Tryptophan modification is currently an unsolved area of bioconjugate chemistry, with its low relative abundance in proteins and poor nucleophilicity make it an ideal, yet difficult target. As a result, previous strategies for modification of Trp typically require harsh conditions or extended reaction times which yielded moderate conversion. We report a method for bioconjugation of Trp that utilizes the readiness of the indole side-chain to undergo photo-induced electron transfer (PET) in the presence of an electron acceptor. Specifically, our method employs <i>N</i>-carbamoyl pyridinium salts as Trp-selective electron acceptors, which is proposed to occur <i>via</i> PET from the Trp residue to the pyridinium salt under UV light irradiation. PET then triggers a fragmentation of the N-N bond, resulting in the transfer of the labelling group to the Trp residue. The modification of several peptides and proteins have shown Trp selectivity with varying structural complexity, with various functional group transfers from the parent pyridinium. The selectivity for Trp over other redox active or photo-labile amino acids has been confirmed using tandem MS experiments. The reaction conditions are tolerant of a variety of concentrations of biomolecule (10µM-5mM), do not require organic cosolvent, show high conversion at short reaction times (&lt;30 min), and are enhanced with the addition of glutathione. These characteristics make it a viable method for modifying Trp residues selectively, with applications in fields such as antibody drug conjugates synthesis, and cellular labeling.

Site-selective modification of tryptophan residues in peptides and proteins using redox active pyridinium salts and light

2019 Fall National Meeting

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