No abstract

Fractionation of the EtOAc extract of a static culture of Aspergillus niger isolated from the Mediterranean sponge Axinella damicornis yielded eight secondary metabolites, out of which seven compounds (2-8) proved to be new natural products, whereas one was identified as the known fungal pigment cycloleucomelone (1). The new compounds included the 3,3'-bicoumarin bicoumanigrin (2), the structurally unusual 4-benzyl-1H-pyridin-6-one derivatives aspernigrins A and B (3 and 4), and pyranonigrins A-D (5-8), the latter featuring a novel pyrano[3,2-b]pyrrole skeleton hitherto unprecedented in nature. All structures were elucidated on the basis of extensive one- and two-dimensional NMR spectroscopic studies ((1)H, (13)C, COSY, HMQC, HMBC, NOE difference spectra) and mass spectral analysis. For the two chiral molecules 4 and 5, the absolute configurations were established by quantum chemical calculations of their circular dichroism (CD) spectra. In each case, two independent methods, i.e., a molecular dynamics approach taking into consideration the molecular flexibility, and a conformational analysis followed by Boltzmann weighting of the single CD spectra calculated for the conformers thus obtained, led to identical results without the need of any empirical comparison of chiroptical data reported for reference compounds. Bicoumanigrin (2) showed moderate cytotoxicity against human cancer cell lines in vitro. In addition, aspernigrin B (4) was found to display a strong neuroprotective effect against glutamic acid.

Labyrinthulids and thraustochytrids are unicellular heterotrophs, formerly considered as fungi, but presently are recognized as members in the stramenopiles of the kingdom Protista sensu lato. We determined the 18S ribosomal RNA gene sequences of 14 strains from different species of the six genera and analyzed the molecular phylogenetic relationships. The results conflict with the current classification based on morphology, at the genus and species levels. These organisms are separated, based on signature sequences and unique inserted sequences, into two major groups, which were named the labyrinthulid phylogenetic group and the thraustochytrid phylogenetic group. Although these groupings are in disagreement with many conventional taxonomic characters, they correlated better with the sugar composition of the cell wall. Thus, the currently used taxonomic criteria need serious reconsideration.