Guanine radical cation

Whereas other experimental methods have been used to obtain values of kti no other method provides values of k-t or equilibrium data. There are, however, several important limitations of our method. First, the method is restricted to relatively fast hole transport processes that can compete with charge recombination of the Sa -G+ radical ion pair (Fig. 6). This precludes the use of strong acceptors which can oxidize A as well as G (Fig. 2a). We find that hole transport cannot compete with charge recombination in such systems, even when a charge gradient is constructed which should favor hole transport [35]. Second, the method is unable to resolve the dynamics of systems in which return hole transport, k t, is very slow (<104 s-1) or systems in which multiple hole transport processes occur. Third, since the guanine cation radical cannot be detected by transient spectroscopy, the method is dependent upon the analysis of the behavior of Sa-. In section 3.4 we de-... 

Therefore, the N(9) radical should be more stable than the N(6) one. That is why both radicals coexist in the system and both N(9) and N(6) deprotonations take place. In the case of the guanine cation-radical, the presence of the carbonyl group in the pyridazine ring brings about two additional effects Deprotonation infringes on this ring exclusively, and double deprotonation leads to the formation of a distonic anion-radical. Scheme 1.25 depicts the differences mentioned. Adhikary et al. (2006) substantiated it experimentally (ESR and UV) and theoretically (B3LYP). 

CT, and possible C4 and C5 sugar radicals have been observed in irradiated hydrated DNA at 77K [66], The CT sugar radical was reported by Razskazovskii et al. in a DNA double helix [69]. The C1 was also produced in double stranded DNA at 77 K by photoexcitation of the guanine cation radical [70], The C3 radical was reported to be 4.5% of the total radical yield in the duplex (d(CTCTCGAGAG)), x-irradiated and observed at 4 K [71], It is very likely that the DNA simulations could be improved with the inclusion of a small percentage of these typical sugar radicals. 

Scheme 20-2. The numbering scheme and prototropic equilibria of one-electron oxidized guanine cation radical (G +), the mono- deprotonated species, (G(N1-H) and G(N2-H), in syn and anti- conformers with respect to the N3 atom) and the di- deprotonated species, G(-2H) . (Reprinted with permission from ref. [189], J. Phys. Chem. (2006) American Chemical Society.)...

Adhikary A, Kumar A, Becker D, Sevilla MD. (2006) The guanine cation radical Investigation of deprotonation states by ESR and DFT. J Phys Chem B 110 24171-24180. 

Adhikary A., Malkhasian A.Y. S., Collins S., Koppen J., Becker D., Sevilla M.D., UVA-Visible photoexcitation of guanine cation radicals produces sugar radicals in DNA and model structures, Nuc. Acid Res., 2005,33,5553-5564. 

Adhikary, A., Collins, S., Khanduri, D., Sevilla, M. D. (2007). Sugar radicals formed by photoexcitation of guanine cation radical in oligonucleotides. The Journal of Physical Chemistry B, 111, 7415. 

Shukla, L. 1., Pazdro, R., Huang, J., DeVreugd, C., Becker, D., 8c Sevilla, M. D. (2004). The formation of DNA sugar radicals from photoexcitation of guanine cation radicals. Radiation Research, 161, 582. 

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