Electronic oxidation reactions

One-Electron Oxidation Reactions of the Pyrimidine and Purine DNA Bases... 

One-Electron Oxidation Reactions of Cytosine and 5-methylcytosine DNA Base... 

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... 

Scheme 4 One-electron oxidation reactions of 2 -deoxyguanosine in aerated aqueous solutions... 

Interestingly, it was recently shown that the one-electron oxidation reaction promoted by photoexcited menadione (MQ) gives rise to A -formylade-nine (52) and A -acetyladenine (53) residues (Scheme 5) in several dinucleoside monophosphates including d(ApA), d(CpA) and d(ApC) [92]. 

Primary and secondary alcohols are oxidized to the corresponding carbonyl compounds by tetra-n-butylammonium persulphate in dichloromethane but, when the reaction is conducted in tetrahydropyran, tetrahydropyranyl ethers (>90%) are formed by a direct one-electron oxidative reaction of tetrahydropyran with the alcohol [9]. Tetrahydrofuranyl ethers have been prepared by an analogous method (10, 11],... 

Ultimately, the catalyst performance of a real fuel cell is of the greatest importance. The DEFC polarization curves for the two PtSn anode catalysts are tested and shown in Fig. 15.9. The characteristic data are summarized in Table 15.4. The PtSn-1 catalyst shows a strongly enhanced electron-oxidation reaction (EOR) activity and much better performance in both the activation-controlled region (low-current density region) and... 

An almost complete description of both OH radical-mediated and one-electron oxidation reactions of the thymine moiety (3) of DNA and related model compounds is now possible on the basis of detailed studies of the final oxidation products and their radical precursors. Relevant information on the structure and redox properties of transient pyrimidine radicals is available from pulse radiolysis measurements that in most cases have involved the use of the redox titration technique. It may be noted that most of the rate constants implicating the formation and the fate of the latter radicals have been also assessed. This has been completed by the isolation and characterization of the main thymine and thymidine hydroperoxides that arise from the fate of the pyrimidine radicals in aerated aqueous solutions. Information is also available on the formation of thymine hydroperoxides as the result of initial addition of radiation-induced reductive species including H" atom and solvated electron. 

Major emphasis has been on the isolation and identification of the main decomposition products arising from one electron oxidation reactions with the pyrimidine and purine bases of isolated DNA and related model compounds13,14D. In recent years, major interest has been devoted on the delineation of the mechanistic features of charge transfer within double stranded DNA. This is mostly achieved using defined-sequence oligonucleotides in which radical cations are generated in most cases by photo-ionization of selected nucleobases and 2-deoxyribose. For more information on these systems, the reader is encouraged to read the recent review article by Cadet et al.134 and other references mentioned there in. 

Electrochemical one- and two-electron oxidation reactions are discussed in Section 3.07.2.4.6. 

Much information can be gained by examining trends in redox potentials within series of compounds. Let us consider such a series of coordination compounds, M0, Mj, M2. .. M , which undergo reversible, one-electron oxidation reactions at potentials E°0, E°lf E°2,... E° respectively, with respect to the same reference electrode. If we define the oxidation of M0 as our standard reaction (equation 8), then we can examine the variation of the free energy difference, F(E°0— E° ), in terms of the structural difference between M0 and each other member M . Such an analysis is directly comparable to the classic approach of Hammett17 which relates a free energy difference term, log(AH/Ax), for equilibrium reactions such as (9) and (10), to the nature of the aryl substituent, X. 

Some insight into the understanding of these later intermediates comes from the observation that the fully oxidized enzyme may undergo a one-electron oxidation reaction, in which the electron donor is probably water and the acceptor ferricytochrome c. The overall product would be a one-electron oxidation product of the fully oxidized centre plus water. Presumably, one-electron reactions in the opposite direction can occur. The transfer of one electron from cytochrome a to the a3/CuB centre in compound C, plus one electron from a3 or CuB will allow a second concerted two-electron reaction with the formation of (Felv=02 CuB—OH-), and ESR visibility of the copper. In the next stage of the reaction antiferromagnetic coupling would be reintroduced. 

Thus, in most OH-induced oxidations short-lived adducts must be considered as intermediates. A case in point in the realm of DNA free-radical chemistry is the oxidation of guanine. From the above, it is evident that OH, despite its high reduction potential, cannot be directly used for the study of one-electron oxidation reactions. However, one can make use of its high reduction potential by producing other reactive intermediates [e.g Tl(II) Chap. 10], which no longer undergo an addition to double bonds or H-abstraction. 

Faraggi M, Klapper MH (1993) Reduction potentials determination of some biochemically important free radicals. Pulse radiolysis and electrochemical methods. J Chim Phys 90 711-744 Faraggi M, Klapper MH (1994) One electron oxidation of guanine and 2 -deoxyguanosine by the azide radical in alkaline solutions. J Chim Phys 91 1062-1069 Faraggi M, Broitman F, Trent JB, Klapper MH (1996) One-electron oxidation reactions of some purine and pyrimidine bases in aqueous solutions. Electrochemical and pulse radiolysis studies. J Phys Chem 100 14751-14761... 

At least two different catalytic sites have been suggested for the one- and two-electron oxidative reactions catalyzed by CPO [89]. Phenols and large substrates unable to gain access to the heme are presumably oxidized at the enzyme surface by (one-electron) LRET pathways similar to that described in Sect. 3.3.3 for lignino-lytic peroxidases, although the catalytic residue exposed to the solvent has not been yet identified. 

The reduction state of the pterin was a point of uncertainty throughout these studies of molybopterin derivatives. The absence of fluorescence in anaerobic molybdopterin samples suggested a reduced pterin. Redox titration of XO and SO both indicated that the pterin could undergo a two-electron oxidation reaction (73, 74). Sulfite oxidase, for example, produced the fluorescence characteristic of an oxidized pterin after addition of 2 equiv of ferricyanide. However, titrating XO was problematic due to interfering redox processes of the iron-sulfur clusters. 

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