Ms-PEG2-t-butyl ester is a PEG derivative containing a mesyl group and a t-butyl protecting group. The mesyl group is a good leaving group for nucleophilic substitution reactions. The t-butyl is an acid labile protecting group. Upon removal, the carboxylic acid can be reacted with primary amines. The hydrophilic PEG linker increases the water solubility of the compound in aqueous media.