Sterically Shielded, Stabilized Nitrile Imine for Rapid Bioorthogonal Protein Labeling in Live Cells

In pursuit of fast bioorthogonal
reactions, reactive moieties have
been increasingly employed for selective labeling of biomolecules
in living systems, posing a challenge in attaining reactivity without
sacrificing selectivity. To address this challenge, here we report
a bioinspired strategy in which molecular shape controls the selectivity
of a transient, highly reactive nitrile imine dipole. By tuning the
shape of structural pendants attached to the ortho position of the N-aryl ring of diaryltetrazoles−precursors of nitrile
imines, we discovered a sterically shielded nitrile imine that favors
the 1,3-dipolar cycloaddition over the competing nucleophilic addition.
The photogenerated nitrile imine exhibits an extraordinarily long
half-life of 102 s in aqueous medium, owing to its unique molecular
shape that hinders the approach of a nucleophile as shown by DFT calculations.
The utility of this sterically shielded nitrile imine in rapid (∼1
min) bioorthogonal labeling of glucagon receptor in live mammalian
cells was demonstrated.