Purines electron affinities

The values of the ionization potentials of the bases were unknown experimentally until quite recently, and the electron affinities are still unknown. The first experimental determination of the ionization potentials of the pyrimidine bases (uracil, thymine), by Akopyan and Vilesov,253 dates from 1965. Two years later Bergmann et al.25i measured the ionization potentials of some purine and pyrimidine bases including cytosine. The cytosine ionization potential as determined by mass spectrometry254 was found to be 8.90 + 0.2 eV. On the other hand the potential determined from the charge transfer spectra by Fulton and Lyons255 was 7.98 eV. 

The following papers may bo consulted for the ionization potentials and electron affinities of cytosine and of its complexes with different partners cytosine,171-173-177-178,180.184.186.200,201,203.207,259,260 guanine-cytosine,171-182 18 -106 2-NH2-purine (amine form)-cytosine, 2-NH2-purine(imine form)-cytosine and 2-NH2-purine(amino form)-cytosine (form 6),190 cytosine-cytosine and cytosine-cytosine-cytosine.171... 

Gua has the lowest reduction potential among the four nucleobases (Table 10.2), and hence it is preferentially oxidized to its radical cation (for the calculation of ionization potentials of the DNA bases see Close 2004 Crespo-Hernandez et al. 2004), and this property makes Gua and its derivatives to stick out of the other nucleobases with respect to its different free-radical chemistry. In contrast, Thy and Cyt are good electron acceptors, while the purines are only poor ones in comparison (for the calculation of electron affinities, see Richardson et al. 2004). This is of special importance in the effects caused by the absorption of ionizing radiation by DNA. 

Concerning the electron affinities of the tautomers, both methods predict that they should be greater for the N (1 )H and N( 3)H tautomers than for the N(9)H and N(7)H ones. These properties are also probably overestimated in the CNDO procedure. No experimental information is available on the electron affinities of purines. 

While adiabatic EAs of U and T are known from experiment to be 0 =b 0.1 eV, the uncertainty in the values for the purines A and G is much greater. A and G clearly have negative adiabatic electron affinities which DFT theory suggests to be ca. —0.35 eV (A) and —0.5 to —0.75 eV (G) with their vertical electron affinities... 

The electron affinities (EA) of the nucleobases have not been determined experimentally. Calculated values for the vertical and adiabatic EA obtained by scaling experimental and calculated values for other aromatic molecules are summarized in Table 1 [33a]. The vertical values follow the order U>T>C>A>G, with U having the largest (most positive) EA. The calculated adiabatic EA for C is less positive than the values for T or U. Chen and Chen [36] have asserted that the electron affinities of the purines are larger than those of the pyrimidines. However, this claim appears to be based upon questionable reduction potential measurements (see p. 114). The nucleobase anion radicals are estimated to be stabilized by c. 3 eV in aqueous solution. 

The biochemical applications involve the electronic nature of the components of DNA and proteins, especially the charge distributions, electron affinities, and gas phase acidities of purines and pyrimidines and amino acids. The role of electron reactions in diverse areas such as cancer, electron conduction, and sequence recognition all depend on fundamental energetic properties such as electron affinities and solution energies. We explain nonadiabatic experimental data from radiation chemistry by excited anionic states of biological molecules [24],... 

In the case of tetracyanoquinodimethane, carbon disulfide, nitromethane, and the purines and pyrimidines, two or more negative-ion states have been observed. In some cases the photoelectron spectrum can be assigned to an excited state and reveal an electron affinity lower than adiabatic electron affinity. In the case of cyclooctatetraene the onset in the PES spectrum is higher than adiabatic electron affinity because of the significant change in the geometry of the anion. 

The values of the electron affinities and rate constants for thermal electron attachment to the purines, pyrimidines, and heterocyclic compounds can be used to predict the temperature dependence of the ECD and NIMS response. These are similar to those made for the chlorinated biphenyls and naphthalenes in... 

ELECTRON AFFINITIES OF PURINES AND PYRIMIDINES 12.2.1 Predictions of Electron Affinities... 

TABLE 12.4 Experimental and Theoretical Electron Affinities (in eV) of Purines and Pyrimidines and Halogenated Uracils... 

Considering the scale of electron affinity of the different nucleosides, pyrimidine is found to be a much better electron acceptor than purine. On other hand, the purine bases, with the electron-rich imidazole, react with hydroxyl radical (OH ) faster than the electron-deficient pyrimidine [127]. This fact reflected the electrophilic nature of the hydroxyl radical. 

From ETS experiments (Aflatooni et al. 1998), Aflatooni et al. showed that energies required to attach an electron into the lowest empty valence molecular orbitals of all the bases are positive. This means that all the bases have negative valence vertical electron affinity (<0). The vertically formed anions of pyrimidines bases (C, T, and U) were found to be more stable than the purines (G and A) by ca. 0.2 eV. The ETS spectra of U, T, C, G, and A are shown in O Fig. 34-7. The spectra in O Fig. 34-7 arise due to the occupation of the lowest empty 7t -MOs of the parent molecules by the electron. The vertical lines on the spectra (O Fig. 34-7) show the position of the vertical attachment energies. For guanine anion, the ETS (shown in O Fig. 34-7) was observed for its enol tautomer (for details see Aflatooni et al. [1998]). For each of these molecules, three VAEs, associated with the three lowest vacant tTi, tT2, and 7T3 MOs, were determined. The VAEs 0.22,1.58, and 3.83 eV for uracil 0.29,1.71, and 4.05 eV for thymine 0.32,1.53, and 4.50 eV for cytosine 0.54,1.36, and 2.17 eV for adenine and 0.46,1.37, and 2.36 eV for guanine(enol) tautomer were determined by ETS. 

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