Assessment of 1, 2, and 6 as probes for cysteine sulfenic acid in HeLa cell lysates. Probes were added to the lysates as solutions in DMSO before the addition of H 2 O 2 . The reactions were carried out for 1 hour at room temperature. All samples were purified using a size exclusion spin column before SDS-PAGE analysis and western blotting. Probe 2 was modified with a biotin-containing azide before SDS-PAGE. Western blots were developed using Alexa Fluor 555 streptavidin conjugate.