Aminyl Radical Generation via Tandem Norrish Type I Photocleavage, β-Fragmentation: Independent Generation and Reactivity of the 2′-Deoxyadenosin- N6-yl Radical

Abstract

Formal hydrogen atom abstraction from the nitrogen-hydrogen bonds in purine nucleosides produces reactive intermediates that are important in nucleic acid oxidation. Herein we describe an approach for the independent generation of the purine radical resulting from hydrogen atom abstraction from the N6-amine of 2′-deoxyadenosine (dA•). The method involves sequential Norrish Type I photocleavage of a ketone (7b) and β-fragmentation of the initially formed alkyl radical (8b) to form dA• and acetone. The formation of dA• was followed by laser flash photolysis, which yields a transient with λmax ~ 340 nm and a broader weaker absorption centered at ~560 nm. This transient grows in at ≥ 2 × 105 s−1; however, computations and reactivity data suggest that β-fragmentation occurs much faster, implying the consumption of dA• as it is formed. Continuous photolysis of 7b in the presence of ferrous ion or thiophenol produces good yields of dA, whereas less reactive thiols afford lower yields presumably due to a polarity mismatch. This tandem photochemical, β-fragmentation method promises to be useful for site-specific production of dA• in nucleic acid oligomers and/or polymers and also for the production of aminyl radicals, in general.

Traceless Tandem Lesion Formation in DNA from a Nitrogen-Centered Purine Radical

“…64 In addition, BME reacts inefficiently with dA•. 36 Hence, we are confident that the reduction in cleavage at dG 11 as a function of BME concentration is a reflection of the competition between the reaction of Tp• with the purine ( k Add ) and the thiol ( k Red ) . The amount of Tp• reduction product was calculated to be the difference in alkali-induced cleavage at dG 11 in the absence of BME and the presence of a given thiol concentration.…”

“…Cleavage was induced either by alkali (piperidine) treatment or incubation with the BER enzyme, formamidopyrimidine glycosylase (Fpg), which cleaves DNA at many purine lesions. 6,36 8-Oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodGuo) was believed to be the major product at dG 11 , based upon its lability to Fpg and the ability of β-mercaptoethanol (BME) to prevent piperidine induced cleavage. 37 However, the structure of the modification(s) at T 12 and the final fate of dA• were undetermined.…”

DNA Damage Emanating From a Neutral Purine Radical Reveals the Sequence Dependent Convergence of the Direct and Indirect Effects of γ-Radiolysis

“…18 Consequently, we anticipated that when flanked by dA, dA• will be at least partially protonated to form dA •+ and result in chemistry in DNA that is attributable to both species (Scheme 2). Site selective, independent dA• generation from 1 20 in DNA enables us to directly probe the nucleobase radical’s reactivity, including the effects of the flanking sequence. All duplexes used in this study contain a dGGG reporter group proximal to the site at which the radical is produced.…”

Biphotonic Ionization of DNA: From Model Studies to Cell

“…The presence of a methyl group at N3 increases the formation of products arising from the hydration pathway in a particular case, suggesting that deprotonation also takes place from the N3 position of the . In the absence of oxygen, the 5‐methyl‐(2′‐deoxyuridylyl) radical undergoes either reduction or oxidation depending on the redox properties of additional compounds . In aqueous aerated solutions, O 2 efficiently competes for the 5‐methyl‐(2′‐deoxyuridylyl) radical leading to the corresponding peroxyl radical.…”

“…In a molar excess of dAdo, however, there was evidence for the formation of novel dAdo‐dAdo dimeric products likely connected though N3 or N6 positions . Recently, near‐UV photolysis of an N6‐substituted alkoxylamine adenine derivative was shown to yield the corresponding N6‐aminyl radical by Norrish type I cleavage . An interesting aspect of this reaction is the lack of formation of secondary radicals.…”