The present invention provides compounds of Formula I: ##STR00001## wherein: R.sub.1 is a label (e.g., a detectable groups; an anti-tumor agent)s; L is present or absent and when present is a linking group; and x represents an integer from 1 to 10; or a pharmaceutically acceptable salt thereof. The compounds are useful for, among other things, identifying cysteine sulfenic acids in proteins and monitoring oxidative damage in proteins and cells.