Bioactive and unusual steroids from Penicillium fungi

Penicillium fungi are represented by various species and can be found worldwide and thrive in a range of environments, such as in the soil, air, and indoors, and in marine environments, as well as food products. Chemical investigation of species of this genus has led to the discovery of compounds from several structural classes with varied bioactivities. As an example, this genus has been a source of bioactive and structurally unusual steroids. The scope of this short review is to cover specialized metabolites of the steroid class and the cytotoxic, antimicrobial, anti-inflammatory as well as phytotoxic activities of these compounds. Other steroids that possess unusual structures, with significant bioactivity yet to determined, will also be discussed to further demonstrate the structural diversity of this compound class from Penicillium fungi, and hopefully inspire the further exploration of such compounds to uncover their activity.

Understanding the Biosynthesis of Paxisterol in Lichen-Derived Penicillium aurantiacobrunneum for Production of Fluorinated Derivatives

The U.S. endemic lichen (Niebla homalea)-derived Penicillium aurantiacobrunneum produced a cytotoxic paxisterol derivative named auransterol (2) and epi-citreoviridin (6). Feeding assay using ¹³C₁-labelled sodium acetate not only produced C-13-labelled paxisterol but also confirmed the biosynthetic origin of the compound. The fluorination of bioactive compounds is known to improve pharmacological and pharmacokinetic effects. Our attempt to incorporate the fluorine atom in paxisterol and its derivatives using the fluorinated precursor sodium monofluoroacetate resulted in the isolation of 7-monofluoroacetyl paxisterol (7). The performed culture experiment, as well as the isolation and structure elucidation of the new fluorinated paxisterol, is discussed herein.

Detecting and Differentiating Monosaccharide Enantiomers by 1H NMR Spectroscopy

Monosaccharides play important roles in living organisms. They are present in essential glycoproteins, nucleic acids, and glycolipids as well as cell walls and bioactive natural product glycosides and polysaccharides. Monosaccharides are optically active, and as a routine, scientists make sure that their absolute configurations are determined when new natural glycosides are isolated. Many determination methods for the absolute configuration of monosaccharides have been reported, and thus far, taking advantage of their optical rotation differences is the most used and efficient method to distinguish enantiomers. This method, however, is not very convenient, because it requires a milligram amount of each pure sample and the availability of a polarimeter. Identification methods dealing with comparison of the retention times of the d- and l-diastereomeric monosaccharide derivatives by GC, TLC Rf values, HPLC, or UPLC have been also reported. Although effective, these methods still require sample preparation and a few milligrams of the test compounds. A new method with simple sample preparation to distinguish enantiomers of monosaccharides by analyzing the 1H NMR spectra of their diastereomeric derivatives has been developed. The monosaccharide components of a commercially available saponin-rich Panax ginseng and monoglycosides have been successfully identified using this procedure.

Spermidine alkaloid and glycosidic constituents of Vietnamese Homalium cochinchinensis

Phytochemical investigation of the aerial parts of Homalium cochinchinensis led to the isolation of secondary metabolites belonging to the spermidine alkaloid, glycoside, depsidone and phenol classes. Of the eleven secondary metabolites isolated in this study, two spermidine alkaloids, dovyalicins H (1) and I (2), which belong to a rare group among this class, and six glycosides (3-8) are previously undescribed. The structures of all new isolates were determined by interpretation of spectroscopic and spectrometric data. In this report, the structural elucidation of these unprecedented secondary metabolites (1-8) is described.

Prioritizing natural product compounds using 1D-TOCSY NMR spectroscopy

Natural product (NP) secondary metabolites are designed evolutionarily to have biological effects in other organisms for defense and the mediation of ecological interactions. Their structural complexity and diversity complement biological systems, allowing them to display unique bioactivities. Although more than half of all pharmaceuticals stem from NPs, pharmaceutical companies have reduced NP-based drug discovery programs due to various time and cost-consuming pitfalls; the re-isolation of already known, bioactive compounds being one of the most common. Dereplication methods minimize cost and speed up the discovery of new, bioactive leads by quickly identifying known small molecules. Liquid chromatography coupled mass spectrometry (LC-MS) is the most widely utilized dereplication technique because of its sensitivity and the open-source availability of MS libraries. However, single-ionization techniques are not able to detect all metabolites in a biological sample. Even more concerning, bioactive isomers cannot be differentiated by their mass alone. In response to these issues, complementary dereplication tools are needed to assist MS. Total correlation spectroscopy (TOCSY) is an NMR experiment that illustrates the connection between all coupled protons in a spin system. Most molecules contain several spin systems, and together, these networks form a unique fingerprint that can be utilized to quickly differentiate and dereplicate known compounds, even those with identical masses. In addition, these fingerprints can be used to identify possible new compounds in a crude NP-extract that are structurally related to known small molecules. From a sample of the U.S. endemic lichen Niebla homalea, five non-cytotoxic, new triterpenoids and three known triterpenoids were isolated in our laboratory. As our goal is to discover both new and cytotoxic compounds, we developed a one-dimensional TOCSY-based dereplication method to quickly identify these non-bioactive triterpenoids. After prioritizing triterpenoid-free fractions that showed antiproliferative activity in various cancer cell lines, the new compound 11 was isolated from another Niebla species.

Semi-synthetic cinnamodial analogues: Structural insights into the insecticidal and antifeedant activities of drimane sesquiterpenes against the mosquito Aedes aegypti

The Aedes aegypti mosquito serves as a major vector for viral diseases, such as dengue, chikungunya, and Zika, which are spreading across the globe and threatening public health. In addition to increased vector transmission, the prevalence of insecticide-resistant mosquitoes is also on the rise, thus solidifying the need for new, safe and effective insecticides to control mosquito populations. We recently discovered that cinnamodial, a unique drimane sesquiterpene dialdehyde of the Malagasy medicinal plant Cinnamosma fragrans, exhibited significant larval and adult toxicity to Ae. aegypti and was more efficacious than DEET–the gold standard for insect repellents–at repelling adult female Ae. aegypti from blood feeding. In this study several semi-synthetic analogues of cinnamodial were prepared to probe the structure-activity relationship (SAR) for larvicidal, adulticidal and antifeedant activity against Ae. aegypti. Initial efforts were focused on modification of the dialdehyde functionality to produce more stable active analogues and to understand the importance of the 1,4-dialdehyde and the α,ß-unsaturated carbonyl in the observed bioactivity of cinnamodial against mosquitoes. This study represents the first investigation into the SAR of cinnamodial as an insecticide and antifeedant against the medically important Ae. aegypti mosquito.

Aedes mosquitoes are the primary carriers of Zika, dengue, chikungunya, and yellow fever viruses around the globe. Given the emergence of insecticide-resistance in this genus and unprecedented ‘globalization’ of mosquito-borne viruses, new chemicals to control these mosquitoes (e.g., insecticides, repellents) are urgently needed. In the continuation of our search for new and safe natural product derived insecticides, we generated semi-synthetic derivatives of cinnamodial (CDIAL), previously identified as an insect antifeedant, repellent and insecticide, to give insights into the important features of the molecule that can contribute to the observed activities. Since the antifeedant and repellent activity of CDIAL are found to be mediated by modulation of a sensory receptor (TRPA1) in the mosquito, we developed a structural model to understand how CDIAL interacts with TRPA1 and to explain the difference in activities of CDIAL and the prepared derivatives. Our findings aid in the development of plant-derived insecticides to control the Ae. aegypti mosquito and justify continued efforts using TRPA1 as a target for new mosquito repellents.

Furoquinoline Alkaloids and Methoxyflavones from the Stem Bark of Melicope madagascariensis (Baker) T.G. Hartley

Melicope madagascariensis (Rutaceae) is an endemic plant species of Madagascar that was first classified as a member of the genus Euodia J. R. & G. Forst (Rutaceae) under the scientific name Euodia madagascariensis Baker. Based on morphological characteristics, Thomas Gordon Hartley taxonomically revised E. madagascariensis Baker to be M. madagascariensis (Baker) T.G. Hartley. Chemotaxonomical studies have long been used to help the identification and confirmation of taxonomical classification of plant species and botanicals. Aiming to find more evidences to support the taxonomical revision performed on E. madagascariensis, we carried out phytochemical investigation of two samples of the plant. Fractionation of the ethanol extracts prepared from two stem bark samples of M. madagascariensis (Baker) T.G. Hartley led to the isolation of seven known furoquinoline alkaloids 1-7 and two known methoxyflavones 8 and 9. The presence of furoquinoline alkaloids and methoxyflavones in the title species is in agreement with its taxonomic transfer from Euodia to Melicope. Antiprotozoal evaluation of the isolated compounds showed that 6-methoxy-7-hydroxydictamnine (heliparvifoline, 3) showed weak antimalarial activity (IC₅₀ = 35 µM) against the chloroquine-resistant strain Dd2 of Plasmodium falciparum. Skimmianine (4) displayed moderate cytotoxicity with IC₅₀ value of 1.5 µM against HT-29 colon cancer cell line whereas 3,5-dihydroxy-3',4',7-trimethoxyflavone (9) was weakly active in the same assay (IC₅₀ = 13.9 µM).