Stigonemapeptin, an Ahp-Containing Depsipeptide with Elastase Inhibitory Activity from the Bloom-Forming Freshwater Cyanobacterium <i>Stigonema</i> sp.

Stigonemapeptin (<b>1</b>), a depsipeptide containing an Ahp (3-amino-6-hydroxy-2-piperidone) residue, was isolated from a bloom sample of the freshwater cyanobacterium <i>Stigonema</i> sp. collected from North Nokomis Lake in the Highland Lake District of northern Wisconsin. The planar structure was determined by 1D and 2D NMR experiments as well as HRESIMS analysis. The absolute configurations of the amino acids were determined using the advanced Marfey’s method after acid hydrolysis. Stigonemapeptin (<b>1</b>), characterized by the presence of the Ahp residue, also contained the modified amino acids Abu (2-amino-2-butenoic acid) and <i>N</i>-formylated Pro. Stigonemapeptin (<b>1</b>) showed <i>in vitro</i> elastase and chymotrypsin inhibitory activity, with IC₅₀ values of 0.26 and 2.93 μM, respectively

Total Synthesis of Scytonemide A Employing Weinreb AM Solid-Phase Resin

The human 20S proteasome inhibitor scytonemide A (<b>1</b>), a macrocyclic imine originally isolated from the cyanobacterium <i>Scytonema hofmanni</i>, was synthesized via a biomimetic solid-phase peptide synthesis (SPPS) approach employing the Weinreb AM resin. Utilizing this approach, cyclization of the protected heptapeptide via formation of the imine bond occurred spontaneously upon cleavage from the resin in the presence of a reducing agent and subsequent aqueous workup. The final deprotection step necessary to produce the natural product was accomplished under slightly basic conditions, facilitating cleavage of the silyl ether group while leaving the macrocycle intact. Purification of the synthetic scytonemide A was accomplished via normal-phase flash column chromatography, potentially facilitating larger scale preparation of the compound necessary for future mechanistic and SAR studies. The structure of the target compound was confirmed by NMR spectroscopy, which also shed light on differences in the spectroscopic data obtained for the synthetic and natural scytonemide A samples for some of the amide and alcohol signals in the ¹H NMR spectrum

Merocyclophanes C and D from the Cultured Freshwater Cyanobacterium <i>Nostoc</i> sp. (UIC 10110)

Merocyclophanes C and D (<b>1</b> and <b>2</b>) were isolated from the cell extract of the cultured cyanobacterium UIC 10110. The structures were determined by one-dimensional nuclear magnetic resonance (NMR) and high-resolution electrospray ionization mass spectrometry and confirmed by 2D NMR techniques. The absolute configurations were determined using electronic circular dichroism spectroscopy. Merocyclophanes C and D represent the first known analogues of the merocyclophane core structure, a recently discovered scaffold of [7,7] paracyclophanes characterized by an α-branched methyl at C-1/C-14; <b>1</b> and <b>2</b> showed antiproliferative activity against the MDA-MB-435 cell line with IC₅₀ values of 1.6 and 0.9 μM, respectively. Partial 16S analysis determined UIC 10110 to be a <i>Nostoc</i> sp., and it was found to clade with UIC 10062 <i>Nostoc</i> sp., the only other strain known to produce merocyclophanes. The genome of UIC 10110 was sequenced, and a biosynthetic gene cluster was identified that is proposed to encode type I and type III polyketide synthases that are potentially responsible for production of the merocyclophanes; however, further experiments will be required to verify the true function of the gene cluster. The gene cluster provides a genetic basis for the observed structural differences of the [7,7] paracyclophane core structures

Ribocyclophanes A–E, Glycosylated Cyclophanes with Antiproliferative Activity from Two Cultured Terrestrial Cyanobacteria

The cell extracts of two cultured freshwater <i>Nostoc</i> spp., UIC 10279 and UIC 10366, both from the suburbs of Chicago, showed antiproliferative activity against MDA-MB-231 and MDA-MB-435 cancer cell lines. Bioassay-guided fractionation led to the isolation of five glycosylated cylindrocyclophanes, named ribocyclophanes A–E (<b>1</b>–<b>5</b>) and cylindrocyclophane D (<b>6</b>). The structure determination was carried out by HRESIMS and 1D and 2D NMR analyses and confirmed by single-crystal X-ray crystallography. The structures of ribocyclophanes A–E (<b>1</b>–<b>5</b>) contain a β-d-ribopyranose glycone in the rare ¹<i>C</i>₄ conformation. Among isolated compounds, ribocyclophane D (<b>4</b>) showed antiproliferative activity against MDA-MB-435 and MDA-MB-231 cancer cells with an IC₅₀ value of less than 1 μM

Chemical Diversity of Metabolites from Fungi, Cyanobacteria, and Plants Relative to FDA-Approved Anticancer Agents

A collaborative project has been undertaken to explore filamentous fungi, cyanobacteria, and tropical plants for anticancer drug leads. Through principal component analysis, the chemical space covered by compounds isolated and characterized from these three sources over the last 4 years was compared to each other and to the chemical space of selected FDA-approved anticancer drugs. Using literature precedence, nine molecular descriptors were examined: molecular weight, number of chiral centers, number of rotatable bonds, number of acceptor atoms for H-bonds (N, O, F), number of donor atoms for H-bonds (N and O), topological polar surface area using N, O polar contributions, Moriguchi octanol–water partition coefficient, number of nitrogen atoms, and number of oxygen atoms. Four principal components explained 87% of the variation found among 343 bioactive natural products and 96 FDA-approved anticancer drugs. Across the four dimensions, fungal, cyanobacterial, and plant isolates occupied both similar and distinct areas of chemical space that collectively aligned well with FDA-approved anticancer agents. Thus, examining three separate resources for anticancer drug leads yields compounds that probe chemical space in a complementary fashion

Trichormamides A and B with Antiproliferative Activity from the Cultured Freshwater Cyanobacterium <i>Trichormus</i> sp. UIC 10339

Two new cyclic lipopeptides, trichormamides A (<b>1</b>) and B (<b>2</b>), were isolated from the cultured freshwater cyanobacterium <i>Trichormus</i> sp. UIC 10339. The strain was obtained from a sample collected in Raven Lake in Northern Wisconsin. The planar structures of trichormamides A (<b>1</b>) and B (<b>2</b>) were determined using a combination of spectroscopic analyses including HRESIMS and 1D and 2D NMR experiments. The absolute configurations of the amino acid residues were assigned by the advanced Marfey’s method after acid hydrolysis. Trichormamide A (<b>1</b>) is a cyclic undecapeptide containing two d-amino acid residues (d-Tyr and d-Leu) and one β-amino acid residue (β-aminodecanoic acid). Trichormamide B (<b>2</b>) is a cyclic dodecapeptide characterized by the presence of four nonstandard α-amino acid residues (homoserine, <i>N</i>-methylisoleucine, and two 3-hydroxyleucines) and one β-amino acid residue (β-aminodecanoic acid). Trichormamide B (<b>2</b>) was cytotoxic against MDA-MB-435 and HT-29 cancer cell lines with IC₅₀ values of 0.8 and 1.5 μM, respectively

Ribocyclophanes A–E, Glycosylated Cyclophanes with Antiproliferative Activity from Two Cultured Terrestrial Cyanobacteria

The cell extracts of two cultured freshwater <i>Nostoc</i> spp., UIC 10279 and UIC 10366, both from the suburbs of Chicago, showed antiproliferative activity against MDA-MB-231 and MDA-MB-435 cancer cell lines. Bioassay-guided fractionation led to the isolation of five glycosylated cylindrocyclophanes, named ribocyclophanes A–E (<b>1</b>–<b>5</b>) and cylindrocyclophane D (<b>6</b>). The structure determination was carried out by HRESIMS and 1D and 2D NMR analyses and confirmed by single-crystal X-ray crystallography. The structures of ribocyclophanes A–E (<b>1</b>–<b>5</b>) contain a β-d-ribopyranose glycone in the rare ¹<i>C</i>₄ conformation. Among isolated compounds, ribocyclophane D (<b>4</b>) showed antiproliferative activity against MDA-MB-435 and MDA-MB-231 cancer cells with an IC₅₀ value of less than 1 μM

Essential Parameters for Structural Analysis and Dereplication by ¹H NMR Spectroscopy

The present study demonstrates the importance of adequate precision when reporting the δ and <i>J</i> parameters of frequency domain ¹H NMR (HNMR) data. Using a variety of structural classes (terpenoids, phenolics, alkaloids) from different taxa (plants, cyanobacteria), this study develops rationales that explain the importance of enhanced precision in NMR spectroscopic analysis and rationalizes the need for reporting Δδ and Δ<i>J</i> values at the 0.1–1 ppb and 10 mHz level, respectively. Spectral simulations paired with iteration are shown to be essential tools for complete spectral interpretation, adequate precision, and unambiguous HNMR-driven dereplication and metabolomic analysis. The broader applicability of the recommendation relates to the physicochemical properties of hydrogen (¹H) and its ubiquity in organic molecules, making HNMR spectra an integral component of structure elucidation and verification. Regardless of origin or molecular weight, the HNMR spectrum of a compound can be very complex and encode a wealth of structural information that is often obscured by limited spectral dispersion and the occurrence of higher order effects. This altogether limits spectral interpretation, confines decoding of the underlying spin parameters, and explains the major challenge associated with the translation of HNMR spectra into tabulated information. On the other hand, the reproducibility of the spectral data set of any (new) chemical entity is essential for its structure elucidation and subsequent dereplication. Handling and documenting HNMR data with adequate precision is critical for establishing unequivocal links between chemical structure, analytical data, metabolomes, and biological activity. Using the full potential of HNMR spectra will facilitate the general reproducibility for future studies of bioactive chemicals, especially of compounds obtained from the diversity of terrestrial and marine organisms