Site-specific covalent metalation of DNA oligonucleotides with phosphorescent platinum(ii) complexes

Abstract

Phosphorescent Pt(II) complexes, composed of a tridentate N^N^C donor ligand and a monodentate ancillary ligand, were covalently attached to DNA oligonucleotides. Three modes of attachment were investigated: positioning the tridentate ligand as an artificial nucleobase via a 2'-deoxyribose or a propane-1,2-diol moiety and orienting it towards the major groove by appending it to a uridine C5 position. The photophysical properties of the complexes depend on the mode of attachment and on the identity of the monodentate ligand (iodido vs. cyanido ligand). Significant duplex stabilization was observed for all cyanido complexes when they are attached to the DNA backbone. The luminescence strongly depends on whether a single or two adjacent complexes are introduced, with the latter showing an additional emission band indicative of excimer formation. The doubly platinated oligonucleotides could be useful as ratiometric or lifetime-based oxygen sensors, as the green photoluminescence intensities and average lifetimes of the monomeric species are drastically boosted upon deoxygenation, whereas the red-shifted excimer phosphorescence is nearly insensitive to the presence of triplet dioxygen in solution.

Chart 1. (a) Structure of the Pt(II) complexes used in this study (X = I, CN) and their different modes of attachment to the oligonucleotide via (b) a GNA moiety, (c) a 2′-deoxyribose, (d) C5-modified 2′-deoxyuridine.

Scheme 1. Synthesis of the Pt(II) precursor complexes PtI and PtCN for the subsequent cycloaddition to a suitably alkyne-modified oligonucleotide. (a) NIS, Pd(OAc)₂, CH₃CN, 90 °C, 2 d; (b) Pt₂(dba)₃, THF, 50 °C, 90 min; (c) KCN, CH₃OH/CH₃CN (1 : 1), 90 °C, 3 h. For further synthetic details, see ESI.

Fig. 1. (a) Melting curves and (b) CD spectra of duplexes III^Tri,GNA, III^PtCN,GNA, III^Tri,DNA and III^PtCN,DNA. Broken lines represent the use of the GNA linker, solid lines of the DNA linker. Data for PtCN-containing DNA are shown in red, for the Pt(II)-free reference strands in black. (c) Melting curves of duplexes I^PtCN,DNA (dotted line) and V (solid line). The inset shows the corresponding CD spectra. Experimental conditions: 1 μM DNA duplex, 5 mM MOPS buffer (pH 7.0), 150 mM NaClO₄, 2.5 mM Mg(ClO₄)₂.

Fig. 2. Photoluminescence spectra and relative intensities of the monomers (with respect to the red-shifted excimers) of doubly platinated (a) duplex V and (b) single-stranded ODN3. Black solid line: air-equilibrated; red solid line: Ar-purged; black dotted line: O₂-saturated. Intensities normalized for clarity at (a) 625 nm and (b) 620 nm.