PEG Hydrazide linkers, Hydrazide reagents is a class of labeling reagents containing a very reactive Hydrazide group which are reactive towards aldehyde, ketones, and carboxyl moieties. Of particular interest is hydrazone bond formation between aldehydes created by periodate-oxidation of sugars in biological samples and PEG linkers at a pH range of 5 to 7. These hydrazone bonds are considerably more stable than a Schiff base formed with a simple amine, which makes them suitable for most protein-labeling applications. The PEG space of the reagent increases the hydrophilicity and fine-tunes the properties of the PEGylated molecules.