Cysteine is commonly modified due to the ease and reversibility of some of the sulfhydryl chemistry and because the amino acid is often capped to protect its sidechain from oxidation and crosslinking. There are countless modifiers used, but usually they fall into the classes described below:
Used for crosslinking proteins to each other with bi-functional modifiers and attaching proteins to solid supports.
Commonly used for capping sulfhydryls and functional studies of free Cys sidechains.
Used to quantify free sulfhydryls; after the reaction an aromatic ion is released which absorbs at 412 nm.
Commonly used to poison sulfhydryl sidechains and for heavy atom labeling in crystallography.