Cleavage of DNA by Proton-Coupled Electron Transfer to a Photoexcited, Hydrated Ru(II) 1,10-Phenanthroline-5,6-dione Complex Steven A. Poteet, § Marek B. Majewski, † Zachary S. Breitbach, § Cynthia A. Griffith, § Shreeyukta Singh, § Daniel W. Armstrong, § Michael O. Wolf, † and Frederick M. MacDonnell* ,§ § Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas 76019, United States † Department of Chemistry, University of British Columbia, Vancouver, BC V6T 1Z1, Canada * S Supporting Information ABSTRACT: Visible light irradiation of a ruthenium(II) quinone-containing complex, [(phen) 2 Ru(phendione)] 2+ (1 2+ ), where phendione = 1,10-phenanthroline-5,6-dione, leads to DNA cleavage in an oxygen independent manner. A combination of NMR analyses, transient absorption spectroscopy, and fluorescence measurements in water and acetonitrile reveal that complex 1 2+ must be hydrated at the quinone functionality, giving [(phen) 2 Ru- (phenH 2 O)] 2+ (1H 2 O 2+ , where phenH 2 O = 1,10-phenan- throline-6-one-5-diol), in order to access a long-lived 3 MLCT hydrate state (τ ∼ 360 ns in H 2 O) which is responsible for DNA cleavage. In effect, hydration at one of the carbonyl functions effectively eliminates the low- energy 3 MLCT SQ state (Ru III phen-semiquinone radical anion) as the predominant nonradiative decay pathway. This 3 MLCT SQ state is very short-lived (<1 ns) as expected from the energy gap law for nonradiative decay, 1 and too short-lived to be the photoactive species. The resulting excited state in 1H 2 O 2+ * has photophysical properties similar to the 3 MLCT state in [Ru(phen) 3 ] 2+ * with the added functionality of basic sites at the ligand periphery. Whereas [Ru(phen) 3 ] 2+ * does not show direct DNA cleavage, the deprotonated form of 1H 2 O 2+ * does via a proton-coupled electron transfer (PCET) mechanism where the peripheral basic oxygen sites act as the proton acceptor. Analysis of the small molecule byproducts of DNA scission supports the conclusion that cleavage occurs via H-atom abstraction from the sugar moieties, consistent with a PCET mechanism. Complex 1 2+ is a rare example of a ruthenium complex which ‘turns on’ both reactivity and luminescence upon switching to a hydrated state. A number of transition metal complexes have attracted attention as potential photodynamic therapy (PDT) agents for cancer treatment, as they often have a high affinity for DNA and accessible photoexcited states in the visible region. 2 Turro, Brewer, Dunbar, and others have shown that complexes of Ru(II), Os(II), and dinuclear Rh(III) cores can be made into effective DNA photocleavage agents. 3-5 Most commonly, the PDT agent works by activation of cellular O 2 to form reactive oxygen species (ROS), such as 1 O 2 or ·OH, which are responsible for inducing apoptosis via damaging reactions such as DNA cleavage. Only a few of these transition metal complexes are able to cleave DNA upon photoexcitation without the need for O 2 ; however, it is clear that such an approach could offer some therapeutic advantages in that many cancer cells are under hypoxic stress. 6-9 Herein, we describe the oxygen independent DNA cleavage activity of a ruthenium(II) complex, [(phen) 2 Ru(phendione)]- Cl 2 (1Cl 2 ) upon visible light irradiation into the MLCT band at 470 nm. Complex 1 2+ is a commonly used synthon in ruthenium polypyridyl chemistry 10-12 and yet, only recently has its unusual solvent and temperature dependent para- magnetism, 13 and now photoreactivity, been described. An investigation into the mechanism of DNA cleavage reveals that complex 1 2+ is in equilibrium with a hydrated species 1H 2 O 2+ (see Scheme 1) and that the latter is the photoactive species. The excited state photophysics of 1 and 1H 2 O 2+ and the mechanism by which DNA cleavage is induced were probed by a combination of transient absorption and luminescence spectroscopy, electrochemistry, and product analysis as described below. The DNA photocleavage activity of 1 under aerobic and anaerobic conditions was examined using a common plasmid cleavage assay. Because of the hydration reaction (vide infra), 1 2+ is present as an equilibrium mixture of 1 2+ and 1H 2 O 2+ which is collectively referred to as RuPD in the following section. Supercoiled pUC18 plasmid DNA was incubated with RuPD (loading ratio: 1 Ru complex per approximately 6 DNA base pairs) and irradiated with 470 nm light for various lengths of time in the presence or absence of O 2 . The plasmid cleavage products were visualized using agarose gel electrophoresis to separate supercoiled (Form I), circular (Form II) and linear Received: October 30, 2012 Published: January 27, 2013 Scheme 1. Hydration Equilibrium for Complex 1 2+ in Water Communication pubs.acs.org/JACS © 2013 American Chemical Society 2419 dx.doi.org/10.1021/ja3106863 | J. Am. Chem. Soc. 2013, 135, 2419-2422