Photochemistry and Photophysics of Coordination Compounds

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242 M.T. Indelli et al. age, thanks to the outstanding oxidant properties of their excited states. The photocleavage ability offers a strategy for the use of these rhodium complexes as DNA targets. The approach used by the Barton’s laboratory is the following: i) upon ultraviolet excitation, the excited state of DNA-bound rhodium complex promotes the scission of DNA sugar-phosphate backbone through oxidative degradation of the sugar moiety; ii) biochemical methods (e.g., gel electrophoresis) are used to determine where the strand scission occurred and therefore where, along the strand, the complex was bound. This method provides a powerful tool to mark specifically the sites of binding [129]. A variety of articles focused on DNA photocleavage by phi complexes containing different ligands in ancillary positions [130, 139–142]. The structural formula of the most extensively characterized complexes are reported below (27, 28, 29, 30). Irradiation with UV light of Rh(phen)2(phi) 3+ (28) andRh(bpy)(phi)2 3+ (30) intercalated in DNA leads to direct DNA strand scission with products
Photochemistry and Photophysics of Coordination Compounds: Rhodium 243 consistent with 3 ′ -hydrogen abstraction from the deoxyribose sugar adjacent to the binding site [140, 141]. The photochemistry of these phi complexes intercalated in DNA has also been studied as a function of irradiation wavelength [143, 144]. Interestingly, the results showed that light of different wavelengths induces selectively different chemical reactions. In particular, the irradiation of the DNA-bound Rh(phi)2(L) 3+ complexes with UV light (λ = 313 nm) leads, as discussed above, to direct scission of the DNA-sugar backbone [141, 144]. If instead the complexes are excited with low-energy light (λ ≥ 365 nm) oxidative damage to the DNA bases is observed. The mechanism of these two photoprocesses is not discussed in great detail [130, 143]. It is proposed, however, which direct DNA scission takes place by hydrogen abstraction from the sugar by the phi ligand radical of a ligand-to-metal charge transfer (LMCT) state [130]. On the other hand, the oxidative damage is attributed to the population of a powerful oxidizing excited state (ILCT [50] or LC [143]) with longer wavelength light. Photocleavage experiments have been used profitably for establishing how the phi complexes are associated to DNA [129, 130, 141]. Confirmation of site selectivity and greater structural definition were obtained later from high-resolution NMR studies [134–138, 145]. The important result is that all the phi complexes bind DNA noncovalently through intercalation in the major groove where the phi ligand is inserted between the base pairs so as to maximize stacking interactions. More recently a full crystal structure of ∆– Rh ((R,R)-Me2trien)2(phi) 3+ ((R,R)-Me2trien = 2R,9R-diamino-4,7diazadecane) bound to a DNA octamer provided a direct evidence of the
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Photochemistry and Photophysics of
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X Preface nation Compounds. The ven
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