DBCO PEG linkers, DBCO reagents is a class of labeling reagents containing a very reactive DBCO ((Dibenzocyclooctyne) group, DBCO linkers is reactive toward azide without Cu catalyst. The reaction can be run either in organic solvent or in aqueous buffer depending on the solubility and property of substrate molecules. The PEG space of the reagent increases the hydrophilicity and fine-tunes the properties of the PEGylated molecules.