Preparation and Biological Evaluation of Di-Hetero-Organometallic-Containing PNA Bioconjugates



Two peptide nucleic acid (PNA) oligomers containing two different organometallic moieties, namely derivatives of azidoferrocene (Fc-N3) and ferrocene carboxylic acid (Fc-COOH) for PNA5 and derivatives of β-cymantrenoylpropionic acid [Cym-CO(CH2)2COOH] and the rhenium bisquinoline tricarbonyl complex [Re(BQ-N3)(CO)3]Br for PNA6, have been prepared on a solid support by using two different synthetic methods: Peptide coupling and click chemistry. PNA5 and PNA6 were unambiguously characterized by ESI-MS or MALDI-TOF MS and their purity checked by LC–MS. As expected, the bioconjugate PNA6 presents two broad and strong absorption bands at 1933 and 2032 cm–1 in its IR spectrum due to the presence of six metal–carbonyl bonds. It has also been demonstrated that the presence of the Mn complex in PNA6 did not significantly alter the fluorescence properties of the Re complex in aqueous solution compared with a previously reported Re–PNA bioconjugate (PNA7). Photoexcitation of PNA6 at 350 nm reveals two distinct emission bands at about 434 and 595 nm in aqueous solutions. PNA6 was successfully electroporated into HeLa cells, as shown by high-resolution continuum-source atomic absorption spectroscopy, which was used to measure the concentrations of both Re and Mn. However, no fluorescence of the Re complex in PNA6 was observed in living cells, even at a concentration 20 times higher than that previously reported for PNA7.