Squaric Acid Ester Amido Polyethylene Glycols are a novel class of reactive  and chemoselective PEGylation reagents. The squaric acid PEG´s are chemoselective towards the amino groups of Lysin in protein PEGylation whereas the guanidine function of Arginine, the hydroxyl group of Serine and the imidazole function of Histidine are not affected.

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