,36

white amorphous powder (MeOH); [α]D +186 (c 0.22, MeOH); 1H NMR (see Table 9 in Supporting Information).

X-ray crystallographic analysis of ambiguine K isonitrile (1)

38 Single crystals for X-ray analysis were grown from a methanol solution. A colorless crystal with dimensions 0.02 × 0.05 × 0.20 mm was used for the analysis. The diffraction data were collected on a Rigaku/MSC RAPID imaging plate area detector equipped with a normal focus sealed X-ray tube and Cu Kα radiation. Crystal data: C26H29ClN2O·CH3OH, MW= 453.0, orthorhombic, space group C2 (5); a =18.730(3)Å, b = 7.0493(11)Å, c = 20.221(3)Å, β = 110.242(9)Å, V = 2505.0(9)Å3; Z = 4, Dc =1.20 mg/m3; μ = 1.54 mm-1; F000 = 968. Total collected reflections 8507, 3626 unique reflections (Rint = 0.0497); final R1= 0.0583 for reflections with I>2σ (I); R1 = 0.0804, wR2 = 0.1756 for all unique data. The integration of intensities and refinement of unit cell parameters and orientation matrix were carried out using CrystalClear.39 The WinGX package was used for completing the structure determination.40 The structure was solved by direct methods, repeated cycling with least-squares refinement on F2 and difference Fourier maps using SHELXL-97.41 All non-hydrogen atoms were refined with anisotropic Gaussian displacement parameters. The Flack parameter, -.00(3), was well-determined, and used to assign the absolute configuration of the structure as C(10) R, C(11) S, C(12) R, C(13) R, and C(15) R.

Bacillus anthracis

The fractions and compounds were tested with concentrations from 100 μg/mL to 48.8 ng/mL using the previously described method.42The minimum inhibitory concentration (MIC) of each sample was calculated as the lowest concentration that prevents visible bacterial growth.

Mycobacterium tuberculosis

The inhibitory activity of fractions and compounds against M. tuberculosis was performed using the Microplate Alamar blue Assay (MABA).43 Virulent H37Rv strain was used in the assay. The MIC value was determined as the lowest drug concentration effecting an inhibition of ≥90%.

Other organisms

The broth microdilution MIC method was used to test the activity of compounds against Staphylococcus aureus44 and Candida albicans.45

Cytotoxicity

The cytotoxicity of compounds were evaluated using green monkey kidney cells (Vero).46 Cell viability was measured using the CellTiter 96 aqueous nonradioactive cell proliferation assay.

figure nihms-111724-f0002

Supplementary Material

1_si_001

Acknowledgement

We thank M. Bishop from Dr. A. Mesecar’s group at UIC for performing the B. anthracis assay. Dr. S. Cho, B. Wang, Y. Wang, and D. Wei from the Institute for Tuberculosis Research (ITR) at UIC for performing antibacterial, antifungal and cytotoxicity assays. We also thank Dr. B.D. Santarsiero, Dr. Y. Wang and Dr. C.A. Crot from the Research Resources Center (RRC) at UIC for X-ray crystallographic analysis and high resolution mass spectrometry, respectively. The 900 MHz NMR spectrometer was funded by NIH P41 grant (GM68944). This research was supported by NIH grant (1R01GM0758556).

Footnotes

Supporting Information available: A photomicrograph of Fischerella ambigua (UTEX 1903), tables with the complete NMR data of 1-5, 1D NMR spectra of 1-5, and 1H NMR data of known compounds reported in this paper. This material is available free of charge via the internet at http://pubs.acs.org.

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