Effect of DNA scaffolding on intramolecular electron transfer quenching ofa photoexcited ruthenium(II) polypyridine naphthalene diimide

Intramolecular emission quenching of a photoexcited ruthenium(II) polypyrid ine by a covalently linked naphthalene diimide (NDI) has been measured in a queous buffer both without and with calf thymus DNA. The complex consists o f a Ru(2,2'-bipyridine)(2)(2,2'-bipyridine-5-carboxamide)(2+) electron dono r covalently attached by way of a -CH2CH2CH2- linker to a 1,4,5,8-naphthale ne diimide acceptor (Ru-NDI, 1). The NDI portion of the complex intercalate s in calf thymus DNA, as indicated by the hypochromism of its optical absor bance bands and observation of an induced circular dichroism spectrum in th e same region. Emission quenching in Ru-NDI has been measured relative to a Ru tris-bpy model lacking the NDI moiety by both lifetime and emission qua ntum yield techniques. Using lifetime averages, the relative emission quenc hing is, respectively, 99.1% and 97.9% in aqueous buffer solutions without and with DNA. The emission quenching is ascribed to intramolecular electron transfer within the Ru-NDI complex with an estimated driving force (-Delta G degrees) of 0.33 eV. In buffer, the emission decays of Ru-NDI alone are fit well with a triexponential model with lifetimes of 0.34 (0.88), 1.99 (0 .11), and 12.6 (0.008) ns (relative amplitude). The emission decays of the DNA-intercalated Ru-NDI complex are also fit well with a triexponential mod el with lifetimes of 0.31 (0.79), 2.00 (0.13), and 11.8 (0.08) ns. Thus, th e fractional amplitudes of the lifetimes change upon DNA intercalation of t he complex, while the lifetimes themselves remain essentially the same. The average rates of electron transfer in aqueous buffer without and with DNA are, respectively, 1.6 x 10(9) and 6.8 x 10(8) s(-1). The striking result o f this study is that the overall character of electron transfer quenching i n Ru-NDI is very similar whether or not it is bound to DNA. Intercalation o f the NDI in DNA apparently has negligible consequences for electron transf er, implying either that the activation energy and electronic coupling in R u-NDI are largely unaffected by this, at first glance, seemingly significan t environmental change or that changes in these parameters on DNA binding c ancel fortuitously.