Biphotonic Photoionization of Cytosine and Its Derivatives with UV Radiation at 248 nm: An EPR Study in Low-Temperature Perchlorate Glasses

Malone, Mark E.; Cullis, Paul M.; Symons, Martyn C. R.; Parker, Anthony W.

doi:10.1021/j100022a052

Cited by 35 publications

(44 citation statements)

References 13 publications

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“…The use of elevated temperatures which more closely correspond to biological conditions is shown to lead to near complete conversion in model systems. The mechanism observed here and in our earlier work (15) has not been recognized in the past and may bear on the conclusions reached in some earlier works [(14,32) and the references therein]. For example, in a study of photolysed 5′-dCMP, it was hypothesized that the radical cations produced by laser two-photon ionization produced sugar radicals via ground-state deprotonation at C1′ or hydrogen abstraction from C5′ (14).…”

Section: Resultsmentioning

confidence: 49%

UVA-visible photo-excitation of guanine radical cations produces sugar radicals in DNA and model structures

Adhikary

Malkhasian

Collins

et al. 2005

Nucleic Acids Research

305

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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.

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Section: Resultsmentioning

confidence: 49%

UVA-visible photo-excitation of guanine radical cations produces sugar radicals in DNA and model structures

Adhikary

Malkhasian

Collins

et al. 2005

Nucleic Acids Research

305

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“…The involvement of such a process in DNA radiation chemistry has been discussed in the literature. A well-established example in model systems is represented by deoxycytidine, where deprotonation of the base-centered radical cation at the C1′ site leads to the C1′-centered deoxycytidine sugar radical (27). However, our results obtained with anthracycline-doped crystals clearly indicate that DNA base radicals are not, to a substantial degree, precursors to base release.…”

Section: Discussionmentioning

confidence: 57%

Direct Radiation Damage to Crystalline DNA: What is the Source of Unaltered Base Release?

2000

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The radiation chemical yields of unaltered base release have been measured in three crystalline double-stranded DNA oligomers after X irradiation at 4 K. The yields of released bases are between 10 and 20% of the total free radical yields measured at 4 K. Using these numbers, we estimate that the yield of DNA strand breaks due to the direct effect is about 0.1 μmol J −1 . The damage responsible for base release is independent of the base type (C, G, A or T) and is not scavenged by anthracycline drugs intercalated in the DNA. For these reasons, reactions initiated by the hydroxyl radical have been ruled out as the source of base release. Since the intercalated anthracycline scavenges electrons and holes completely but does not inhibit base release, the possibility for damage transfer from the bases to the sugars can also be ruled out. The results are consistent with a model in which primary radical cations formed directly on the sugar-phosphate backbone react by two competing pathways: deprotonation, which localizes the damage on the sugar, and hole tunneling, which transfers the damage to the base stack. Quantitative estimates indicate that these two processes are approximately equally efficient.

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“…Heavy-atom bond lengths and bond angles were optimized at the 3-21G SCF level. For 1-MeT, 1-MeC, 1-methyluracil, indole, tryptophan, uridine, 2Ј-deoxythymidine, 2-deoxyribose, 5Ј-dTMP Ϫ , and 5Ј-dCMP Ϫ , the C-H bond lengths were 1.08 and 1.09 Å for sp 2 , and 5Ј-dAMP Ϫ were the same as those used earlier in calculations of gas-phase IPs (11,13,15,17,31). Heavy-atom bond lengths and bond angles for 5Ј-dTMP Ϫ and 5Ј-dCMP Ϫ were obtained from crystallographic B-DNA dodecamer data (36) on the geometries of 5Ј-dTMP Ϫ at position 8 in strand B and 5Ј-dCMP Ϫ at position 9 of strand A.…”

Section: Methodsmentioning

confidence: 99%

“…Nucleotide ionization plays an important role in mechanisms resulting in DNA damage caused by high-energy photons (1)(2)(3)(4). However, there is little information about threshold energies required for DNA ionization in aqueous solution, except that they lie in the range of 4 to 7 eV (5,6).…”

mentioning

confidence: 99%

Conduction-band-edge ionization thresholds of DNA components in aqueous solution

Fernando

Papadantonakis²,

Kim

et al. 1998

Proc. Natl. Acad. Sci. U.S.A.

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Numerous investigations have focused on DNA damage induced by ionizing radiation; however, photoionization threshold energies of nucleic acid components in aqueous solution are not known. Herein, data from gas-phase photoelectron experiments have been combined with results from selfconsistent field and post-self-consistent field molecular orbital calculations and with theoretical Gibbs free energies of hydration to describe aqueous ionization energies of 2-deoxythymidine 5-phosphate (5-dTMP ؊ ) and 2-deoxycytidine 5-phosphate (5-dCMP ؊ ). For the test molecules, indole and tryptophan, this approach yields aqueous ionization energies (4.46 and 4.58 eV, respectively) in agreement with experimental values (4.35 and 4.45 eV). When uridine and 2-deoxythymidine ionization energies are evaluated, the results agree with recent data from 193-nm laser measurements indicating that uridine ionization occurs via a one-photon event. For 5-dCMP ؊ and 5-dTMP ؊ , a comparison of aqueous ionization energies with gas-phase ionization potentials (IPs) indicates that hydration alters the relative energies of ionization events. In the gas phase, phosphate vertical IPs are ϳ1.3 eV smaller than base IPs. In aqueous solution, the base and phosphate ionization energies are more similar, and only differ by ϳ0.5 eV. For 5-dCMP ؊ and 5-dTMP ؊ , the increased favorableness of base ionization, which accompanies hydration, is consistent with experimental data indicating that, at 77 K in aqueous perchlorate glasses, the primary photoionization pathway involves base ionization followed by deprotonation.Nucleotide ionization plays an important role in mechanisms resulting in DNA damage caused by high-energy photons (1-4). However, there is little information about threshold energies required for DNA ionization in aqueous solution, except that they lie in the range of 4 to 7 eV (5, 6). In earlier work, vertical and adiabatic ionization potentials (IPs) were provided for gas-phase nucleotide components and model compounds (7-10). More recently, experimental data on nucleotide component model compounds has been combined with theoretical results to evaluate IPs for intact gas-phase nucleotides (11)(12)(13)(14). Herein, gasphase UV photoelectron data were used with results from molecular orbital calculations at the self-consistent field (SCF) and post-SCF levels to provide valence electron IPs of the gas-phase anionic nucleotides 2Ј-deoxycytidine 5Ј-phosphate (5Ј-dCMP Ϫ ) (12, 15), 2Ј-deoxythymidine 5Ј-phosphate (5Ј-dTMP Ϫ ) (15), 2Ј-deoxyguanosine 5Ј-phosphate (5Ј-dGMP Ϫ ) (11,13,15,16), and 2Ј-deoxyadenosine 5Ј-phosphate (5Ј-dAMP Ϫ ) (14, 15). In all cases, the lowest IP is associated with the negatively charged phosphate group. However, these earlier gas-phase results do not provide information about nucleotide ionization under physiological conditions.Water and counterion interactions are among the most important that DNA encounters in biological environments, and the effects of these interactions on nucleotide IPs were examined in gas-pha...

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Biphotonic Photoionization of Cytosine and Its Derivatives with UV Radiation at 248 nm: An EPR Study in Low-Temperature Perchlorate Glasses

Cited by 35 publications

References 13 publications

UVA-visible photo-excitation of guanine radical cations produces sugar radicals in DNA and model structures

UVA-visible photo-excitation of guanine radical cations produces sugar radicals in DNA and model structures

Direct Radiation Damage to Crystalline DNA: What is the Source of Unaltered Base Release?

Conduction-band-edge ionization thresholds of DNA components in aqueous solution

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