Emily Mevers

Biologically active secondary metabolites from marine cyanobacteria

Marine cyanobacteria are a rich source of complex bioactive secondary metabolites which derive from mixed biosynthetic pathways. Recently, several marine cyanobacterial natural products have garnered much attention due to their intriguing structures and exciting anti-proliferative or cancer cell toxic activities. Several other recently discovered secondary metabolites exhibit insightful neurotoxic activities whereas others are showing pronounced anti-inflammatory activity. A number of anti-infective compounds displaying activity against neglected diseases have also been identified, which include viridamides A and B, gallinamide A, dragonamide E, and the almiramides.

Homodimericin A: A Complex Hexacyclic Fungal Metabolite

Microbes sense and respond to their environment with small molecules, and discovering these molecules and identifying their functions informs chemistry, biology, and medicine. As part of a study of molecular exchanges between termite-associated actinobacteria and pathogenic fungi, we uncovered a remarkable fungal metabolite, homodimericin A, which is strongly upregulated by the bacterial metabolite bafilomycin C1. Homodimericin A is a hexacyclic polyketide with a carbon backbone containing eight contiguous stereogenic carbons in a C20 hexacyclic core. Only half of its carbon atoms have an attached hydrogen, which presented a significant challenge for NMR-based structural analysis. In spite of its microbial production and rich stereochemistry, homodimericin A occurs naturally as a racemic mixture. A plausible nonenzymatic reaction cascade leading from two identical achiral monomers to homodimericin A is presented, and homodimericin A’s formation by this path, a six-electron oxidation, could be a response to oxidative stress triggered by bafilomycin C1.

Lyngbyabellins K–N from Two Palmyra Atoll Collections of the Marine Cyanobacterium Moorea bouillonii

Five lipopeptides of the lyngbyabellin structure class, four cyclic (1-3, and 5) and one linear (4), were isolated from the extracts of two collections of filamentous marine cyanobacteria obtained from Palmyra Atoll in the Central Pacific Ocean. Their planar structures and absolute configurations were elucidated by combined spectroscopic and chromatographic analyses as well as chemical synthesis of fragments. In addition to structural features typical of the lyngbyabellins, such as two thiazole rings and a chlorinated 2-methyloctanoate residue, these new compounds possess several unique aspects. Of note, metabolites 2 and 3 possessed rare mono-chlorination on the 3-acyloxy-2-methyloctanoate residue while lyngbyabellin N (5) had an unusual N,N-dimethylvaline terminus. Lyngbyabellin N also possessed a leucine statine residue, and showed strong cytotoxic activity against HCT116 colon cancer cell line (IC50 = 40.9 ± 3.3 nM).

Unequivocal determination of complex molecular structures using anisotropic NMR measurements

Picking structures out of a lineup Pharmaceutical research relies critically on determining the correct structures of numerous complex molecules. When well-ordered crystals are not available for x-ray analysis, nuclear magnetic resonance (NMR) spectroscopy is the most common structure-elucidation method. However, sometimes it is hard to distinguish isomers with similar spectra. Liu et al. showcase a protocol that combines computer modeling with anisotropic NMR data acquired using gel-aligned samples. Because of its uniform sensitivity to relative bond orientations across the whole molecular framework, the method overcomes common pitfalls that can lead to invalid structure assignments. Science, this issue p. eaam5349 A nuclear magnetic resonance method applied to aligned molecules helps to elucidate their complex structures. INTRODUCTION Single-crystal x-ray diffraction studies represent the gold standard for unequivocal establishment of molecular structure and configuration. For molecules that will not crystallize or that form poorly-diffracting crystals, alternative methods must be used. Crystalline sponges and atomic force microscopy are techniques with increasing potential, although nuclear magnetic resonance (NMR) spectroscopy methods provide the primary viable alternative means to determine molecular structures. However, misinterpretation of NMR data—as a result of poor data quality, inappropriate experiment selection, or investigator bias—has led to burgeoning numbers of structure revision reports. Clearly, the development of a method to more effectively use NMR data and simultaneously quell reports of incorrect structures would be highly beneficial. RATIONALE Combining computer-assisted structure elucidation (CASE) algorithms and density functional theory (DFT) calculations with measured anisotropic NMR parameters, specifically residual dipolar coupling (RDC), and residual chemical shift anisotropy (RCSA) holds strong promise as an effective alternative means of assigning three-dimensional (3D) molecular structures. Anisotropic NMR data provide a spatial view of the relative orientations between bonds (RDCs) and chemical shielding tensors (RCSAs), regardless of the separation between the bonds and atoms, respectively. Hence, these data are sensitive reporters of global structural validity. The combination of DFT calculations and anisotropic NMR data represents an orthogonal approach to conventional NMR data interpretation that is not subject to the interpretational biases of human investigators and, as such, mitigates the risk of incorrect structure assignments. RESULTS Anisotropic NMR data can be used directly to evaluate the validity of investigator-proposed structures or can be combined with a CASE program in conjunction with DFT calculations for both structural proposal and validation. The RDC data are typically used to structurally define C-H bond vectors, whereas the RCSA data report on the chemical shift tensors of both protonated and nonprotonated carbons, the latter only accessible by long-range RDC data that are difficult to measure and interpret. These data are used to evaluate a given structure proposal on the basis of the agreement between the experimentally measured data and theoretical values calculated for the corresponding 3D DFT models. When structures generated by a CASE program are being considered, the method only requires a multidimensional NMR data set of sufficient quality and sophistication to allow the CASE program to generate a set of proposals that contains the correct structure of the molecule. The molecules being studied should also be amenable to modern DFT calculations for 3D model building. The CASE program output is sorted on the basis of cumulative error between experimental and calculated 13C data for the ensemble of structures generated, and the best-fitting molecules are subsequently subjected to DFT calculation for analysis. Results obtained using the proposed method demonstrate its applicability to a diverse range of complex molecules, each of which challenged the investigators originally reporting the structures. CONCLUSION The technique described here represents a potential paradigm shift from conventional NMR data interpretation and can provide an unequivocal and unbiased confirmation of interatomic connectivity and relative configuration for organic and natural product structures. The principle of residual dipolar coupling (RDC)–based model differentiation is shown using aquatolide as an example. The revised structure of aquatolide is shown on the top left, with the originally reported structure shown on the bottom left. The selected C-H bond vectors in the two structures have different orientations, as is evident after translating them to the same origin in the middle diagrams. Theoretical RDC values associated with these vectors can be calculated for each model on the basis of the experimentally determined alignment tensor. Correlation data are shown for only the four highlighted CH groups, although the alignment tensor was actually determined using all available data. The originally proposed (incorrect) structure clearly shows poorer agreement between the calculated and experimental data. Assignment of complex molecular structures from nuclear magnetic resonance (NMR) data can be prone to interpretational mistakes. Residual dipolar couplings and residual chemical shift anisotropy provide a spatial view of the relative orientations between bonds and chemical shielding tensors, respectively, regardless of separation. Consequently, these data constitute a reliable reporter of global structural validity. Anisotropic NMR parameters can be used to evaluate investigators’ structure proposals or structures generated by computer-assisted structure elucidation. Application of the method to several complex structure assignment problems shows promising results that signal a potential paradigm shift from conventional NMR data interpretation, which may be of particular utility for compounds not amenable to x-ray crystallography.

Lipopeptides from the tropical marine cyanobacterium Symploca sp.

A collection of the tropical marine cyanobacterium Symploca sp., collected near Kimbe Bay, Papua New Guinea, previously yielded several new metabolites including kimbeamides A–C, kimbelactone A, and tasihalide C. Investigations into a more polar cytotoxic fraction yielded three new lipopeptides, tasiamides C–E (1–3). The planar structures were deduced by 2D NMR spectroscopy and tandem mass spectrometry, and their absolute configurations were determined by a combination of Marfey’s and chiral-phase GC-MS analysis. These new metabolites are similar to several previously isolated compounds, including tasiamide (4), grassystatins (5, 6), and symplocin A, all of which were isolated from similar filamentous marine cyanobacteria.

Parguerene and Precarriebowmide, Two Classes of Lipopeptides from the Marine Cyanobacterium Moorea producens

Two new marine cyanobacterial natural products, parguerene (1) and precarriebowmide (2), were isolated from a collection of Moorea producens obtained from La Parguera, Puerto Rico. The planar structures of both were deduced by 2D NMR spectroscopy and mass spectrometry. Parguerene is a modified acyl amide with some structural similarity to the bacterial metabolite stipiamide (3), whereas precarriebowmide is a lipopeptide and represents a minor modification compared to two other known metabolites, carriebowmide (4) and carriebowmide sulfone (5). The identification of 2 led to an investigation into whether carriebowmide and carriebowmide sulfone were true secondary metabolites or isolation artifacts.

A Maldiisotopic Approach to Discover Natural Products: Cryptomaldamide, a Hybrid Tripeptide from the Marine Cyanobacterium Moorea producens

Genome sequencing of microorganisms has revealed a greatly increased capacity for natural products biosynthesis than was previously recognized from compound isolation efforts alone. Hence, new methods are needed for the discovery and description of this hidden secondary metabolite potential. Here we show that provision of heavy nitrogen 15N-nitrate to marine cyanobacterial cultures followed by single-filament MALDI analysis over a period of days was highly effective in identifying a new natural product with an exceptionally high nitrogen content. The compound, named cryptomaldamide, was subsequently isolated using MS to guide the purification process, and its structure determined by 2D NMR and other spectroscopic and chromatographic methods. Bioinformatic analysis of the draft genome sequence identified a 28.7 kB gene cluster that putatively encodes for cryptomaldamide biosynthesis. Notably, an amidinotransferase is proposed to initiate the biosynthetic process by transferring an amidino group from arginine to serine to produce the first residue to be incorporated by the hybrid NRPS-PKS pathway. The maldiisotopic approach presented here is thus demonstrated to provide an orthogonal method by which to discover novel chemical diversity from Nature.

Small Molecule DFPM Derivative-Activated Plant Resistance Protein Signaling in Roots Is Unaffected by EDS1 Subcellular Targeting Signal and Chemical Genetic Isolation of victr R-Protein Mutants

The small molecule DFPM ([5-(3,4-dichlorophenyl)furan-2-yl]-piperidine-1-ylmethanethione) was recently shown to trigger signal transduction via early effector-triggered immunity signaling genes including EDS1 and PAD4 in Arabidopsis thaliana accession Col-0. Chemical genetic analyses of A. thaliana natural variants identified the plant Resistance protein-like Toll/Interleukin1 Receptor (TIR)-Nucleotide Binding (NB)-Leucine-Rich Repeat (LRR) protein VICTR as required for DFPM-mediated root growth arrest. Here a chemical genetic screen for mutants which disrupt DFPM-mediated root growth arrest in the Col-0 accession identified new mutant alleles of the TIR-NB-LRR gene VICTR. One allele, victr-6, carries a Gly216-to-Asp mutation in the Walker A domain supporting an important function of the VICTR nucleotide binding domain in DFPM responses consistent with VICTR acting as a canonical Resistance protein. The essential nucleo-cytoplasmic regulator of TIR-NB-LRR-mediated effector-triggered immunity, EDS1, was reported to have both nuclear and cytoplasmic actions in pathogen resistance. DFPM was used to investigate the requirements for subcellular EDS1 localization in DFPM-mediated root growth arrest. EDS1-YFP fusions engineered to localize mainly in the cytoplasm or the nucleus by tagging with a nuclear export signal (NES) or a nuclear localization signal (NLS), respectively, were tested. We found that wild-type EDS1-YFP and both the NES and NLS-tagged EDS1 variants were induced by DFPM treatments and fully complemented eds1 mutant plants in root responses to DFPM, suggesting that enrichment of EDS1 in either compartment could confer DFPM-mediated root growth arrest. We further found that a light and O2-dependent modification of DFPM is necessary to mediate DFPM signaling in roots. Chemical analyses including Liquid Chromatography-Mass Spectrometry and High-Resolution Atmospheric Pressure Chemical Ionization Mass Spectrometry identified a DFPM modification product that is likely responsible for bioactivity mediating root growth arrest. We propose a chemical structure of this product and a possible reaction mechanism for DFPM modification.

Identification of Potent Ebola Virus Entry Inhibitors with Suitable Properties for in Vivo Studies

Previous studies identified an adamantane dipeptide piperazine 3.47 that inhibits Ebola virus (EBOV) infection by targeting the essential receptor Niemann-Pick C1 (NPC1). The physicochemical properties of 3.47 limit its potential for testing in vivo. Optimization by improving potency, reducing hydrophobicity, and replacing labile moieties identified 3.47 derivatives with improved in vitro ADME properties that are also highly active against EBOV infection, including when tested in the presence of 50% normal human serum (NHS). In addition, 3A4 was identified as the major cytochrome P450 isoform that metabolizes these compounds, and accordingly, mouse microsome stability was significantly improved when tested in the presence of the CYP3A4 inhibitor ritonavir that is approved for clinical use as a booster of anti-HIV drugs. Oral administration of the EBOV inhibitors with ritonavir resulted in a pharmacokinetic profile that supports a b.i.d. dosing regimen for efficacy studies in mice.

Chemical Interaction among Termite-Associated Microbes

Bacteria and fungi in shared environments compete with one another for common substrates, and this competition typically involves microbially-produced small molecules. An investigation of one shared environmental niche, the carton material of the Formosan subterranean termite Coptotermes formosanus, identified the participants on one of these molecular exchanges. Molecular characterization of several termite-associated actinobacteria strains identified eleven known antimicrobial metabolites that may aid in protecting the C. formosanus colony from pathogenic fungal infections. One particular actinobacterial-derived small molecule, bafilomycin C1, elicited a strong chemical response from Trichoderma harzianum, a common soil saprophyte. Upon purification and structure elucidation, three major fungal metabolites were identified, t22-azaphilone, cryptenol, and homodimericin A. Both t22-azaphilone and homodimericin A are strongly upregulated, 123- and 38-fold, respectively, when exposed to bafilomycin C1, suggesting each play a role in defending T. harzianum from the toxic effect of bafilomycin C1.