4-(3-tosyl-2-(tosylmethyl)propanoyl)benzoic acid is a strong covalent linker featuring a free carboxylic acid and two tosyl groups. Each tosyl group can be displaced by thiol or amine nucleophiles via a Michael addition, and the inclusion of two on this molecule allow this reaction to proceed twice. This may be used to "staple" two reduced cysteine residues on a given protein to reform disulfide bridges. The carboxylic acid is free to react to form amides or esters.
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