Cbz-N-amido-PEG4-acid is a PEG derivative containing a CBZ-protected amino group and a terminal carboxylic acid. The terminal carboxylic acid can be reacted with primary amine groups in the presence of activators (e.g. EDC, or DCC) to form a stable amide bond. The hydrophilic PEG spacer increases solubility in aqueous media. The protected amine can be deprotected by acidic conditions.