Interaction of chiral lanthanide complexes with nucleic acids
Enantiopure A and A lanthanide complexes, bearing a phenanthridinium or a dipyridoquinoxaline chromophore as a sensidser, have been designed with the aim of developing structural and reactive probes for nucleic acids. Their interaction with DNA was studied using various spectroscopic techniques. A certain degree of stereoselectivity in DNA binding was discerned. The A enantiomers of the Eu tetramide and of the EuPh(_3)dpq complexes interacted with nucleic acids in a predominantly intercalative binding mode, by inserting their planar aromatic chromophore between the base-pairs. The former showed a preference for C-G sites, while the latter bound preferentially to A-T base-pairs. A rather different binding mode, probably involving the minor groove, was revealed in the interaction of the A enantiomer of the EuPh(_3)dpq complex with nucleic acids, with a higher affinity for C and G bases. In the presence of nucleic acid, a charge transfer process occurred in each case, which quenched the singlet excited state of the phenanthridinium moiety or the lanthanide excited state (in Ph(_3)dpq complexes). In the unique case of the EuNp(_3)dpq complexes, the interaction resulted in an increase in the metal emission intensity and lifetime, as a consequence of the protection of the molecule, probably accommodated in the DNA minor groove, from a quenching process. This light switch' effect can be exploited in the development of spectroscopic probes. The TbNp(_3)dpq, on the other hand, was found to generate singlet oxygen efficiently and could therefore act as a reactive probe. Moreover, the EuPh(_3)dpq and TbPh(_3)dpq complexes showed extraordinarily high emission quantum yields in aqueous media, due to the favourable photophysical properties of the dpq antenna as well as the nonadentate nature of the Ph(_3)dpq ligand. This makes them valuable luminescent probes.