Utilization of Phosphorogenic Iridium(III) Nitrone Complexes as Synthetic Probes for the SNAP-tag Protein

The SNAP-tag is a self-labeling protein that is used to study the expression and functions of proteins of interest (POI) in live cells. This protein relies on the specific reaction with synthetic probes containing a benzylguanine (BG) moiety. Despite the large number of organic probes developed, luminogenic probes with high signal-to-noise ratio are limited. In this study, a new class of phosphorogenic iridium(III) nitrone complexes are designed for two-step labeling of SNAP-tag to confer luminogenic properties and controllable singlet oxygen ( 1 O2) generation on the system. In this strategy, a strained alkyne-modified substrate BCN-BG was used to label SNAP-tag, which was then reacted with the iridium(III) nitrone complexes. These complexes were weakly emissive with minimized 1 O2 generation due to quenching of the nitrone moiety. However, they displayed a significant increase in emission intensity upon reactions with the strained alkyne BCN-OH (ca. 7.2 – 47.1) and BCN-modified BSA (ca. 82.9 – 327.1). The emission enhancement was associated with significant 1 O2 generation (Φ∆ up to 0.91) of the complexes. Confocal images revealed that CHO-K1 cells expressing organelle-specific SNAP-tag exhibited intense and long-lived emission only in presence of both BCN-BG and the complexes.