A Synthetic Camel Anti‐Lysozyme Peptide Antibody (Peptibody) with Flexible Loop Structure Identified by High‐Resolution Affinity Mass Spectrometry

We describe the synthesis and characterisation of the fully functional molecular recognition structure of a 26‐amino acid residue peptide antibody, referred to as peptibody , designed from a monoclonal single‐domain antibody fragment derived from a camel heavy‐chain antibody. The CDR3 region (CDR=complementarity determining region) of the cAbLys3 camel antibody fragment, which binds to the active site of hen eggwhite lysozyme (HEL) and acts as a potent enzyme inhibitor by mimicking an oligosaccharide substrate, was prepared by solid‐phase peptide synthesis. To obtain a closed loop‐like structure resembling that in the crystal structure, N‐ and C‐terminal cysteine residues were added to the linear peptide and oxidised to a cyclic disulfide‐bridged peptide by using dimethylsulfoxide. A further, internal cysteine‐12 residue was acetamidomethyl‐protected to prevent possible oxidative byproducts. Affinity separation on a lysozyme microcolumn combined with MALDI‐TOF mass spectrometry revealed that the peptide resumed high affinity to lysozyme only after deprotection of Cys‐12, suggesting the importance of this paratope sequence for epitope recognition. The complex of lysozyme and active peptibody was characterised directly by conducting high‐resolution ESI‐FTICR mass spectrometry, which provided a molecular comparison of affinities for linear and cyclic peptibodies.