Table 3.

Representative sulfur compounds and their oxidation state. RSS are in bold letters. Modified from [79].

Oxidation State	Representative Compound and Formula
+6	Sulfate, SO42−
+6 and −2	Thiosulfate, S2O32−
+5 and −2	Polythionates (-O3S-Sn-SO3-)2− Dithionate, S2O62− Trithionate, S3O62− Tetrathionate, S4O62−
+4	Sulfur dioxide, SO2 Sulfite, SO32− Disulfite, S2O52− Sulfonic acid (RSO3H) from ROS-mediated protein sulfonylation. Sulfone, OS(S) the oxidation product of sulfoxides.
+3	Dithionite, S2O42− Disulfide-S-dioxide (thiosulfonate) RS(O2)SR
+2	Carbonyl Sulfide (COS), OCS. Sulfinic acid (RSO2H) from ROS-mediated protein sulfinylation.
+1	Disulfide-S-monoxide (thiosulfinate) RS(O)SR
0	S0 (sulfane sulfur), elemental sulfur, mainly S8 (cycloocta-S). Sulfoxide (R-S(-O)-R such as the dimethyl sulfoxide (DMSO). Oxidized derivatives of sulfide and sulfenic acid (RSOH) from ROS-mediated protein sulfenylation. Near the six electrons, valence S0 never exists by itself. Sulfane sulfur (S0, S-S, or S2) is labile. There are a variety of compounds such as S8, thiosulfate, polysulfanes, and polysulfides, that contain S0
−1	Disulfide (RSSR) is a persulfide −S-S− found in the linkages between two cysteine residues in proteins. RSSH denotes persulfides (also called hydrosulfides or hydropersulfides) obtained by the action of H2S on cysteine residues (R-SH). Thioethers and thiols can be oxidized to disulfides. Persulfides such as CysSSH, GSSH, and protein-SSH act as signaling compounds in organisms. Major products of the decomposition of persulfides are polysulfanes Disulfide-S-monoxide (thiosulfinate) RS(O)SR Disulfide-S-dioxide (thiosulfonate) RS(O2)SR Thiyl-radical HS· or RS·
−2	Sulfide, S2− and organic polysulfides, S22−, S32−, S52− Disulfides (R-S-S-R) Carbon disulfide (CS2) FeS2 NaHS and Na2S are sources of S2− and of its conjugated acids SH− and H2S. Organic and inorganic polysulfides (with Sn > 2) contain S0 atoms, which allows a diversity of oxidation states.
−2	Hydrogen sulfide (H2S), disulfane or hydrogen persulfide (H2S2), H2S3, other inorganic polysulfides (H2Sx) x ≥ 1, and polysulfanes (RSSnH, RSSnSR, n > 2). Polysulfanes contain S0 atoms, which allows a diversity of oxidation states.
−2	Thioethers (C-S-C) such as dimethyl sulfide (DMS), CH3-S-CH3 and dimethyl disulfide (DMDS), CH3-S-S-CH3.
−2	Thiols (R-SH) such as glutathione (GSH) and methyl mercaptan, CH3-SH. Thiols are derived from the sulfhydryl group -SH of cysteine that enables multiple oxidation states (−2 to +6). Thiolates are anionic derivatives of thiols in which a metal or other cation replaces H.