Photoinduced electron transfer quenching of excited Ru(II) polypyridyls bound to DNA: the role of the nucleic acid double helix

In the presence of double helical polynucleotides (sodium poly(dA-dT)-poly(dA-dT) or calf thymus DNA), the efficiency of oxidative or reductive electron transfer between photoexcited ruthenium(II) chelates Ru(tap)2(hat)^(2+) or Ru(phen)^(⅔+) (where tap =1,4,5,8-tetraazaphenanthrene, hat = 1,4,5,8,9,12-hexaazatriphenylene, and phen = 1,10-phenanthroline) and appropriate cationic quenchers (ethidium, Ru(NH3)3/6^+, methyl viologen, or M(phen)3/3^+, where M = Co, Rh, Cr) increases 1–2 orders of magnitude compared to the efficiency of the same quenching in microhomogeneous aqueous medium (k_q= 0.3-1.8 × 10^9 M's^(−1)). The enhancement is more pronounced when the binding constant of the quencher (10^3 < Kh < 10^6 M^(−1)) is large. Similar trends are found when the biopolymers are replaced by sodium poly(styrenesulfonate) (PSS). The accelerated electron transfer process is proposed to be due mainly to the effect of accumulation of the reagents in the electrostatic field of the polymer; if corrections for this effect are introduced (e.g. ratioing [quencher]/[polynucleotide]), the reaction rate becomes essentially independent of the polymer concentration. Based upon a model for electron transfer reaction of the complexes within a small cylindrical interface around the DNA helix, calculations of the bimolecular electron transfer rate constants are computed to be 10′ times smaller when the reactants are bound to the double-stranded polynucleotides and decreased mobility of the cationic species is apparent. The effect is less pronounced if a simpler polyelectrolyte (PSS) is employed. Emission lifetimes of the Ru(II) polypyridyls bound to the DNA (0.32–2μs, double exponential decays) are discussed as well