| Summary: | Polysulfides (H<sub>2</sub>S<sub>x</sub>) represent a class of reactive sulfur species (RSS) which includes molecules such as H<sub>2</sub>S<sub>2</sub>, H<sub>2</sub>S<sub>3</sub>, H<sub>2</sub>S<sub>4</sub>, and H<sub>2</sub>S<sub>5,</sub> and whose presence and impact in biological systems, when compared to other sulfur compounds, has only recently attracted the wider attention of researchers. Studies in this field have revealed a facet-rich chemistry and biological activity associated with such chemically simple, still unusual inorganic molecules. Despite their chemical simplicity, these inorganic species, as reductants and oxidants, metal binders, surfactant-like “cork screws„ for membranes, components of perthiol signalling and reservoirs for inorganic hydrogen sulfide (H<sub>2</sub>S), are at the centre of complicated formation and transformation pathways which affect numerous cellular processes. Starting from their chemistry, the hidden presence and various roles of polysulfides in biology may become more apparent, despite their lack of clear analytical fingerprints and often murky biochemical footprints. Indeed, the biological chemistry of H<sub>2</sub>S<sub>x</sub> follows many unexplored paths and today, the relationship between H<sub>2</sub>S and its oxidized H<sub>2</sub>S<sub>x</sub> species needs to be clarified as a matter of “unmistaken identity„. Simultaneously, emerging species, such as HSSeSH and Se<sub>n</sub>S<sub>8−n</sub>, also need to be considered in earnest.
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