No abstract

The spectroelectrochemical properties of a novel light switch for DNA, Ru(phen) 2 (PHEHAT) 2+ (phen ) 1,10phenanthroline; PHEHAT ) 1,10-phenanthrolino [5,6-b]1,4,5,8,9,12-hexaazatriphenylene), are examined and compared to those of Ru(phen) 2 (DPPZ) 2+ (DPPZ ) dipyrido[3,2-a;2′,3′-c]phenazine) and Ru(phen) 2 (HAT) 2+ (HAT ) 1,4,5,8,9,12-hexaazatriphenylene). The excited Ru(phen) 2 (PHEHAT) 2+ luminesces in organic solvents but not in water. It is shown that the orbitals involved in the absorption and luminescence spectroscopy are not the same as those in the electrochemistry. In aqueous solution, this complex luminesces upon intercalation of the PHEHAT ligand into the stacking of the DNA bases. Two modes of distribution of the complex on DNA can be evidenced from the titration curves of the complex with DNA. Laser flash photolysis experiments show that the excited state is able to abstract an electron from GMP (guanosine-5′-monophosphate) with a rather low efficiency, leading to the reduced complex and oxidized GMP. However, this process is not accompanied by the formation of photoproduct with GMP and cannot be detected with DNA on the time scale of the experiments.

A combination of synthetic methods involving mononuclear precursors of predetermined chirality {Delta- or Lambda-[Ru(pp)(2)(CO)(2)](2+); pp = 2,2'-bipyridine or 1,10-phenanthroline} and chromatographic techniques has allowed the isolation of the meso (DeltaLambda) and rac (DeltaDelta/LambdaLambda) diastereoisomers of the dinuclear complexes [{Ru(pp)(2)}(2)(&mgr;-HAT)](4+) {HAT = 1,4,5,8,9,12-hexaazatriphenylene}. The enantiomers of the rac forms have been separated, and characterization of all species has been achieved by NMR and CD studies. Additonally, the homochiral (Delta(3)/Lambda(3)) and heterochiral (Delta(2)Lambda/Lambda(2)Delta) diastereoisomers of the trinuclear complexes [{Ru(pp)(2)}(3)(&mgr;-HAT)](6+), and the enantiomers of both forms, have been isolated and identified. Emission studies of all the dinuclear species at room temperature indicate the relative luminescence quantum yields and the emission lifetimes significantly decrease for the meso compared with the rac diastereoisomers. No significant differences were detected at room temperature in the diastereoisomeric forms of the trinuclear compounds. However, in a glass at low temperature (80 K), the luminescence lifetimes of the trinuclear heterochiral diastereoisomer were slightly shorter than those of the homochiral form.

Abstract— The luminescence quenching of ruthenium‐tris‐l,4,5,8‐tetraazaphenanthrene [Ru(tap)32+] by nucleotides approaches the diffusion rate only with guanosine‐5′‐monophosphate (GMP), the most reducing nucleotide, and leads to an electron transfer with the production of the monoreduced complex and the oxidized base. The resulting deprotonated GMP(‐H)radical recombines with the monoreduced complex according to a bimolecular equimolar process. The pH dependence of the decay of the transient reduced complex, in the presence of an oxidant (oxygen or benzoquinone) indicates the formation of Ru(tap)2(tapH)2+i.e. the reduced protonated species, subsequent to the electron transfer, with a pKa of 7.6 as confirmed from pulse radiolysis experiments. As the non‐protonated reduced complex, Ru(tap)2(tap2)+ has a higher reducing power than the protonated one, oxygen is able to reoxidize only the non‐protonated species, whereas benzoquinone reoxidizes both species but with different rate constants. The flash photolysis of Ru(tap),2+ in the presence of DNA and the effect of Mg2+ ions and GMP as supplementary additives also show the existence of a photo‐induced electron transfer with the nucleic acid, which can be correlated to the photosensitized cleavage of DNA by this complex.