Rhenium(I) polypyridine dibenzocyclooctyne complexes as phosphorescent bioorthogonal probes: Synthesis, characterization, emissive behavior, and biolabeling properties

• Phosphorescent rhenium(I) complexes containing a DIBO unit have been synthesized.
• The complexes exhibit interesting photophysical properties and cytotoxic activity.
• The complexes undergo facile reactions with azide derivatives.
• The complexes serve as bioorthogonal probes for cells treated with Ac4ManNAz.

We report the development of rhenium(I) polypyridine complexes appended with a dibenzocyclooctyne (DIBO) moiety as bioorthogonal probes for azide-modified biomolecules. Three phosphorescent rhenium(I) polypyridine DIBO complexes [Re(N^N)(CO)3(py-C6-DIBO)][CF3SO3] (py-C6-DIBO = 3-(N-(6-(3,4:7,8-dibenzocyclooctyne-5-oxycarbonylamino)hexyl)aminocarbonyl)pyridine; N^N = 1,10-phenanthroline (phen) (1a), 3,4,7,8-tetramethyl-1,10-phenanthroline (Me4-phen) (2a), 4,7-diphenyl-1,10-phenanthroline (Ph2-phen) (3a)) and their DIBO-free counterparts [Re(N^N)(CO)3(py-C6-BOC)][CF3SO3] (py-C6-BOC = 3-(N-(6-(tert-butoxycarbonylamino)hexyl)aminocarbonyl)pyridine; N^N = phen (1b), Me4-phen (2b), Ph2-phen (3b)) were synthesized and characterized. Upon photoexcitation, all the complexes displayed intense and long-lived yellow triplet metal-to-ligand charge-transfer (3MLCT) (dπ(Re) → π*(N^N)) emission. The DIBO complexes underwent facile reactions with benzyl azide in methanol at 298 K with second-order rate constants (k2) in the range of 0.077 to 0.091 M− 1 s− 1. As revealed from SDS-PAGE analysis, the DIBO complexes can selectively label azide-modified proteins and the resulting bioconjugates displayed strong phosphorescence upon photoexcitation. Results of inductively coupled plasma mass spectrometry (ICP-MS) and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays indicated that the DIBO complexes accumulated in Chinese Hamster Ovary (CHO) cells with considerable cytotoxic activity. Upon incubation of CHO cells with these complexes, relatively weak intracellular emission was observed. In contrast, upon pretreatment of the cells with 1,3,4,6-tetra-O-acetyl-N-azidoacetyl-d-mannosamine (Ac4ManNAz), intense emission was observed from the cell membrane and some internal compartments. The results suggest that the DIBO complexes are promising candidates for imaging azide-labeled biomolecules.

A series of rhenium(I) polypyridine dibenzocyclooctyne (DIBO) complexes was developed as a new class of phosphorescent labels for azide-modified biomolecules. Various experiments revealed that these complexes can specifically react with azide-containing compounds. One of the DIBO complexes was used to image CHO cells pretreated with 1,3,4,6-tetra-O-acetyl-N-azidoacetyl-d-mannosamine (Ac4ManNAz).Figure optionsDownload as PowerPoint slide