Synthesis, Properties, and Live-Cell Imaging Studies of Luminescent Cyclometalated Iridium(III) Polypyridine Complexes Containing Two or Three Biotin Pendants

• Published in: Inorganic chemistry Vol. 48; no. 13; pp. 6011 - 6025
• Main Authors: Zhang, Kenneth Yin, Lo, Kenneth Kam-Wing
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 06-07-2009
• Subjects: Biotin - chemistry; Chromatography, Gel; Chromatography, High Pressure Liquid; Iridium - chemistry; Microspheres; Oxidation-Reduction; Pyridines - chemical synthesis; Pyridines - chemistry
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic900412n

Three luminescent cyclometalated iridium(III) bis-biotin complexes [Ir(N∧C)2(N∧N)](PF6) (HN∧C = 2-(4-(N-(6-(biotinamido)hexyl)aminomethyl)phenyl)pyridine, HppyC6B, N∧N = 2,2′-bipyridine, bpy (1); HN∧C = 2-phenylpyridine, Hppy, N∧N = 4,4′-bis((2-(biotinamido)ethyl)aminocarbonyl)-2,2′-bipyridine, bpyC2B2 (2); HN∧C = Hppy, N∧N = 4,4′-bis((2-((6-(biotinamido)hexanoyl)amino)ethyl)aminocarbonyl)-2,2′-bipyridine, bpyC2C6B2 (3)) and one tris-biotin complex [Ir(ppyC6B)2(bpyC6B)](PF6) (bpyC6B = 4-((6-(biotinamido)hexyl)aminocarbonyl)-4′-methyl-2,2′-bipyridine) (4) have been synthesized and characterized. The biotin-free complex [Ir(ppy)2(bpyC4)](PF6) (bpyC4 = 4,4′-bis(n-butylaminocarbonyl)-2,2′-bipyridine) (5) has also been prepared for comparison studies. Upon photoexcitation, all the complexes displayed intense and long-lived greenish-yellow to red triplet metal-to-ligand charge-transfer (3MLCT) (dπ (Ir) → π*(N∧N)) emission in fluid solutions at room temperature and in low-temperature glass. Cyclic voltammetric studies revealed iridium(IV/III) oxidation at about +1.21 to + 1.29 V and diimine-based reductions at about −1.07 to −1.39 V versus SCE. The interactions of the bis-biotin and tris-biotin complexes with avidin have been studied by 4′-hydroxyazobenzene-2-carboxylic acid (HABA) assays, emission titrations, and dissociation assays. The possibility of these complexes as cross-linkers for avidin has been examined by microscopy studies using avidin-conjugated green fluororescent microspheres and size-exclusion HPLC analysis. Utilization of these luminescent iridium(III) biotin complexes in signal amplification has been demonstrated using avidin-coated nonfluorescent microspheres and complex 3 as an example. Additionally, the lipophilicity of all the complexes has been determined by reversed-phase HPLC. The cytotoxicity of these iridium(III) complexes toward the human cervix epithelioid carcinoma (HeLa) cell line has been evaluated by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide (MTT) assays. Furthermore, the cellular uptake of the complexes has been examined by ICP-MS, laser-scanning confocal microscopy, and flow cytometry.