title: Efficient UV-induced charge separation and recombination in an 8-oxoguanine-containing dinucleotide.
abstract: During the early evolution of life, 8-oxo-7,8-dihydro-2'-deoxyguanosine (O) may have functioned as a proto-flavin capable of repairing cyclobutane pyrimidine dimers in DNA or RNA by photoinduced electron transfer using longer wavelength UVB radiation. To investigate the ability of O to act as an excited-state electron donor, a dinucleotide mimic of the FADH2 cofactor containing O at the 5'-end and 2'-deoxyadenosine at the 3'-end was studied by femtosecond transient absorption spectroscopy in aqueous solution. Following excitation with a UV pulse, a broadband mid-IR pulse probed vibrational modes of ground-state and electronically excited molecules in the double-bond stretching region. Global analysis of time- and frequency-resolved transient absorption data coupled with ab initio quantum mechanical calculations reveal vibrational marker bands of nucleobase radical ions formed by electron transfer from O to 2'-deoxyadenosine. The quantum yield of charge separation is 0.4 at 265 nm, but decreases to 0.1 at 295 nm. Charge recombination occurs in 60 ps before the O radical cation can lose a deuteron to water. Kinetic and thermodynamic considerations strongly suggest that all nucleobases can undergo ultrafast charge separation when π-stacked in DNA or RNA. Interbase charge transfer is proposed to be a major decay pathway for UV excited states of nucleic acids of great importance for photostability as well as photoredox activity.
authors: Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT 59717;
authors: Yuyuan
authors: Y
authors: Zhang
authors: Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT 59717;
authors: Jordan
authors: J
authors: Dood
authors: Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT 59717;
authors: Ashley A
authors: AA
authors: Beckstead
authors: Department of Chemistry, University of Utah, Salt Lake City, UT 84112; and.
authors: Xi-Bo
authors: XB
authors: Li
authors: Department of Chemistry, University of Utah, Salt Lake City, UT 84112; and.
authors: Khiem V
authors: KV
authors: Nguyen
authors: Department of Chemistry, University of Utah, Salt Lake City, UT 84112; and kohler@chemistry.montana.edu burrows@chem.utah.edu robimp@unina.it.
authors: Cynthia J
authors: CJ
authors: Burrows
authors: Consiglio Nazionale delle Ricerche, Istituto di Biostrutture e Bioimmagini, 80136 Naples, Italy kohler@chemistry.montana.edu burrows@chem.utah.edu robimp@unina.it.
authors: Roberto
authors: R
authors: Improta
authors: Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT 59717; kohler@chemistry.montana.edu burrows@chem.utah.edu robimp@unina.it.
authors: Bern
authors: B
authors: Kohler
mesh_terms: D020029:Base Pairing; D004579:Electron Transport / Q000528:radiation effects; D006147:Guanine / Q000031:analogs & derivatives* / Q000737:chemistry; D007700:Kinetics; D008956:Models, Chemical; D009841:Oligonucleotides / Q000737:chemistry* / Q000528:radiation effects*; D055668:Photochemical Processes; D011789:Quantum Theory; D013054:Spectrophotometry, Atomic; D017550:Spectroscopy, Fourier Transform Infrared; D013816:Thermodynamics; D014466:Ultraviolet Rays
chemical_list: D009841:Oligonucleotides; C024829:8-hydroxyguanine; D006147:Guanine
keywords: DNA charge transfer states; DNA photophysics; ab initio calculations; photoreactivation; time-resolved vibrational spectroscopy