N3-PEG3-CH2COOtBu

Azido-PEG3-CH2CO2-t-Bu is a N3 (azide) containing t-butyl ester. The hydrophilic PEG spacer increases solubility in aqueous media. The azide group can react with alkyne, BCN, DBCO via Click Chemistry to yield a stable triazole linkage. The t-butyl protected carboxyl group can be deprotected under acidic conditions.