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Acid-PEG4-t-butyl ester is a PEG linker containing a t-butyl protected carboxyl group with a terminal carboxylic acid. 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. The hydrophilic PEG spacer increases solubility in aqueous media. The t-butyl protected carboxyl group can be deprotected under acidic conditions.