Amino-PEG4-CH2CO2H is a PEG derivative containing an amino group with a terminal carboxylic acid. The hydrophilic PEG spacer increases solubility in aqueous media. The amino group is reactive with carboxylic acids, activated NHS esters, carbonyls (ketone, aldehyde) etc. 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.