Structural Manipulation of Ruthenium(II) Polypyridine Nitrone Complexes to Generate Phosphorogenic Bioorthogonal Reagents for Selective Cellular Labeling.

We report a new class of ruthenium(II) polypyridine complexes functionalized with a nitrone group as phosphorogenic bioorthogonal probes. These complexes were very weakly emissive owing to rapid C=N isomerization of the nitrone moiety, but exhibited significant emission enhancement upon strain-promoted alkyne-nitrone cycloaddition (SPANC) reaction with bicyclo[6.1.0]nonyne (BCN)-modified substrates. The modification of nitrone with a dicationic ruthenium(II) polypyridine unit at the α-C-position and a phenyl ring at the N-position led to remarkably accelerated reaction kinetics, which are substantially greater (up to ≈278 fold) than those of other acyclic nitrone-BCN systems. Interestingly, the complexes achieved specific cell membrane/cytosol staining upon specific labeling of an exogenous substrate, BCN-modified decane (BCN-C10), in live cells. Importantly, the in situ generation of the more lipophilic isoxazoline adduct in the cytoplasm resulted in increased cytotoxicity, highlighting a novel approach to apply the SPANC labeling technique in drug activation.