Direct Enzymatic Synthesis of a Deep‐Blue Fluorescent Noncanonical Amino Acid from Azulene and Serine

We report a simple, one-step enzymatic synthesis of the blue fluorescent noncanonical amino acid β-(1-azulenyl)-l-alanine (AzAla). By using an engineered tryptophan synthase β-subunit (TrpB), stereochemically pure AzAla can be synthesized at scale starting from commercially available azulene and l-serine. Mutation of a universally conserved catalytic glutamate in the active site to glycine has only a modest effect on native activity with indole but abolishes activity on azulene, suggesting that this glutamate activates azulene for nucleophilic attack by stabilization of the aromatic ion.