Radical cations trapping

Starting radical cation Trap Product kb Refs. 

Heteroatom (O and N) attachment to the C8-site of dG to form 8-oxo-dG and C8-arylamine adducts lowers the oxidation potential relative to dG. The oxidation potential of 8-oxo-dG is 0.74 V versus NHE. Consequently, 8-oxo-dG can act as a deep radical cation trap within duplex DNA. Depending on the DNA sequence, an 8-oxo-dG lesion will be the preferential site of further oxidation and will protect isolated Gs and GG steps from oxidation the oxidation of 8-oxo-dG by G(—H) occurs with a rate of 4.6 x 10 /M/s. Thus, there is speculation that GC-rich domains outside the coding regions of genes serve to protect the genome from mutagenesis by oxidation. ... 

Carbazole oxidized by nickel peroxide in the absence of light and in the presence of 2-methyl-2-nitrosopropane gave the radical 71, an observation taken as additional evidence for the intermediacy of radical cations, trapped in this case by the nitrosoalkane, in oxidative dimerization of carbazoles (see Section II,A,2). 

Dimer formation can be quenched by conducting the experiment in a nucleophilic solvent, and the product obtained is characteristic of radical cation trapping. The anti-Markovnikov addition of acetone across the C-C single bond of the methylated analogue, eq. 41 (116,117),... 

It has been suggested that the amine radical cation (46) is not directly involved in initiating chains and that most polymerization is initiated by benzoyloxy radicals.179 However, Sato et a ." employed spin trapping (3.5.2.1) to demonstrate that the anilinomethyl radical (45) was formed from the radical cation (46) by loss of a proton and proposed that the radical 45 also initiates polymerization. Overall efficiencies for initiation by amine-peroxide redox... 

Morkovnik et al. (1989) found experimentally that the addition of an equimolar amount of 4-morpholino- or 4-dimethylaminoaniline to a suspension of nitrosyl perchlorate in 100 % acetic acid, dioxan, or acetonitrile yields a mixture of the diazonium perchlorate and the perchlorate salt of the amine radical cation, with liberation of gaseous nitric oxide. Analogous results in benzene, including evidence for radicals by ESR spectroscopy and by spin trapping experiments, were obtained by Reszka et al. (1990). 

Photo-induced Diels Alder reaction occurs either by direct photo activation of a diene or dienophile or by irradiation of a photosensitizer (Rose Bengal, Methylene Blue, hematoporphyrin, tetraphenylporphyrin) that interacts with diene or dienophile. These processes produce an electronically excited reagent (energy transfer) or a radical cation (electron transfer) or a radical (hydrogen abstraction) that is subsequently trapped by the other reagent. 

Scheme 2 Competition between electron transfer (7 8) and water trapping (7 9+10) of sugar radical cation 7...

Fig-1 Influence of the pH value on the ratio of the electron transfer, forming enol ether 8, and the water trapping, yielding products 9+10, of the enol radical cation 7 in a DNA double strand... 

The efficiency of this injection system depends upon the reaction conditions. 02) which traps the first formed radical 6, reduces the yield of enol ether 8. Therefore, we are using our assay under anaerobic conditions. Also, the pH of the solution influences the product ratio because it changes the nucleophilicity of the water. Figure 1 shows how the efficiency of the electron transfer is reduced as the pH value increases from 5.0 to 7.0 [5]. This is in accord with an increase of the nucleophilicity of water, which traps the radical cation (7—>9+10) in competition to the electron transfer step (7—>8). 

Fig. 7 Histogram showing the products PG and PGgg> formed after charge injection into G, water trapping of the guanine radical cations and subsequent strand cleavage...

Fig. 11 Yields of the water trapping products Pq at the single guanines, and reaction profile diagram for the electron transfer as well as water trapping of the guanine radical cations...

In the perfectly paired double strand 22, the yield of product PGgg> which indicates the amount of charge that has reached the hole trap GGG, is 68%. But if the intermediate G C base pair is exchanged by a G T mismatch, the efficiency of the charge transport drops to 23%. With an abasic site (H) opposite to G the hole transport nearly stops at this mismatched site (Fig. 15). We have explained this influence of a mismatch on the efficiency of the charge transport by a proton transfer from the guanine radical cation (G2 +)... 

Fig. 1 Schematic mechanism for the long-distance oxidation of DNA. Irradiation of the anthraquinone (AQ) and intersystem crossing (ISC) forms the triplet excited state (AQ 3), which is the species that accepts an electron from a DNA base (B) and leads to products. Electron transfer to the singlet excited state of the anthraquinone (AQ 1) leads only to back electron transfer. The anthraquinone radical anion (AQ ) formed in the electron transfer reaction is consumed by reaction with oxygen, which is reduced to superoxide. This process leaves a base radical cation (B+-, a hole ) in the DNA with no partner for annihilation, which provides time for it to hop through the DNA until it is trapped by water (usually at a GG step) to form a product, 7,8-dihydro-8-oxoguanine (8-OxoG)...

Fig. 4 Schematic representation of long-distance radical cation migration in DNA. In AQ-DNA(l), irradiation of the anthraquinone group linked at the 5 -terminus leads to reaction at GG steps that are 27 A and 44 A from the site of charge injection. The amount of reaction observed at each guanine is represented approximately by the length of the solid arrow. In UAQ-DNA(2), irradiation of the anthraquinone leads to reaction at each of the eight GG steps. However, replacement of a G by 7,8-dihydro-8-oxoguanine (8-OxoG) introduces a deep trap that inhibits reaction at guanines on the same side of the DNA as the trap...

The relative amount of strand cleavage at each site of AQ-DNA(l) is indicated by the length of the solid vertical arrow shown in Fig. 4. As is often observed, the 5 -G of the GG steps react more often than do the 3 -G. In the case of AQ-DNA(l), the relative reactivity is ca. 1 3, but this ratio depends upon the specific base pair sequence surrounding a GG step, which may be an indication of radical cation delocalization to bases adjacent to the GG sequence. It is worth pointing out again that these reactions are carried out under single-hit conditions where the relative strand cleavage efficiency seen at various locations of AQ-DNA(l) reflect the statistical probability that the radical cation will be trapped by H20 at that site. 

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