The use of 3-bromopropionic acid for the determination of protein thiol groups.

S-Carboxymethylcysteine, formed by the reaction of iodoacetic acid with cysteine, was found to undergo intramolecular cyclization to yield 3-oxo-(2H,3H,5H,6H-1,4-thiazine)-5-carboxylic acid. The cyclization was studied under various conditions and the product was isolated and characterized. S-Carboxyethylcysteine, formed by the reaction of 3-bromopropionic acid with cysteine, did not undergo the cyclization reaction. The use of 3-bromopropionic acid was examined as an alternative to iodoacetic acid for the protection and determination of protein thiol groups.