(2-pyridyldithio)-PEG4-propargyl is a PEG derivative which contains a pyridyl disulfide moiety and a propargyl group. The pyridyl disulfide group can react with thiol groups of proteins over a wide range of pH levels. The propargyl group can be reacted with azide-bearing compounds or biomolecules via copper catalyzed azide-alkyne Click Chemistry to yield a stable triazole linkage. The hydrophilic PEG linker increases the water solubility of compounds in aqueous media.