Radical anion cytosine

The high pKa-value of the cytosine radical anion, however, will enforce rapid protonation in water and maybe also by the guanine counterbase in a DNA double strand, as depicted in Scheme 3 [38]. In fact, it is currently be-... 

Hayon s suggestion 44, 45) and species 50 in alkaline solution (Hayon s suggestion 46). The calculation (the spectra as well as the INDO calculation446 of electron densities of uracil and cytosine radical anion) suggested that the solvated electron reacts with uracil and cytosine by addition to the lowest unoccupied molecular orbital, followed at neutral pH by protonation to form the C-4 ketyl radical of uracil and the C-2 ketyl radical of cytosine. The calculation on transition energies presented in ref. 446 cannot be treated as evidence that the species above is the one formed on pulse rediolysis of aqueous solutions of the pyrimidines. 

Another example relates to OH -aided one-electron oxidation of cytosine. With cytosine, the OH reaction proceeds by addition to C(5), a process that has a selectivity of 90% [24]. The 5-hydroxy-6-yl radical is an excellent reductant, and the same is true for the ionized 6-yl radical formed by deprotonation from N(l). This radical anion now contaim sufficient electron density to eliminate the OH group at C(5) as OH". The result is the cytosine-l-yl radical which is oxidizing, probably due to appreciable spin density at the hetero atoms N(l) and... 

Reaction between C in methanol and RTCNQ in acetonitrile yielded three kinds of ionic solids (1) insulators composed of methoxy substituted RTCNQ anions such as (CHC )[F4TCNQ-0Me ](H20) (Fig. 6) [136], (2) semiconducting CT solids with fully ionic RTCNQ radical anions such as (CHC )(TCNQ ) [137, 138], and (3) conducting CT solids of partially ionic or mixed valent RTCNQ radical anions such as (CHC"XMeTCNQ° >2 [138], where CHC" is the hemiprotonated cytosine pair (Fig. 6b). Cation units in aU products were found to be protonated cytosine species, most commonly CHC, where comes from methanol. This result suggests that the intrinsic transport properties of DNA should be studied not in protic solvents but under strictly dried conditions. 

Murata T, Saito G (2006) Properties of reaction products between cytosine and F4TCNQ in MeOH two hemiprotonated cytosine salts with F4TCNQ radical anion and methoxy adduct anion. Chem Lett 35 1342-1343... 

The 77-SCF MO Cl method has also been used446 to interpret spectral transitions of a series of possible intermediates in the reaction of uracil and cytosine with the solvated electrons eaq, produced by radiolysis of water. Experimentally this reaction has been investigated by Hayon,447 who used the technique of flash radiolysis. Hayon measured the optical-absorption spectra of the transient species in the UV range to obtain information on the site of attack of eaq on the pyrimidine base. At pH 5.0 the solvated electrons react with the pyrimidine molecules mainly at the C-2 and C-4 carbonyls, and the intermediates are rapidly protonatcd to give the corresponding ketyl radicals. For uracil Hayon found two absorption maxima (at 305 and < 280 nm) at pH 5.1 and one peak at 310 nm at pH 11.7. In this last case, on ionization of one of the chromophores the ketyl radical anion of the other nondissociated carbonyl is formed. Several species, 44, 45, 46, have been suggested by... 

Naumov S, Beckert D (2002) Reply to the Comment on A Fourier transform EPR study of uracil and thymine radical anions in aqueous solution)/ by DM Close. Phys Chem Chem Phys 4 45 Naumov S, Barthel A, Reinhold J, Dietz F, Geimer J, Beckert D (2000) Calculation of spin densities of radicals of pyrimidine-type bases by density functional theory. Influence of solvent and comparison with EPR results. Phys Chem Chem Phys 2 4207-4211 Naumov S, Hildenbrand K, von Sonntag C (2001) Tautomers of the N-centered radical generated by reaction of SO4 - with N(1)substituted cytosines in aqueous solution. Calculation of isotropic hyperfine coupling constants by a density functional method. J Chem Soc Perkin Trans 2 1648-1653... 

In a report by Gu et al. 2 -deoxycytidine-3 -monophosphate is charge neutralized by a single proton on the phosphate [109], The vertical electron attachment energy is calculated to be 0.15 eV, suggesting that 3 -dCMPH can capture near 0 eV electrons. The SOMO of the radical anion is seen to reside solely on the cytosine base. 

The cytosine anion is a strong base (pK > 13Y and is therefore expected to rapidly protonate in solution. Hissung and von Sonntag have shown by conductance techniques that the radical anion of cytosine is protonated by water (most likely at N3 or 02) in less than 4 ns. 

Protonated form of radical anion formed in cytosine 4- e reaction. 

Murata T, Nishimura K, Saito G (2007) Organic conductor based on nucleobase structural and electronic properties of a charge transfer solid composed of TCNQ anion radical and hemiprotonated cytosine. Mol Cryst Liq Cryst 466 101-112... 

These ideas have been illustrated in a recent study of the co-crystalline complex of 1-meCyt 5-FUra [19]. Using model calculations, it was shown how the hydrogen-bonding network of the crystal is able to sustain a proton shuttle which leads to the selective formation of certain radicals. Calculations predict that the site of reduction would be the cytosine base, yielding the N3 protonated cytosine anion, Cyt(N3-I-H), while the uracil base would be the site of oxidation, yielding the N1 deprotonated uracil cation, Ura(Nl—H) ... 

The previous section outlined the typical e loss and e gain products observed in the nucleic acid bases in the solid state. These studies can be applied to the study of the radiation chemistry of DNA. The relevance of the study of model systems is shown by considering the following remarkable observations. Years ago, Ehrenberg et al. showed the EPR spectra of the 5,6-dihydrothymine-5-yl radical observed in thymine, thymidine, and DNA. The spectra are nearly identical [46]. The reduction product observed in cytosine monohydrate is the N3 protonated anion. In solution, this reduction product gives rise to a 1.4-mT EPR doublet. The same feature is present in irradiated DNA at 77 K. Likewise, the result of e loss in guanine bases is characterized by a broad EPR singlet. The same feature is also evident in the EPR spectrum of DNA irradiated and observed at 77 K. 

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