Electron loss centers

Swarts SG, Becker D, Sevilla MD, Wheeler KT. (1996) Radiation-induced DNA damage as a function of hydration. II. Base damage from electron-loss centers. Radiat Res 145 304-314. 

Concerning cellular DNA, a two-component hypothesis has been developed. According to this hypothesis, the electron loss centers (radical cations) end up with the purines, particularly with the guanine moiety, whereas the final site of deposition of the ejected electron is with the pyrimidines, particularly with thymine [127]. The two-components hypothesis implies that in DNA there are mechanism of electron and positive hole transfer by which the initially generated and randomly distributed electron gain and loss centers are tunneled into the T and G traps respectively. 

On the very basic level, kinetic stability of 1,2-dications can be explained by molecular orbital (MO) theory. When loss of two electrons leading to the formation of a dication (double oxidation) occurs from an antibonding MO, it results in the formation of a normal (two-electron/two-center) chemical bond (Scheme 1). 

The cyclobutadiene derivatives [Co(Tj -C4Fc Ph4 )Cp] (n = 1-3) (Cp = 17 -cyclopentadienyl) undergo one-electron oxidation to stable cations (27) which show charge-transfer bands in the near infrared region. An analysis of the spectra led to the conclusion that the cations are Class II mixed-valence compounds and that the charge-transfer transition arises from a weak interaction between Co(I) and Fe(III) rather than between Fe(II) and Fe(III). Each compound showed further oxidations at more positive potentials, including an irreversible process ascribed to electron loss from the cobalt center. 

The debate over the site of electron loss in DNA is much less pronounced since it has been estimated that over 90% of the cations generated in DNA are centered on guanine [67] and guanine end products account for 90% of the electron loss products in DNA [70]. However, the spectra of G recorded in solid-state studies of nucleotides and nucleosides do not correspond to the spectrum recorded in fiiU DNA [71] and investigations of the strand-break specificity determined that some adenine cations could be generated [64]. Thus, it is also possible that other cations are formed, primarily A. ... 

Fig. 5.6. The theoretical ratios of the ionization cross sections for protons and antiprotons incident on hydrogen. The calculations are of the one and a haif centered FHBS type, with averages of the perturbative and unitary results taken, Martir et al. [1.8]. They were performed at energies 30,60, 100 and 200keV/amu. The dotted and broken lines are direct ionization. The solid line is for electron loss for proton impact, i.e., charge transfer added to direct ionization. The approximate symmetry about the Bom result for protons and antiprotons is resurrected if both mechanisms are considered.

Ahn, C.C., and O.L. Krivanek, 1982, A library of electron loss spectra at I eV resolution obtainable from Center for Solid State Science (Arizona State University). 

The decay of the anions (or the electron excess center) appears to proceed through several known steps. In carboxylic acids, protonation of the anion is a highly favored first step above 4.2 K. At 77 K or higher the protonated anion decays to form the acetyl radical by loss of water. This is followed by proton abstraction of a type CH2R. Such a scheme is illustrated in equation (4)... 

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