Electron transfer from guanosine to the lowest triplet excited state of 4-nitroindole through hydrogen-bonded complex

The photochemical reaction of 4-nitroindole and guanosine was studied with the transient absorption spectroscopy as guanine is an effective hole trap during the charge transfer in DNA, and 4-nitroindole is a universal base. Excitation of 4-nitroindole generates the lowest triplet excited state 4-nitroindole (3HN-NO2) within 10 ns in the quantum yield 0.41. 3HN-NO2 has increased basicity compared to its ground state. Consequently, its nitro group exhibits the hydrogen bond accepting ability. 3HN-NO2 can interact with guanosine (G) to form the hydrogen-bonded 3HN-NO2…G complex with the rate constant k=(8.7 ± 0.3)×109 M−1⋅s−1. The hydrogen-bonded complex is identified based on the blue shift evolvement of the absorption maximum of 3HN-NO2 in alcoholic solutions. The reduction potential of 3HN-NO2 is Ered(3HN-NO2) = 1.23 V vs. SCE. The electron transfer occurs in the 3HN-NO2…G complex and generates G+• and HN-NO2−• followed by the proton transfer from N1 and N2 of G producing radicals HN-NO2H•, G(N1-H)• and G(N2-H)•.