Radical Anions and their Protonated Forms

The radical anions may be formed by reacting the nucleobases with eaq which may be either generated radiolytically or in a two-step reaction, e.g in the laser flash photolysis of anthraquinonedisulfonate in the presence of pyrimidines (yielding the pyrimidine radical cation and an anthraquinonedisulfonate radical anion) and subsequent photoionization of the anthraquinonedisulfonate radical anion (Lii et al. 2001). The latter approach, combined with Fourier transform EPR spectroscopy, yielded detailed information as to the conformation of the radical anions of Ura and Thy in aqueous solution (for a discussion see Close 2002 Naumov and Beckert 2002). Similarly valuable EPR information has been obtained from y-irradiated single crystals (cf. Box and Budzinski 1975 Boxet al. 1975 Sagstuen et al. 1998). 

Pyrimidines. The pyrimidines react with eaq at practically diffusion-controlled rates (Table 10.6). The ensuing reactions of the radical anions of Ura and Thy are very well understood, while with that of Cyt some open questions still remain. 

Thy and Ura behave like typical carbonyl compounds, and the first intermediate is a radical anion [reaction (163), in the case of Thy/Thd] which is in equilibrium with its O-protonated conjugate acid [equlibrium (164)]. 

The other functional groups, especially the second carbonyl function, withdraws considerable electron density, and hence the pK3 values of these O-pro-tonated radical anions are much lower than those of the simple a-hydroxyalkyl radicals (Thy pKa = 6.9 (Hayon 1969) Ura pfCa = 7.2 (Shragge and Hunt 1974) the value of 4.3 reported for the pfCa for the protonated 5,6Me2Ura radical anion (Aravindakumar et al. 1998) is surprising low and difficult to explain on the basis of the effects of methyl substituents on the pfCa values of a-hydroxyalkyl radi- 

The radical anion can also be repaired by polyphenols such as rutin or quer-citin (Shi et al. 2000b Zhao et al. 2003). Yet, this repair is not related to their phenolic functions but is rather due to an ET to their carbonyl (flavone) functions. 

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Table 7.4. Absorption maxima and molar absorption coefficients of cyclic disulfide radical anions and their protonated forms. According to von Sonntag (1990).The lipoic acid value has been taken from Hoffman and Hayon (1972). Note that the eguilibria (cf. apparent pKa values) include the ring-open forms which are not detectable...

The properties of the purine radical anions formed by their reaction with eaq resemble those of the Cyt radical anion. Because the pKa values of the heteroatom-protonated conjugate acid of the purine radical anions are very high, the purine radical anion are rapidly protonated by water (Hissung et al. 1981a Visscher et al. 1987 von Sonntag 1991 Candeias and Steenken 1992a Candeias et al. 1992 Aravindakumar et al. 1994). For example, the dAdo radical anion... 

A subsequent study ° from the Arnold group showed an intriguing stereoelectronic effect in oxidative benzylic carbon-hydrogen bond cleavage reactions of substrates 8 and 9 (Scheme 3.7). In this study, electron transfer reactions were conducted in the presence of a nonnucleophilic base. Radical cation formation also weakens benzylic carbon-hydrogen bonds, thereby enhancing their acidity. Deprotonation of benzylic hydrogens yields benzylic radicals that can be reduced by the radical anion of dicyanobenzene to form benzylic anions that will be protonated by solvent. This sequence of oxidation, deprotonation, reduction, and protonation provides a sequence by which epimerization can be effected at the benzylic center. In this study, tram isomer 10 showed no propensity to isomerize to cis isomer 11 (equation 1 in Scheme 3.7), but 11 readily converted to 10 (equation 2 in Scheme 3.7). The reactions were repeated in deuterated solvents to assure that these observations resulted from kinetic rather than thermodynamic factors. Trans isomer 9 showed no incorporation of deuterium (equation 3 in Scheme 3.7) whereas cis isomer 11 showed complete deuterium incorporation. The authors attributed this difference in reactivity to... 

Aqueous solutions of the anion-radicals of p-nitrophenol, namely HOC6H4NOo"- and HOC6H4N(V- as well as their protonated forms H0C6H4N02H- and H0C6H4N03H were studied using the technique of pulse radiolysis. 

A more specific classification can be made in terms of the types of radical ion produced in the initial electron transfer (le) step (i.e., in reactions not of the electrocatalytic type), viz., radical anions or radical cations, and their subsequent reactions to form stable products, which may include processes such as a further le transfer step, a dimerization after proton transfer, a rearrangement or cleavage, etc. 

Controlled one-electron reductions transform l,2,3,4-tetraphenyl-l,3-cyclopentadiene or 1,2,3, 4,5-pentaphenyl-l,3-cyclopentadiene into mixtures of the dihydrogenated products and the corresponding cyclopentadienyl anions (Famia et al. 1999). The anion-radicals initially formed are protonated by the substrates themselves. The latter are thermodynamically very strong acids because of their strong tendency to aromatization. As with the cyclopentadiene anion-radicals, they need two protons to give more or less stable cyclopentadienes. The following equations represent the initial one-electron electrode reduction of l,2,3,4,5-pentaphenyl-l,3-cyclopentadiene (CjHAtj) and explains the ratio and the nature of the products obtained at the expense of the further reactions in the electrolytic pool ... 

Handy et al. 2006). Anion-radicals are bases and their basicity is freqnently very high. When the anion-radicals are formed in alkylimidazolinm ionic liqnids, there is some danger of their nndesir-able protonation. 

Visscher KJ, de Haas MP, Loman H, Vojnovic B, Warman JM (1987) Fast protonation of adenosine and of its radical anion formed by hydrated electron attack a nanosecond optical and dc-conduc-tivity pulse radiolysis study. Int J Radiat Biol 52 745-753 Visscher KJ, Spoelder HJW, Loman H, Hummel A, Horn ML (1988) Kinetics and mechanism of electron transfer between purines and pyrimidines, their dinucleotides and polynucleotides after reaction with hydrated electrons a pulse radiolysis study. Int J Radiat Biol 54 787-802 von Sonntag C (1980) Free radical reactions of carbohydrates as studied by radiation techniques. Adv Carbohydr Chem Biochem 37 7-77... 

In this account, we will focus on the transient analysis of these systems, which has strongly contributed to a deeper understanding of the diverse reaction modes (Patemo-Buchi, proton abstraction, cycloaddition). In general, aromatic ketones were selected as electron acceptors for reasons of suitable excitation and long wavelength absorption of the radical anion intermediates. Among them, fluorenone 3 is particularly well suited since the concentration, solvent, temperature, and cation radius dependence of the absorption spectra of pairs formed with metal cations are already known [29]. Hogen-Esch and Smid [30, 10] pointed out that a differentiation between CIP and SSIP is possible for fluorenone systems. On the other hand, FRI s and SSIP s cannot be differentiated simply by their UV/Vis absorption spectra, whereas for instance conductance measurements may be successful. However, the portion of free radical ions in fluorenyl salt solutions was shown to be less important [9, 31]... 

Flash photolysis of misonidazole, metronidazole, and nitrobenzothiazoles has been carried out in [1369-1371], Laser flash-photolysis (355 nm) allows to determine relatively stable anion-radicals of misonidazole and metronidazole in aqueous solutions [1370], Solvated electrons have been formed at harder irradiation, the result of which interaction with nitroimidazole molecules is generation of their radical anions [1372], The authors [1372] have also found that fluorescence intensity of metronidazole is about 20 times more than that of misonidazole in same conditions. Photochromic properties of benzothiazole derivatives containing nitro and methyl groups in the ortho positions with respect to each other were studied by flash photolysis [1371], The application of the thermodynamic approach to predict the kinetic stability of formed nitronic acids is limited owing to specific intramolecular interactions. The lifetime of photoinduced nitronic acid anions tends to increase with rise in the chemical shift of the methyl protons. The rate constants photoinduced nitronic acids and their anions increase as the CH3C-CN02 bond becomes longer [1371],... 

The radical anions of cyanoaromatic compounds, cyanoalkenes and aromatic hydrocarbons, which are generated by photoinduced electron transfer, can be protonated by protic solvents or by radical cations of amines to form their neutral radicals. Disproportionation of the radicals yields the reduction products [114]. 

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