Radical Cations with Biological Relevance

8 Radical Cations with Biological Relevance. - The radical cation of 1-ben-zyl-l,4-dihydronicotinamide, an analogue of NADH, was generated by oxidation with Fe(bpy)33+ and Ru(bpy)33+ (bpy = 2,2 -bipyridine) in deaerated acetonitrile solutions using a rapid-mixing flow apparatus. The ESR spectra reveal a keto structure of the radical cation, rationalized by DFT calculations. Upon photo-induced electron transfer, also the enol form could be established, which, however, relaxes to the keto form131,132. 

Novel porphyrin-based photosensitizers, bacteriochlorins exhibiting long-wavelength absorptions between 788 and 831 nm, are promising candidates for 

Radical cations may serve as acids by deprotonation and formation of the corresponding neutral radicals. This has been often shown in rigid matrices. An analogous reactivity was established in the radical cations of vitamin E type137 and carotenoids (see above)138. The latter lead to the formation of didehyd-rodiiners. 

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. 

The use of alanine in dosimetry has been established for many years. A theoretical study of the primarily produced radical cations and their transformations was performed in a large-cluster approach (central molecule with 12 surrounding molecules). Whereas deprotonation is a preferred process, decarboxylation is highly improbable141. 

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Whereas little is known about ketone and enol radical cations in solution, the related one-electron oxidation of phenols has been extensively studied [72]. Nowadays, anodic oxidation of phenols constitutes a valuable synthetic access to phenoxenium ions [73] which are important intermediates for carbon-carbon bond formation processes [74-76] and to various natural products [77]. In light of the biological relevance of phenol oxidation [78,79] redox potentials of phenols [72,80] and phenolates [80-85] as well as pK values of phenol radical cations [80,86,87] are documented in various solvents. Some of the data will be quoted later in comparison with enol systems. 

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