Adenine radical cation

The adenine radical cation was observed in a single crystal of adenine hydrochloride hemihydrate [43]. In this crystal, the adenine is protonated at Nl. After electron loss, the molecule deprotonates at Nl, giving Ade(Nl -l-H, Nl-H). This produces a radical that is structurally equivalent to the cation of the neutral adenine molecule with spin density on C8 and N6 [p(C8) = 0.17 and p(N6) = 0.25]. The adenine radical cation is strongly acidic (pi a< 1) [22]. This strong driving force makes the reaction independent of environmental conditions. In single crystals of adenosine [42] and anhydrous deoxyadenosine [44], the N6 deprotonated cation [Ade(N6-H) ] is observed which is characterized by p(C8) = 0.16 and p(N6) = 0.42. The experimental isotropic hyperfine couplings are N6-H = 33.9 MHz and C8-H = 12.4 MHz. 

Huttermann J, Ward JF, Myers LS Jr (1971) Electron spin resonance studies of free radicals in irradiated single crystals of 5-methylcytosine. Int J Radiat PhysChem 3 117-129 Huttermann J, Ohlmann J, Schaefer A, Gatzweiler W (1991) The polymorphism of a cytosine anion studied by electron paramagnetic resonance spectroscopy. Int J Radiat Biol 59 1297-1311 Hwang CT, Stumpf CL, Yu Y-Q, Kentamaa HI (1999) Intrinsic acidity and redox properties of the adenine radical cation. Int J Mass Spectrom 182/183 253-259 Ide H, Otsuki N, Nishimoto S, Kagiya T (1985) Photoreduction of thymine glycol sensitized by aromatic amines in aqueous solution. J Chem Soc Perkin Trans 2 1387-1392 Idris Ali KM, Scholes G (1980) Analysis of radiolysis products of aqueous uracil + N20 solutions. J Chem Soc Faraday Trans 176 449-456... 

Reuther A, Iglev FI, Laenen R, Lauberau A (2000) Femtosecond photo-ionization of nucleic acid bases electronic lifetimes and electron yields. Chem Phys Lett 325 360-368 Reynisson J, Steenken S (2002) DFT calculations on the electrophilic reaction with water of the guanine and adenine radical cations. A model for the situation in DNA. Phys Chem Chem Phys... 

A + Adenine radical cation (in general also with respect to dAdo, Ado and DNA)... 

Adenine has a considerably higher oxidation potential than guanine [111], and for this reason it is not as readily oxidized, even by strongly oxidizing radicals, S04 " excepted. The p a of the adenine radical cation, at less than 1 [5], lies below that of its guanine analogue. 

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]. 

Fig. 14 Reaction profile diagram for the hole transfer from a guanine radical cation (G +) to a distant GGG sequence via the activated hopping mechanism, which also involves adenines (A) as charge carriers...

Intramolecular electron transfer leading to a bromouracil radical anion is supposed to take place upon irradiation of the duplex d(GCABrUGC)2 in sodium cacodylate buffer at pH 7.0437. The resulting uracilyl-5-yl radical can abstract the adjacent C-l hydrogen of the adenosine radical cation. Adenine is then released by hydrolytic cleavage of the A-glycosidic bond. 

The fluorescence polarization excitation spectrum has been measured for thymine in aqueous solution. " The depolarization at the red edge is attributed to the hidden n, ir transition. Ionization of the lowest excited singlet and triplet states have been determined by the effect of pH on the absorption, fluorescence, and phosphorescence spectra of purines and pyrimidines. " Spectral, polarization, and quantum yield studies of cytidylyl-(3, 5 )-adenosine have also been published. Intermediates in the room-temperature flash photolysis of adenine and some of its derivatives have been identified hydrated electron, radical cations and anions, and neutral radicals resulting from their reactions have been assigned. Photoionization occurs via the triplet state. FMN encapsulated in surfactant-entrapped water pools interacts with polar head groups, entrapped water molecules, and outer apolar solvent. ... 

The composition and sequence of the base pairs in DNA should have an important effect on the ET and HT mechanisms. With regard to HT, guanine (G) is much more easily oxidized than adenine A) or thymine (T) consequently, hole injection into the DNA helix will generate a G+ radical cation. The hole may now randomly... 

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