Photoinduced Electron Transfer Quenching of Excited Ru(II) Polypyridyls Bound toDNA: The Role of the Nucleic Acid Double Helix

摘要

In the presence of double helical polynucleotides the efficiency of oxidative orreductive electron transfer between photoexcited ruthenium(II) chelates Ru(tap) (hat) 2+ or Ru (phen) 2+ and appropriate cationic quenchers increases 1-2 orders of magnitude compared to the efficiency of the same quenching in microheterogenous aqueous medium. The enhancement is more pronounced when the binding constant of the quencher 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, 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 10,000 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 (0.32-2 us, double exponential decays) are discussed as well.