A procedure for alkylation of cysteine residues with sodium 2-bromoethanesulfonate is described. The reaction is performed under mild conditions and complete modification is obtained without side reactions.

S-Sulfoethylated proteins are comparable to performic acid oxidized or S-sulfonated proteins with respect to solubility properties and behaviour in ion exchange chromatography. S-Sulfoethylcysteine was found to be stable during acid hydrolysis. S-Sulfoethylated peptides were suitable for automatic Edman degradation due to their polarity, since losses during extraction are reduced, especially after the coupling stage, as shown by degradation of modified peptides.