Photoinduced Electron Transfer in an Oligodeoxynucleotide Duplex:  Observation of the Electron-Transfer Intermediate

Kinetics of a photoinduced electron-transfer reaction between phenothiazine (PTZ) and Ru(bpy) 2 (4-m-4‘-pa-bpy) 2+ [bpy = 2,2‘-bipyridine and 4-m-4‘-pa-bpy = 4-methyl-4‘-carbonylpropargylamine] covalently bound to a DNA duplex is investigated by transient absorption spectroscopy. The electron donor, PTZ, is attached to the 5‘ terminal of one oligodeoxynucleotide strand, and the chromophore, Ru(bpy) 2 (4-m-4‘-pa-bpy) 2+, is covalently linked to a uridine of the complementary strand. Electron transfer between the donor, PTZ, and photoexcited acceptor, *Ru(bpy) 2 (4-m-4‘-pa-bpy) 2+, occurs, and the product of the forward electron-transfer reaction, PTZ + •, is observed. The rate of the back electron-transfer reaction ( k b = 3.94 × 10 6 s -1; Δ G ∼ 2.0 eV) is determined by monitoring the decay of the electron-transfer intermediate, PTZ + •, by transient absorption spectroscopy.