Figure 5.

Protein cyclization utilizing unnatural amino acids: (a) Schematic overview of protein 21 with an unnatural amino acid (uaa) which undergoes an intramolecular reaction with a natural amino acid to form cyclic protein 22. (b) Overview of unnatural amino acids used for protein cyclization: O‐(2‐mercaptoalkyl)‐L‐tyrosine (21 a), (S)‐2‐amino‐6‐(6‐bromohexanamido)‐hexanoic acid (21 b), O‐2‐bromoethyl tyrosine (21 c) and (p‐benzoylphenyl)alanine (21 d). They react with ‐SH (cysteine) or ‐YH (cysteine, histidine or lysine) to form the new bridges 22 a–d.