Using Metal Complex Reduced States to Monitor the Oxidation of DNA

Abstract

Metallointercalating photooxidants interact intimately with the base stack of double-stranded DNA and exhibit rich photophysical and electrochemical properties, making them ideal probes for the study of DNA-mediated charge transport (CT). The complexes [Rh(phi)2(bpy′)]3+ (phi = 9,10-phenanthrenequinone diimine; bpy′ = 4-methyl-4′-(butyric acid)-2,2′-bipyridine), [Ir(ppy)2(dppz′)]+ (ppy = 2-phenylpyridine; dppz′ = 6-(dipyrido[3,2-a:2′,3′-c]phenazin-11-yl)hex-5-ynoic acid), and [Re(CO)3(dppz)(py′)]+ (dppz = dipyrido[2,3-a:2′,3′-c]phenazine; py′ = 3-(pyridin-4-yl)-propanoic acid) were each covalently tethered to DNA in order to compare their photooxidation efficiencies. Biochemical studies show that upon irradiation, the three complexes oxidize guanine by long-range DNA-mediated CT with the efficiency: Rh > Re > Ir. Comparison of spectra obtained by spectroelectrochemistry after bulk reduction of the free metal complexes with those obtained by transient absorption (TA) spectroscopy of the conjugates suggests that the reduced metal states form following excitation of the conjugates at 355 nm. Electrochemical experiments and kinetic analysis of the TA decays indicate that the thermodynamic driving force for CT, variations in the efficiency of back electron transfer, and coupling to DNA are the primary factors responsible for the trend observed in the guanine oxidation yield of the three complexes.

Charge Photoinjection in Intercalated and Covalently Bound [Re(CO)3(dppz)(py)]+–DNA Constructs Monitored by Time-Resolved Visible and Infrared Spectroscopy

“…The observation of damage at the 5′-GG-3′ site indicates that long-range photoinduced hole injection from the Re label to DNA indeed occurs, consistent with results obtained for a similar Re-DNA conjugate. 66 However, the extent of damage is consistently greater in the case of Re-25(I) than Re-25(G).…”

“…For example, each complex exhibits absorption maxima at 364 and 382 nm (ε ≈ 11,000 M −1 cm −1 ), 41,65 with a tail that extends into the visible region. 66 The emission spectra of both complexes show maxima at 554 and 595 nm. At 570 nm, Re′-OH and Re′-OEt each show a biexponential emission decay in acetonitrile, with lifetimes on the order of 200 ns (~10%) and 10 μs (~90%), tentatively attributed to emission from different 3 IL states.…”

“…At 570 nm, Re′-OH and Re′-OEt each show a biexponential emission decay in acetonitrile, with lifetimes on the order of 200 ns (~10%) and 10 μs (~90%), tentatively attributed to emission from different 3 IL states. 66 Tethering the Re species to DNA, therefore, is expected to have negligible influence on the energetics of the complex.…”

“…67 The exact value of E 0,0 is unknown, but it is estimated to lie between the energy at which the excitation and emission spectra coincide (480 nm, 2.58 eV) and the energy of the emission maximum in aqueous solution (570 nm, 2.18 eV). For Re′-OEt in acetonitrile, E ° (Re + /Re 0 ) was reported as −850 mV vs. NHE, 66 predicting the excited-state reduction potential to lie between 1.33 and 1.73 eV. As an oxidant, electronically excited Re′-OEt is clearly strong enough to oxidize guanine, and it may be strong enough to oxidize adenine [ E ° (A •+ /A) = 1.42 V vs. NHE].…”

DNA Charge Transport: from Chemical Principles to the Cell

“…Examples of metallointercalators that have been used to probe the redox properties of DNA include [Rh(phi) 2 (bpy′)] 3+ , [Ru(phen)(bpy′)(dppz)] 2+ , [Re(CO) 3 (dppz)(py′)] + , and [Ir(ppy) 2 (dppz′)] + , where phi = 9,10-phenanthrenequinone diimmine, bpy′ = 4-methyl-4′-(butyric acid)-2,2′-bipyridine, phen = 1,10-phenanthroline, dppz = dipyrido[2,3-a:2′,3′-c]phenazine, py′ = 3-(pyridin-4-yl)-propanoic acid, ppy = 2-phenylpyridine, and dppz′ = 6-(dipyrido[3,2-a:2′,3′-c]phenazin-11-yl)hex-5-ynoic acid) (Olmon et al, 2011; Shao and Barton, 2007; Williams et al, 2004). The yield of oxidative DNA damage produced by metallointercalators has been found to depend primarily on the thermodynamic driving force for CT, the efficiency of back electron transfer (ET) processes, and, importantly, the degree of electronic coupling to the DNA π-stack (Olmon et al, 2011). These complexes can be covalently tethered to DNA by utilizing modified ligands (dppz′, bpy′, phen′) in order to localize the complex to one end of the DNA (Holmlin et al, 1999).…”