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 ((HN)-H-3-NO2) within 10 ns in the quantum yield 0.41. (HN)-H-3-NO2 has increased basicity compared to its ground state. Consequently, its nitro group exhibits the hydrogen bond accepting ability. (HN)-H-3-NO2 can interact with guanosine (G) to form the hydrogen-bonded (HN)-H-3-NO2 center dot center dot center dot G complex with the rate constant k=(8.7 +/- 0.3)x10(9) M-1.s(-1). The hydrogen-bonded complex is identified based on the blue shift evolvement of the absorption maximum of (HN)-H-3-NO2 in alcoholic solutions. The reduction potential of (HN)-H-3-NO2 is E-red((HN)-H-3-NO2) = 1.23 V vs. SCE. The electron transfer occurs in the (HN)-H-3-NO2 center dot center dot center dot G complex and generates G(+center dot) and HN-NO2-center dot followed by the proton transfer from N-1 and N-2 of G producing radicals HN-NO2H center dot, G(N-1-H)(center dot) and G(N-2-H)(center dot).