Alkylating reagents Iodoacetamide and chloroacetamide are thiol-reactive alkylating reagents used to block reduced cysteine residues for protein characterization and peptide mapping. Alkylation with iodoacetamide after reduction of cystine results in the covalent addition of a carbamidemethyl group (57.07 Da), preventing disulfide bond formation. The reducing agent added after alkylation will react with excess iodoacetamide.