Fluorescence labelling of DNA by carboxylic polypyridyl-Ru complexes containing bpy and DIP ligands: a study revisited.

The coordination complexes (DIP)(2)Ru(CH(3)bpyCOOH) and (DIP)(2)Ru(COOHbpyCOOH), where DIP and bpy are diphenylphenanthroline and bispyridine, have been recently proposed as fluorescent markers of nuclear DNA (Musatkina et al., J. Inorg. Biochem. 101:1086-1089, 2007), but no DNA binding investigation and no quantitative fluorescence evaluations had been done. Both complexes, as well as the smaller ones with bpy's in place of DIP's, have been investigated here by spectroscopic DNA titrations (UV-vis absorption, fluorescence, circular dichroism) and by in vitro cellular studies (flow cytometry and fluorescence imaging). Contrary to previous reports, neither the carboxylic function nor the more extended DIP ligand ensures any appreciable binding to DNA. This is clearly illustrated by the appearance of an isosbestic point of a second kind and by the proportionality of the fluorescence maximum intensity to the absorbance at the excitation wavelength. Above all, the lack of enhanced fluorescence in the presence of DNA definitively rules out the use of such complexes as DNA markers. Moreover, there is no detectable nuclear uptake. However, the fluorescent complexes with the DIP ligands, especially (DIP)(2)Ru(CH(3)bpyCOOH), are massively incorporated into the cytoplasm while preserving cell integrity, which could suggest other types of biological application.