Fig. 1. Labeling of cysteine redox forms with chemoselective probes.

IPM is an iodoacetamide-based alkyne probe for cysteinyl thiol (–SH)⁶⁶; BTD is a benzothiazine-based alkyne probe for cysteine sulfenic acid (–SOH)²⁹; DiaAlk is a diazene-based alkyne probe for cysteine sulfinic acid (–SO₂H)¹⁹. The alkyne group can be conjugated to azide-bearing tags via click chemistry for detection and enrichment. In mammalian cells, –SO₂H within peroxiredoxins can be reduced by a sulfiredoxin⁶⁷. In C. elegans no known sulfiredoxin is present, and –SO₂H is presumably irreversible³³. Oxidation and reduction processes are depicted in red and blue arrows, respectively.