Thiophenylalanine: A novel handle for peptide synthesis, stapling and protein labelling.

Chemoselective functionalisation of peptides and bio-conjugation of proteins has led to great advances in the synthesis of clinically relevant biomolecules and our understanding of cellular processes.

Thiophenylalanine represents an interesting handle owing to the increased acidity of the aryl thiol which would mean a higher percentage of reactive thiolate would persist even at physiological pH, allowing chemoselective reactions to be performed.

Despite the interesting qualities of the residue, there is a notable gap in its use in peptide and protein chemistry which can be attributed to the difficult and harsh conditions required involved in the synthesis, often incompatible with common protecting groups.

Herein, we exploit a mild, palladium catalysed method to obtain Fmoc-SPhe(Trt) in one step from commercially available reagents to fully explore the possibilities of incorporating the building block into peptide and protein synthesis.

Namely, we will demonstrate the use of SPhe 1) as a chemoselective method for peptide stapling, 2) to perform on-resin selective disulfide formation and attempt a regioselective one-pot double disulfide formation of a novel antimicrobial peptide, 3) as a method for discretely labelling protein residues through affinity-binding proximal transfer and 4) in the form of other aryl thiol building blocks to expand the scope of this work.

The development of a novel handle for bioconjugation will have an immense impact on the field of chemical biology and provide a tool for peptide analogue synthesis and elucidation of cellular processes.