Singlet guanine

Devasagayam, T.P.A., Steenken, S., Obendorf, M.S.W., Schultz, W.A. and Sies, H. (1991). Formation of 8-hydroxy(deoxy)guanosine and generation of strand breaks at guanine residues in DNA by singlet oxygen. Biochemistry 30, 6283-6289. 

Table 11-5. Vertical excitation energies in eV relative to the ground state minimum of the singlet electronic excited states in guanine...

Shukla MK, Leszczynski J (2005) Effect of hydration on the lowest singlet jiji excited-state geometry of guanine a theoretical study. J Phys ChemB 109 17333-17339... 

Fig. 2 a Energetics of photo oxidation of G and A by singlet Sa. b Dynamics of charge separation (kcs) and charge recombination (kCY) for Sa-linked hairpins possessing a single guanine... 

CT-induced damage, consistent with theoretical predictions that the HOMO of guanine doublets is localized on the 5 -G [90]. Conversely, non-specific oxidation of both guanines is indicative of an alternative chemistry, for instance reaction with singlet oxygen. 

H-atom abstraction has been demonstrated to be the mechanism of action of excited uranyl ions, and in this case negligible base oxidation is found. Nucleo-base (especially guanine) oxidation is the principal reaction caused by singlet oxygen and this reactive species can be generated by a number of the complexes (e.g. many Ru(II)polypyridyls and porphyrins). It is worth pointing out, however, that the yield of may be lower when the sensitiser is bound to DNA, and it is the authors view that some of the reactions claimed to proceed via 62 may be caused by direct reaction of the photo-oxidised sensitiser with the DNA. 

Guanine is a preferential DNA target to several oxidants it shows the lowest ionization potential among the different purine and pyrimidine nucleobases and it is the only nucleic acid component that exhibits significant reactivity toward singlet oxygen ( O2) at neutral pH. ... 

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. 

Deprotonation of the dG radical cations in double-stranded DNA is evident from the EPR spectra of guanine radicals recorded in neutral solutions at room temperature [80, 81]. The EPR signal assigned to the neutral G(-H) radical derived from the deprotonation of G shows the singlet with g 2.004 and half width at a half height -0.8 mT. However, the deprotonation rate of G cannot be estimated from the conventional EPR spectra, and further time-resolved EPR studies with laser pulse generation of the radicals are required to address this problem. 

Perturbation of the lanthanide singlet excited state, in which quenching by electron- or charge-transfer processes may be altered by the binding of analytes to the metal centre. For example the W-alkylphenanthridinium unit in 11.32 binds selectively to the electron-rich guanine-cytosine... 

P.C. Mishra et al., Interaction of singlet oxygen and superoxide radical anion with guanine and formation of its mutagenic modification 8-oxoguanine. Int. J. Quantum Chem. 102, 282-301 (2005)... 

Figure 13-8. Relevant schematic potential energy curves for the near UV photophysics of the most stable tautomers of guanine. The region of the first singlet tht excited state surface accessible by Franck-Condon excitation is indicated in bold. Excited state internal conversion through a conical intersection (Cl) with S0 is illustrated by curved arrows. Vertical arrows indicate fluorescence emission. The eventual role of excited nir singlet states cannot be ruled out, especially at high energy excess in the excited state (see text)...

Table 14-2. Computed barrier height (kcal/mol) for guanine and hypoxanthine corresponding to the keto-enol tautomerism in the ground and the lowest singlet tt-jt excited state obtained at the B3LYP/6-311++G(d,p) and TD-B3LYP/6-311++G(d,p)//CIS/X levels, respectively in the gas phase and in aqueous solution [163, 165]a...

Table 14-3. Selected dihedral angles (°) of the guanine in the isolated and hydrated forms and that in the GC and GG base pairs in lowest singlet tttt excited state obtained at the CIS/6-311G(d,p) level [167]...

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