Enzymatic Late‐Stage Halogenation of Peptides

The late‐stage site‐selective derivatisation of peptides has many potential applications in structure‐activity relationship studies and postsynthetic modification or conjugation of bioactive compounds. The development of orthogonal methods for C−H functionalisation is crucial for such peptide derivatisation. Among them, biocatalytic methods are increasingly attracting attention. Tryptophan halogenases emerged as valuable catalysts to functionalise tryptophan (Trp), while direct enzyme‐catalysed halogenation of synthetic peptides is yet unprecedented. Here, it is reported that the Trp 6‐halogenase Thal accepts a wide range of amides and peptides containing a Trp moiety. Increasing the sequence length and reaction optimisation made bromination of pentapeptides feasible with good turnovers and a broad sequence scope, while regioselectivity turned out to be sequence dependent. Comparison of X‐ray single crystal structures of Thal in complex with d‐Trp and a dipeptide revealed a significantly altered binding mode for the peptide. The viability of this bioorthogonal approach was exemplified by halogenation of a cyclic RGD peptide.