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Keywords:

  • cholera toxin;
  • single-domain antibodies;
  • thermal stability

Variable heavy domains derived from the heavy-chain-only antibodies found in camelids (VHH domains) are known for their thermal stability. Here, the structure of A9, an anti-cholera toxin VHH domain (Kd = 77 ± 5 nM) that has an unusually low melting temperature of 319.9 ± 1.6 K, is reported. The CDR3 residues of A9 form a β-hairpin that is directed away from the former VH–VL interfacial surface, exposing hydrophobic residues to the solvent. A DALI structural similarity search showed that this CDR3 conformation is uncommon.