Radical cations deprotonation

The differential absorption spectra obtained in the presence of these two nucleotides are indeed similar to those obtained after reduction electrolysis of the complex in the first reduction wave, and obtained by pulse radiolysis. The prerence of the deprotonated radical cation GMI —H) can also be detected by recording the transient absorption after reaction of the reduced complex with O2. 

A number of papers have reported studies on pyrimidine radical cations. 1-Methylthymine radical cations generated via a triplet-sensitized electron transfer to anthraquinone-2,6-disulfonic acid were detected by Fourier transform electron paramagnetic resonance (FTEPR). The parent 1-methylthymine radical cation, and its transformation to the N(3)-deprotonated radical cation, were observed. Radical cations formed by addition of HO and POs" at C(6) were also detected depending on the pH. Similarly, pyrimidine radical cations deprotonated at N(l) and C(5)-OH were detected from the reaction of 804 with various methylated pyrimidines." These radicals are derived from the initial SO4 adducts of the pyrimidines. Radical cations of methylated uracils and thymines, generated by electron transfer to parent ions of... 

Shi Y, Fluang C, Wang W, Kang J, Yao S, Lin N, Zheng R (2000a) Electron transfer from purine de-oxynucleotides to deoxynudeotides deprotonated radical cations in aqueous solution. Radiat... 

This radical cation may then ionize a solvent molecule by proton transfer, if the proton affinity of the solvent molecule is higher than that of the deprotonated radical cation. It seems that the solvent acts as aggregates, having then a higher proton affinity. These protonated solvent molecules may then ionize analyte molecules by proton transfer if these last have a higher proton affinity than the solvent molecules ... 

Formation of an intimate ion pair of OH " and aminium radical cation was also proposed for the intermediate step before deprotonation. The presence of the above radical was verified through UV analysis of the polymer formed with the characteristic band on the end group. Through chromatographic analysis of the TBH-DMT reaction products, H2O was detected as the above mechanism proposes after deprotonation. 

Deoxycytidine (dCyd) (14 in Scheme 2) is also an excellent target for one-electron oxidation reactions mediated by triplet excited menadione. On the basis of extensive identification of dCyd photooxidation products, it was concluded that this nucleoside decomposes by competitive hydration and deprotonation reactions of cytosine radical cations with yields of 52% and 40%, respectively [53]. It was also found, on the basis of 180 labeling experiments, that hydration of cytosine radical cations (15) predominantly occurs... 

It may be noted that competitive deprotonation of 29 at C-l gives rise to 2-deoxyribonolactone (27) with the concomitant release of free 5-methylcy-tosine as minor processes. Interestingly, competitive hydration of 5-MedCyd radical cations (29) occurs exclusively at C-6 as inferred from labeling experiments with 1802 (36) [61]. Thus, mass spectrometry analysis of the four cis and trans diastereomers of 5-MedCyd glycols 36 showed that incorporation of 1802 takes place exclusively at C-5 of 6-hydroxy-5,6-dihydro-2 -deoxycy-tyd-5-yl radicals (34). 

One-electron oxidation of the adenine moiety of DNA and 2 -deoxyadenos-ine (dAdo) (45) gives rise to related purine radical cations 46 that may undergo either hydration to generate 8-hydroxy-7,8-dihydroadenyl radicals (47) or deprotonation to give rise to the 6-aminyl radicals 50. The formation of 8-oxo-7,8-dihydro-2 -deoxyadenosine (8-oxodAdo) (48) and 4,6-diamino-5-formamidopyrimidine (FapyAde) (49) is likely explained in terms of oxidation and reduction of 8-hydroxy-7,8-dihydroadenyl precursor radicals 47, respectively [90]. Another modified nucleoside that was found to be generated upon type I mediated one-electron oxidation of 45 by photoexcited riboflavin and menadione is 2 -deoxyinosine (51) [29]. The latter nucleoside is likely to arise from deamination of 6-aminyl radicals (50). Overall, the yield of formation of 8-oxodAdo 48 and FapyAde 49 upon one-electron oxidation of DNA is about 10-fold-lower than that of 8-oxodGuo 44 and FapyGua 43, similar to OH radical mediated reactions [91]. 

Scheme 6 Deprotonation of a guanine radical cation with and without a complementary base...

Experiments with methyl guanine (27), in which the acidic proton of the radical cation is exchanged by a methyl group, support this explanation [22]. With this base in a mismatch situation (strand 25) the hole transfer becomes efficient again because a deprotonation cannot occur (Fig. 15). 

The radical cations of urazole-annelated azoalkanes 65 were generated by pulse radiolysis and the transients characterized spectrally and kinetically by time-resolved optical monitoring. The initial distonic 1,3 radical cations 66 were detected, and the methyl-substituted 66 further deprotonates to radical 67 (Scheme 1) <1997JA10673>. 

Fig. 9 Deprotonation of cations radicals of synthetic analogs of NADH by oxygen or nitrogen bases in acetonitrile. Correlation between the intrinsic barrier and the homolytic bond dissociation energy of the cation radical (AH + —> A+ + H ).

Although several examples of anodic dimerizations involving the coupling of cation radicals have been described,34 the most systematic mechanistic analyses have concerned electropolymerizations leading to conducting polymers.35 They involve a series of successive cation radical-cation radical coupling steps as represented in Scheme 2.33 for the case of pyrrole. Deprotonation of the first... 

Scheme 9 Cyclization and deprotonation of an alkene radical cation...

The regiochemistry of nucleophilic addition to alkene radical cations is a function of the nucleophile and of the reaction conditions. Thus, water adds to the methoxyethene radical cation predominantly at the unsubstituted carbon (Scheme 3) to give the ff-hydroxy-a-methoxyethyl radical. This kinetic adduct is rearranged to the thermodynamic regioisomer under conditions of reversible addition [33]. The addition of alcohols, like that of water, is complicated by the reversible nature of the addition, unless the product dis-tonic radical cation is rapidly deprotonated. This feature of the addition of protic nucleophiles has been studied and discussed by Arnold [5] and Newcomb [84,86] and their coworkers. 

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