Metallation-Induced Heterogeneous Dynamics of DNA Revealed by Single-Molecule FRET.

The metallation of nucleic acids is key to wide-ranging applications, from anticancer medicine to nanomaterials, yet there is a lack of understanding of the molecular-level effects of metallation. Here, we apply single-molecule fluorescence methods to study the reaction of an organo-osmium anticancer complex and DNA. Individual metallated DNA hairpins are characterised using Förster resonance energy transfer (FRET). Although ensemble measurements suggest a simple two-state system, single-molecule experiments reveal an underlying heterogeneity in the oligonucleotide dynamics, attributable to different degrees of metallation of the GC-rich hairpin stem. Metallated hairpins display fast two-state transitions with a two-fold increase in the opening rate to ≈2 s-1 , relative to the unmodified hairpin, and relatively static conformations with long-lived open (and closed) states of 5 to ≥50 s. These studies show that a single-molecule approach can provide new insight into metallation-induced changes in DNA structure and dynamics.

Mitochondria-targeted spin-labelled luminescent iridium anticancer complexes.

Mitochondria generate energy but malfunction in many cancer cells, hence targeting mitochondrial metabolism is a promising approach for cancer therapy. Here we have designed cyclometallated iridium(iii) complexes, containing one TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) spin label [C43H43N6O2Ir1·PF6]˙ (Ir-TEMPO1) and two TEMPO spin labels [C52H58N8O4Ir1·PF6]˙ (Ir-TEMPO2). Electron paramagnetic resonance (EPR) spectroscopy revealed spin-spin interactions between the TEMPO units in Ir-TEMPO2. Both Ir-TEMPO1 and Ir-TEMPO2 showed bright luminescence with long lifetimes (ca. 35-160 ns); while Ir-TEMPO1 displayed monoexponential decay kinetics, the biexponential decays measured for Ir-TEMPO2 indicated the presence of more than one energetically-accessible conformation. This observation was further supported by density functional theory (DFT) calculations. The antiproliferative activity of Ir-TEMPO2 towards a range of cancer cells was much greater than that of Ir-TEMPO1, and also the antioxidant activity of Ir-TEMPO2 is much higher against A2780 ovarian cancer cells when compared with Ir-TEMPO1. Most notably Ir-TEMPO2 was particularly potent towards PC3 human prostate cancer cells (IC50 = 0.53 µM), being ca. 8× more active than the clinical drug cisplatin, and ca. 15× more selective towards cancer cells versus normal cells. Confocal microscopy showed that both Ir-TEMPO1 and Ir-TEMPO2 localise in the mitochondria of cancer cells.