Selective Modification of Tryptophan Residues in Peptides and Proteins Using a Biomimetic Electron Transfer Process.

We report here a photochemical process for the selective modification of tryptophan (Trp) residues in peptides and small proteins using electron-responsive -carbamoylpyridinium salts and UV-B light. Preliminary mechanistic experiments suggest that the photoconjugation process proceeds through photoinduced electron transfer (PET) between Trp and the pyridinium salt, followed by fragmentation of the pyridinium N-N bond and concomitant transfer of this group to Trp. The reaction displays excellent site selectivity for Trp and is tolerant to other, redox-active amino-acid residues. Moreover, the reaction proceeds in pure aqueous conditions without the requirement of organic cosolvents or photocatalysts, is enhanced by glutathione, and operates efficiently over a wide range of peptide concentrations (10-700 μM). The scope of the process was explored through the labeling of 6-Trp-containing peptides and proteins ranging from 1 to 14 kDa. We demonstrate the versatility of the -carbamoylpyridinium salt both by tuning the electrochemical and photochemical properties of the pyridinium scaffold to enable challenging photoconjugation reactions and by using the carbamoyl moiety to tether a plethora of productive functional groups, including reactive handles, purification tags, and removable protecting groups.