t-Boc-N-amido-PEG1-CH2CO2H is a PEG derivative containing a terminal carboxylic acid and Boc-protected amino group. The hydrophilic PEG spacer increases solubility in aqueous media. 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 Boc group can be deprotected under mild acidic conditions to form the free amine.