Guanine, oxidation

In DNA RNA hybrids, where intrastrand base stacking is comparable to B-DNA, we observed efficient long range guanine oxidation when using ethidium as an intercalating photooxidant [130]. Conversely, a Rh(III) com-... 

Fig. 9 Illustrations of the charge transport in duplex 37/38, a in the absence and b in the presence of BamH I. Horizontal arrows and the numbers shown on the site of guanine oxidation indicate the band intensity relative to that of G24 in the protein bound duplex... 

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. 

The guanine radical cation is a weak acid = 3.9) [22]. Therefore deprotonation will depend on the environment. Bachler and Hildenbrand [38] have studied the guanine oxidation product in aqueous solution of 5 -dGMP. The best fit to their EPR spectra seems to be from the radical cation (Gua " ). 

The effect of photodynamic degradation on the viscosity of DNA132 was shown to be the result initially of single chain breaks, followed eventually by double chain scission this is in agreement with the idea that guanine oxidation is the cause of the viscosity change. 

Base Sequence Effects on the Rates of Guanine Oxidation. 144... 

The transient absorption spectra of duplexes with [2AP]A4GGAs are depicted in Fig. 5. At a delay time of 100 ns, the transient absorption spectrum is attributed to the superposition of the spectra of the 2AP(-H) and G /G (-H) radical products and the hydrated electrons. The structureless tail of the eh absorption in the 350-600 nm region decays completely within At<500 ns. The formation of G VG(-H) radicals monitored by the rise of the 310-nm absorption band and associated with the decay of the 2AP V 2AP(-H) transient absorption bands at 365 and 510 nm (Fig. 5) occurs in at least three well-separated time domains (Fig. 6). The prompt (<100 ns) rise of the transient absorption at 312 nm due to guanine oxidation by 2AP was not resolved in our experiments. However, the ampHtude, A((=ioo), related to the prompt formation of the G /G(-H) radicals (Fig. 6a) can be estimated using the extinction coefficients of the radical species at 312 and 330 nm (isosbestic point) [11]. The kinetics of the G VG(-H) formation in the yits and ms time intervals were time-resolved and characterized by two well-defined components shown in Fig. 6a (0.5 /zs) and Fig. 6b (60 /zs). 

The effects of the number of intervening bases on the rate constant of guanine oxidation by the 2AP(-H) radicals is described by the following equation ... 

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