Tos-PEG4-acid is a PEG linker containing a tosyl group and a terminal carboxylic acid. The hydrophilic PEG spacer increases solubility in aqueous media. The tosyl group is a very good leaving group for nucleophilic substitution reactions. The terminal carboxylic acid can react with primary amine groups in the presence of activators (e.g. EDC, or HATU) to form a stable amide bond.
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