Isolated from a New Caledonian collection of the poecilosclerid sponge Echinochalina bargibanti, arsenicin A (C3H6As4O3) was described as the first natural organic polyarsenic compound. Continuing on bioassay guided fractionation of the organic extracts of this sponge, we describe here the isolation and structural elucidation of the first sulfur-containing organic polyarsenicals ever found in nature. The novel metabolites, called arsenicin B and arsenicin C, are built on a noradamantane type framework and they further distinguish from the adamantane type framework of arsenicin A by a different heteroatom composition and by the presence of an unusual As-As bonding. Extensive 1D and 2D-NMR measurements, in combination with tandem APCI mass spectra, allowed to establish the structure of arsenicin B (C3H6As4S2) as 2. The scarcity of arsenicin C and and its intrinsic chemical instability, only allowed to collect partial spectral data which prevented the full structural definition. After extensive computational testing of several putative structures, structure 3 for arsenicin C (C3H6As4OS) was inferred by comparing experimental with DFT-calculated 1H and 13C NMR spectra. Finally, the absolute configuration of 2 and 3 were determined with a combined use of experimental and TD-DFT calculated ECD spectra and observed specific rotations. These findings pose great challenges for both the biosynthesis of these metabolites and the cycle of Arsenic in nature. Arsenicin B and arsenicin C showed strong antifungal and antibacterial activities, especially against S. aureus comparable to the antibiotic gentamycin.
Combined use of NMR, distance geometry, and restrained energy minimization for the conformational analysis of 8-lysine–vasopressin