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Title: UV-Vis Photodissociation Action Spectroscopy Reveals Cytosine-Guanine Hydrogen Transfer in DNA Tetranucleotide Cation Radicals upon One-Electron Transfer
ABSTRACT: We report the generation and spectroscopic study of hydrogen-rich DNA tetranucleotide cation radicals (GATC+2H)+• and (AGTC+2H)+•. The radicals were generated in the gas phase by one-electron reduction of the respective dications (GATC +2H)2+ and (AGTC+2H)2+ and characterized by collision-induced dissociation and photodissociation tandem mass spectrometry and UV−vis photodissociation action spectroscopy. Among several absorption bands observed for (GATC+2H)+•, the bands at 340 and 450 nm were assigned to radical chromophores. Timedependent density functional theory calculations including vibronic transitions in the visible region of the spectrum were used to provide theoretical absorption spectra of several low-energy tetranucleotide tautomers having cytosine-, adenine-, and thymine- based radical chromophores that did not match the experimental spectrum. Instead, the calculations indicated the formation of a new isomer with the 7,8-H-dihydroguanine cation radical moiety. The isomerization involved hydrogen migration from the cytosine N-3−H radical to the C-8 position in N-7-protonated guanine that was calculated to be 87 kJ mol−1 exothermic and had a low-energy transition state. Although the hydrogen migration was facilitated by the spatial proximity of the guanine and cytosine bases in the low-energy (GATC+2H)+• intermediate formed by electron transfer, the reaction was calculated to have a large negative activation entropy. Rice−Ramsperger−Kassel−Marcus (RRKM) and transition state theory kinetic analysis indicated that the isomerization occurred rapidly in hot cation radicals produced by electron transfer with more » the population-weighed rate constant of k = 8.9 × 103 s−1. The isomerization was calculated to be too slow to proceed on the experimental time scale in thermal cation radicals at 310 K. « less
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The journal of physical chemistry
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National Science Foundation
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