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 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.
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Guanine–adenine interactions in DNA tetranucleotide cation radicals revealed by UV/vis photodissociation action spectroscopy and theory
Hydrogen-rich cation radicals (GATT + 2H) + ˙ and (AGTT + 2H) + ˙ represent oligonucleotide models of charged hydrogen atom adducts to DNA. These tetranucleotide cation radicals were generated in the gas phase by one-electron reduction of the respective (GATT + 2H) 2+ and (AGTT + 2H) 2+ dications in which the charging protons were placed on the guanine and adenine nucleobases. We used wavelength-dependent UV/Vis photodissociation in the valence-electron excitation region of 210–700 nm to produce action spectra of (GATT + 2H) + ˙ and (AGTT + 2H) + ˙ that showed radical-associated absorption bands in the near-UV (330 nm) and visible (400–440 nm) regions. Born–Oppenheimer molecular dynamics and density-functional theory calculations were used to obtain and rank by energy multiple (GATT + 2H) dication and cation-radical structures. Time-dependent density functional theory (TD-DFT) calculations of excited-state energies and electronic transitions in (GATT + 2H) + ˙ were augmented by vibronic spectra calculations at 310 K for selected low-energy cation radicals to provide a match with the action spectrum. The stable product of one-electron reduction was identified as having a 7,8-dihydroguanine cation radical moiety, formed by intramolecular hydrogen atom migration from adenine N-1–H. The hydrogen migration was calculated to have a transition state with a low activation energy, E a = 96.5 kJ mol −1 , and positive activation entropy, Δ S ‡ = 75 J mol −1 K −1 . This allowed for a fast isomerization of the primary reduction products on the ion-trap time scale of 150 ms that was substantially accelerated by highly exothermic electron transfer.
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- Award ID(s):
- 1661815
- NSF-PAR ID:
- 10180469
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 22
- Issue:
- 29
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 16831 to 16842
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/d0cp02362k
