Radical anion adenine

The adenine anion has a = 3.5 [22]. After electron capture, the negative charge of the adenine radical anion resides mainly on N1, N3, and N7 and therefore protonation likely occurs at one of these nitrogens. The results show that in single crystals examined at 10 K, that reduction of adenine leads to the N3 protonated adenine radical anion, Ade(N3 -I- H), with spin densities of ca. p(C2) = 0.41, p(C8) = 0.14, and p(N3) = 0.12 [42]. 

The adenine anion has a pK, = 3.5. After electron capture the negative charge of the adenine radical anion resides mainly on Nl, N3, and N7 and therefore... 

Although the reduction potentials argue for thymine, as the most easily reducable base in protic solvents like water, subsequent protonation reactions need to be considered as well. The coupling of single electron reduction with a subsequent protonation step will strongly affect the ease of single electron reduction. Table 2 contains the pKa-values of some nucleobases in their reduced and neutral states [37]. It is clear that the thymine radical anion, due to its rather neutral pKa-value of about 7 is unlikely to become pro-tonated either by water or by the adenine counter base in the DNA strand. 

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... 

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. 

A pulse of 2-10 Mev electrons from a linear accelerator, passed through aqueous solutions of pyridinium ions leads rapidly to substantial concentrations of pyridinyl radicals , including those derived from the coenzyme, nicotinamide adenine dinucleotide . The pyridinyl radicals disproportionate, are protonated or dimerize. The reducing agent is a solvated electron or the COj" radical anion (Eq. 4) . 

The reaction of cinnamic acid and its derivatives with HO radical in neutral medium results in the formation of hydroxylation products (either on the benzene ring or on the exocyclic chain) and decarboxylation products. The latter products are also obtained from the reaction of the same substrates with the 804 radical anion. Evidence for the protecting effects of cinnamic acid and hydroxylated derivatives against oxidative reaction mediated by HO is provided. The use of adenine as a model compound suggests that the high protective effect is due to an antioxidizing cascade process." ... 

FADH% flavin adenine dinucleotide, neutral semiquinone radical FAD" , flavin adenine dinucleotide, anion semiquinone radical FMN, flavin mononucleotide ... 

Another approach by Hiittermann et al. [50] has been to use pulsed EPR techniques to study the radicals present in DNA fibers equilibrated in D2O and then irradiated and observed at 77 K. This work supports the conclusions that the primary radiation-induced defects are Cyt and Gua. Also reported are contributions from Thy and an allyl radical found on thymine [Thy(Me—H) ]. Also discussed are three components tentatively assigned as adenine and guanine anions and a species whose dominant hyperfine interaction involves the N1 of cytosine. 

Moorthy PN, Hayon E (1975) Free-radical intermediates produced from the one-electron reduction of purine, adenine and guanine derivatives in water. J Am Chem Soc 97 3345-3350 Mori M, Teshima S-l, Yoshimoto H, Fujita S-l, Taniguchi R, Hatta H, Nishimoto S-l (2001) OH Radical reaction of 5-substituted uracils pulse radiolysis and product studies of a common redox-ambivalent radical produced by elimination of the 5-substituents. J Phys Chem B 105 2070-2078 Morin B, Cadet J (1995) Chemical aspects of the benzophenone-photosensitized formation of two lysine - 2 -deoxyguanosine cross-links. J Am Chem Soc 117 12408-12415 Morita H, Kwiatkowski JS,TempczykA(1981) Electronic structures of uracil and its anions. Bull Chem Soc Jpn 54 1797-1801... 

Sevilla MD, Becker D, Yan M, Summerfield SR (1991) Relative abundances of primary ion radicals in y-irradiated DNA cytosine vs thymine anions and guanine vs adenine cations. J Phys Chem 95 3409-3415... 

In non-aqueous media (DMF, DMSO, acetonitrile), purine and some of its 6-sub-stituted derivatives (adenine, 6-methylpurine, 6-methylaminopurine, 6-methoxypurine, 6-dimethylaminopurine) undergo an initial le reduction to form the corresponding anionic free radical, followed by dimerization with rate constantsof 103—10s lmol l l64. The free radical nature of the le reduction product was independently established by ESR spectroscopy l69>. This behaviour is to be contrasted with the 2e and 4e reductions undergone by these compounds in aqueous media. s6-99-155. ... 

Figure 18-9. Radical I, the N3 protonated adenine anion A(N3+H) (Reprinted with permission from ref. [30], Radiation Research (1981) Radiation Research Society.)...

From these results one can see the incredible power of the combined EPR/ENDOR experiment. While the EPR spectrum of irradiate adenosine had rather narrow lines, the spectrum was unresolved due to the overlap of several radicals. The ENDOR spectra were easy to follow for complete rotations about all three crystallographic axes. Analysis of the ENDOR data yielded accurate anisotropic hyperfine tensors that could be related to two different free radicals. From these results one can confidently say that Radical I is the N3 protonated adenine anion A(N3+H) and Radical II is the N6 deprotonated adenine cation A(N6-H) With ENDOR data one is able to determine the protonation state of a radical, and if care is taken in the analysis, to even discern slight deviations from planarity of radicals. 

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. ... 

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