Heterobifunctional crosslinker enabling the connection of an azide containing compound or biomolecule to one with a primary amine. The succinimidyl ester group will react preferentially with primary amines to form a covalent adduct and the terminal alkyne will react with azides via copper-catalyzed 1,3-dipolar cycloaddition click chemistry. The hydrophilic PEG spacer adds to the water solubility of this reagent, allowing for easy modification amine containing biomolecules such as antibodies, proteins and peptides with the terminal alkyne. The PEG spacer also reduces aggregation of modified proteins stored in solution by adding hydrophilic character.