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Azide/azido functionalized polyethylene glycol (N3-PEG-X) is a bifunctional PEG derivative that can be used to modify proteins, peptides and other materials. Azide group can react with alkyne in aqueous solution catalyzed by copper. It can also be reduced into amine group easily while another functional group can be react with other molecules or functional groups. PEGylation can increase solubility and stability and reduce immunogenicity of peptides and proteins. It can also suppress the non-specific binding of charged molecules to the modified surfaces.