Iodoacetic Acid (IAA) can react with several protein functional groups but is typically used for specific S-carboxymethylation of sulfhydryls (reduced cysteines).
Iodoacetic acid reacts with sulfhydryls on cysteines, imidazolyl side chain nitrogens of histidines, the thioether of methionine and the primary amine group of lysines.The rate of reaction and specificity is dependent on the ionization level, which can be manipulated by the pH of the reaction condition.
React at slightly alkaline pH for specific S-carboxymethylation of free sulfhydryls
React at low pH for specific carboxymethylation of methionines
React at high pH to favor carboxymethylation of histidines and lysines
Methylate reduced cysteine peptide fragments in protease digests for mass spectrometry