From model compounds to protein binding: Syntheses, characterizations and fluorescence studies of [Ru^II(bipy)(terpy)L]²⁺ complexes (bipy = 2,2′-bipyridine; terpy = 2,2′:6′, 2″-terpyridine; L = imidazole, pyrazole and derivatives, cytochrome c)

Compounds [Ru^II(bipy)(terpy)L](PF₆)₂ with bipy = 2,2′-bipyridine, terpy = 2,2′:6′,2″-terpyridine, L = H₂O, imidazole (imi), 4-methylimidazole, 2-methylimidazole, benzimidazole, 4,5-diphenylimidazole, indazole, pyrazole, 3-methylpyrazole have been synthesized and characterized by ¹H NMR, ESI-MS and UV/Vis (in CH₃CN and H₂O). For L = H₂O, imidazole, 4,5-diphenylimidazole and indazole the X-ray structures of the complexes have been determined with the crystal packing featuring only few intermolecular C-H⋯π or π-π interactions due to the separating action of the PF₆-anions. Complexes with L = imidazole and 4-methylimidazole exhibit a fluorescence emission with a maximum at 662 and 667 nm, respectively (λ_exc = 475 nm, solvent CH₃CN or H₂O). The substitution of the aqua ligand in [Ru(bipy)(terpy)(H₂O)] ²⁺ in aqueous solution by imidazole to give [Ru(bipy)(terpy)(imi)]²⁺ is fastest at a pH of 8.5 (as followed by the increase in emission intensity). Coupling of the [Ru(bipy)(terpy)] ²⁺ fragment to cytochrome c (Yeast iso-1) starting from the Ru-aqua complex was successful at 35°C and pH 7.0 after 5 d under argon in the dark. The [Ru(bipy)(terpy)(cytc)]-product was characterized by UV/Vis, emission and mass spectrometry. The location where the [Ru(bipy)(terpy)] complex was coupled to the protein was identified as His44 (corresponding to His39 in other numbering schemes) using digestion of the Ru-coupled protein by trypsin and analysis of the tryptic peptides by HPLC-high resolution MS.