Among ROS, H
2O
2 has the longest half-life and the highest capacity for diffusion [
41], which makes it highly suitable for redox signaling. Indeed, H
2O
2 acts as a second messenger in plants by diffusing in cells and across membranes via aquaporins, thereby allowing both autocrine and paracrine signaling [
45]. H
2O
2 is relatively stable and its reaction with reduced free Cys or glutathione (GSH) is slow compared to some other ROS and RCS [
49]. H
2O
2 generally reacts more easily with protein Cys but its reactivity for thiol oxidation is highly dependent on a favorable protein microenvironment that reduces activation energy [
49]. This dependency on protein structure for reactivity allows specificity of H
2O
2 mediated redox signal [
49]. Thiolates, which are more reactive than thiols towards H
2O
2, can perform a nucleophilic attack on H
2O
2, causing a reversible two electrons oxidation to a sulfenic acid (-SOH) [
51], potentially altering enzymes function and activity (
Figure 1) [
42]. It is noteworthy that in addition to H
2O
2, natural or artificial hydroperoxides and peroxynitrite can also cause thiol oxidation to sulfenic acid [
31,
52]. The latter is usually considered highly unstable and acts as an intermediate towards several Cys redox PTMs (
Figure 1A), including, as discussed below,
S-sulfinylation,
S-sulfonylation,
S-glutathionylation, S-S formation, or persulfidation [
22]. As discussed in the section below, there is also an enzymatic pathway responsible for the generation of
S-sulfenylated Cys. This occurs in instances where a
S-sulfinylated Cys can be reduced by a sulfiredoxin (SRX) [
53]. The stability of the
S-sulfenylated Cys is mainly determined by its molecular environment. Improvement of sulfenic acid stability is determined by decreased solvent accessibility, absence of a proximal Cys that could induce the formation of a S-S and stabilization of the sulfenate by an H-bond network with adjacent amino acids [
22]. In addition, the sulfenic acid has a unique reactivity since it can act both as a nucleophile and an electrophile [
31]. For instance, the nucleophilic reactions of sulfenic acid include its overoxidation to sulfinic acid [
31]. For this, the sulfenic acid performs a nucleophilic attack on H
2O
2, leading to irreversible sulfinic and sulfonic forms of oxidation discussed below. Electrophilic sulfenic acid reactions lead for example to Cys persulfidation which involves a reaction with H
2S and cannot occur with a non-oxidized thiolate (see section below, [
54]). As well, sulfenic acid can react with a thiol to create an intramolecular or intermolecular S-S or a mixed disulfide [
31] (see S-S and
S-glutathionylation sections below). The ability of sulfenic acid to act as an electrophile has also been exploited by using its reactivity towards 5,5-dimethyl-1,3-cyclohexanedione (dimedone). This highly selective reaction has been used to develop chemoselective dimedone-based probes enabling the detection of sulfenylated proteins in cells [
55]. More recently, a more reactive benzo[c][
1,
2]thiazine-based (BTD) probe [
56] was used to identify the
Arabidopsis thaliana (Arabidopsis) sulfenome [
57]. Chemoselective methods for surveying and identifying the different levels of Cys thiol oxidation have been recently reviewed [
58].