Luminescent Conjugates between Dinuclear Rhenium Complexes and Peptide Nucleic Acids (PNA): Synthesis, Photophysical Characterization, and Cell Uptake

Different PNA decamers have been appended to a luminescent [Re-2(mu-Cl)(2)(CO)(6)(mu-1,2-diazine)] complex, to obtain conjugates suitable for cellular imaging. The new compounds can be dissolved in water in the presence of an amount of DMSO as small as 0.4-0.6 mol %. The conjugation with PNA did not perturb the photoluminescence behavior of the organometallic fragment: emission from (MLCT)-M-3 excited states, centered at ca. 610 nm, was observed, with satisfactory photoluminescence quantum yields (Phi = ca. 0.01, in aerated water). Moreover, the emission could be stimulated also by two-photon excitation. Experiments of cell uptake, performed with different cell lines and under different experimental conditions, showed that the nature of the PNA oligomer strongly affects the biointeraction, while no major differences were observed among the cell lines investigated. The Re-PNA conjugates containing neutral PNA decamers (either homothymine or a standard sequence of the four nucleobases) showed a marked tendency to concentrate in the nuclear region, whereas nucleus penetration was more difficult for the free dirhenium complex. Staining of nucleus and cytoplasm with different colors was generally observed. No nucleus penetration was instead observed for a water-soluble Re-PNA homothymine decamer end-capped with four lysine residues, which localized in endosome-like compartments.