Ascidians (tunicates) contain compounds with a large range of pharmacological activities. With the most common of chemical ingredients, elemental sulfur and dopamine, we hypothesize that an ascidian has developed a means to protect itself from predators using in essence chemical warfare. A novel aspect of this work is the idea that tunicates can filter elemental sulfur, transform solid insoluble Sx, and transfer sulfur atoms chemically into a phenolic molecule for use as a protecting agent. Our work focused on a biomimetic HSx- reaction with o-quinone as a biosynthetic generator of antimicrobial (anticancer) o-benzopolysulfanes. Experiments using HPLC and SIM GC/MS revealed the presence of benzotrithiole, pentathia-benzocycloheptene, and heptathia-benzocyclononane compounds. The benzo-S3, -S5, and -S7 products co-exist in equilibrium, where their relative concentrations are influenced by solvent. Detection of heptathia-benzocyclononane is an exciting new result that had been predicted by our density functional theory (DFT) computation. This suggests that natural product o-benzopolysulfanes exist as a mixture of the S3-, S5-, and S7-homologs. Factors controlling the formation and interconversion of the S3-, S5-, and S7-polysulfanes and possible biological implications will be discussed.
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