This work reports ESR studies that identify the favored site of deprotonation of the guanine cation radical (G• + ) in an aqueous medium at 77 K. Using ESR and UV-visible spectroscopy, one-electron oxidized guanine is investigated in frozen aqueous D 2
This work presents evidence that photo-excitation of guanine radical cations results in high yields of deoxyribose sugar radicals in DNA, guanine deoxyribonucleosides and deoxyribonucleotides. In dsDNA at low temperatures, formation of C1′• is observed from photo-excitation of G•+ in the 310–480 nm range with no C1′• formation observed ≥520 nm. Illumination of guanine radical cations in 2′dG, 3′-dGMP and 5′-dGMP in aqueous LiCl glasses at 143 K is found to result in remarkably high yields (∼85–95%) of sugar radicals, namely C1′•, C3′• and C5′•. The amount of each of the sugar radicals formed varies dramatically with compound structure and temperature of illumination. Radical assignments were confirmed using selective deuteration at C5′ or C3′ in 2′-dG and at C8 in all the guanine nucleosides/tides. Studies of the effect of temperature, pH, and wavelength of excitation provide important information about the mechanism of formation of these sugar radicals. Time-dependent density functional theory calculations verify that specific excited states in G•+ show considerable hole delocalization into the sugar structure, in accord with our proposed mechanism of action, namely deprotonation from the sugar moiety of the excited molecular radical cation.
To better understand the cause of the diversity in reported values of the electron affinities (EAs) for DNA bases, we performed a series of DFT (B3LYP functional) calculations at different basis set sizes. Through investigation of (1) trends in the values of EAs, (2) the excess electron spin distribution of the anion radical dependence on basis set size, (3) effect of the excess electron on ZPEs, we are able to identify the features of a basis set that allows for dipole-bound and continuum states to compete with molecular states for the electron. Smaller basis sets that confine the excess electron to the molecule allow for reasonable estimates of relative valence electron affinities excluding dipole-bound states and suggest the order of adiabatic valence electron affinities to be U ≈ T > C ≈ I (hypoxanthine) > A > G with G nearly 1 eV less electron affinic than U. Combining the best estimates from theory and experiment we place the adiabatic valence electron affinities of the pyrimidines as zero to +0.2 eV, whereas the purines A and G are predicted to be clearly negative with electron affinities of ca. -0.35 and -0.75 eV, respectively. The virtual states (i.e., negative electron affinities) for A and G in the gas-phase become relevant to biology when their energies are lowered to bound states in solvated systems. Indeed, our calculations performed including the effect of solvation (PCM model) show that all EAs for the DNA bases are positive and have the same relative order as found with the compact basis sets in the gas-phase calculations.
Proton-transfer reactions in two DNA base pair anion and cation radicals are treated by density functional theory to aid our understanding of the possible contributions of these reactions to electron and hole transfer in DNA. The proton-transfer transition structures for both the GC and IC anion and cation radicals are found. For both anion and cation radicals, it is the proton at the N1 guanine (G) site, or hypoxanthine (I) site, that transfers to cytosine. The forward and reverse activation energies as well as reaction enthalpies and free energy changes are calculated. These calculations show that small activation energies of 1 and 3 kcal/mol are present for the GC anion and cation, respectively. The predicted free energy change for the proton transfer is favorable for GC anion radical (-3 kcal/mol) but is slightly unfavorable for the GC cation radical (1.4 kcal/mol). Both of these values compare well with experimental estimates. Remarkably, the IC anion radical system shows no activation energy toward proton transfer and a large free energy change favoring the proton transferred state (-7 kcal). Electron affinities (EA) and ionization potentials (IP) of the two base pairs are also calculated and reported.
In this work, it is shown that the incorporation of an 8-deuteroguanine (G*) moiety in DNA-oligomers allows for direct determination at 77 K of (i) the location of holes (i.e., the radical site) within dsDNA at specific base sites, even within stacks of G, as well as (ii) the protonation state of the hole at that site. These findings are based on our work and demonstrate that selective deuteration at C-8 on guanine moiety in dGuo results in an ESR signal from the guanine cation radical (G*• + ) which is easily distinguishable from that of the undeuterated guanine cation radical (G• + ). G*• + is also found to be easily distinguishable from its conjugate base, the N1-deprotonated radical, G*(−H)•. Our ESR results clearly establish that at 77 K (i) one-electron oxidized guanine in double stranded DNAoligomers exists as the deprotonated neutral radical G(−H)• as a result of facile proton transfer to the hydrogen bonded cytosine, and (ii) the hole is preferentially located at the 5′-end in several ds DNAoligomers with a GGG sequence.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.