Aldehydes are common in carbohydrates (latent, either free or as hemi-aminal), carbohydrate containing proteins, oxidizable matrices, most native antibodies and many enzymes - as in HRP. Amines from solvent accessible lysines in any proteins can either be use for conjugation via active esters or also be converted to aldehydes and used for conjugation purposes using Phthalimidooxy PEG-based cross linkers. They form stable oximes with aldehydes and their reactivity is sufficiently mild, for maintaining activity and tertiary structure of sensitive biologics.

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• Versatile and efficient synthesis of protein-polysaccharide conjugate vaccines using aminooxy reagents and oxime chemistry; A. Lees, G. Sen, A. Lopez Acosta; Vaccine 2006; 24: 716-729.