Conjugation of Peptides to Carrier Protein via Carbodiimide
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One of the most commonly used carbodiimide reagents is the water-soluble reagent, 1-ethyl-3-(dimethylaminopropyl) carbodiimide, also known as EDAC. Like other carbodiimides, EDAC couples an amino group to a carboxyl group (although side reactions involving cysteine sulfhydryl and tyrosine aryl hydroxyl are also reported). For peptide conjugation, usually the carboxyl group comprises the C-terminal of the peptide, and the amino group is an ɛ-amino group on a lysine residue contained in the carrier protein. Thus, peptides with more than one carboxyl group (i.e., peptides containing aspartate or glutamate) are not recommended for EDAC conjugation. Furthermore, the peptide preparations should be quite free of residual acetic acid and trifluoroacetic acid from synthesis, cleavage, and work-up.