T-Butyl ester-PEG4-CH2COOH

T-Butyl ester-PEG4-CH2COOH is a PEG linker characterized by the presence of a tert-butyl group and a terminal carboxylic acid. The terminal carboxylic acid can react with primary amine groups in the presence of an activator (e.g., HATU) to form a stable amide bond. The hydrophilic PEG spacer improves solubility in aqueous media. The tert-butyl group can be deprotected under acidic conditions.