Independent Generation of Reactive Intermediates Leads to an Alternative Mechanism for Strand Damage Induced by Hole Transfer in Poly(dA–T) Sequences

Abstract

Purine radical cations (dA and dG) are the primary hole carriers of DNA hole migration due to their favorable oxidation potential. Much less is known about the reactivity of higher energy pyrimidine radical cations. The thymidine radical cation (T) was produced at a defined position in DNA from a photochemical precursor for the first time. T initiates hole transfer to dGGG triplets in DNA. Hole localization in a dGGG sequence accounts for ∼26% of T formed under aerobic conditions in 9. Reduction to yield thymidine is also quantified. 5-Formyl-2'-deoxyuridine is formed in low yield in DNA when T is independently generated. This is inconsistent with mechanistic proposals concerning product formation from electron transfer in poly(dA-T) sequences, following hole injection by a photoexcited anthraquinone. Additional evidence that is inconsistent with the original mechanism was obtained using hole injection by a photoexcited anthraquinone in DNA. Instead of requiring the intermediacy of T, the strand damage patterns observed in those studies, in which thymidine is oxidized, are reproduced by independent generation of 2'-deoxyadenosin- N6-yl radical (dA). Tandem lesion formation by dA provides the basis for an alternative mechanism for thymidine oxidation ascribed to hole migration in poly(dA-T) sequences. Overall, these experiments indicate that the final products formed following DNA hole transfer in poly(dA-T) sequences do not result from deprotonation or hydration of T, but rather from deprotonation of the more stable dA, to form dA, which produces tandem lesions in which 5'-flanking thymidines are oxidized.

Ultrafast Processes Occurring in Radiolysis of Highly Concentrated Solutions of Nucleosides/Tides

“…After formation of the “hole” (i.e., the unpaired spin) via direct-type effect, the hole is localized on the guanine base via base-to-base and backbone-to-base hole transfer processes [30,31,38,39,43,44,45,46,66]. Reactions of the localized hole on the guanine base in DNA are schematically shown as well [29,30,31,43,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,69]. For successful formation of a neutral sugar radical, a rapid deprotonation must occur from the one-electron oxidized sugar–phosphate backbone before a competitive backbone-to-base hole transfer takes place [30,31,43,45,46].…”

“…A substantial volume of work has been focused on radiation-induced hole-mediated damage in DNA via the oxidation pathway, as this pathway may lead to cellular death [29,30,31,38,39,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68]. A fast base-to-base hole transfer process [29,30,31,38,39,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72] and backbone-to-base hole transfer process [30,31,38,39,43,…”

“…are the most stable hole localization sites after long range hole transfer [29,30,31,38,39,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72]. Previous laser spectroscopic measurements or the use of one-electron oxidants (SO 4 •− ), which had often been used to model the pathways involved in DNA damage via the direct effect, were not able to probe these dynamics immediately following the ionizing radiation [29,30,31,38,39,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,…”

Independent Generation and Time‐Resolved Detection of 2′‐Deoxyguanosin‐N2‐yl Radicals

“…[9] However,p hotochemical conversion of 3 to dG(N2-H)C is too inefficient for laser flash photolysis examination of dG(N2-H)C.M ore recently,w e reported on amethod for generating 2'-deoxyadenosin-N6-yl radical (2)f rom ak etone precursor (4,S cheme 2). [10] Upon photolysis, 4 undergoes Norrish Type Ip hotocleavage followed by rapid b-fragmentation. Using acetone as triplet photosensitizer greatly increased the conversion efficiency of 4 and allowed us to obtain the spectrum of 2.H owever, photosensitization of 3 by ketones was not attempted, because photodissociation of tetraphenylhydrazine occurs from the excited singlet.…”

“…Tabelle 1: Product studies upon anaerobicp hotolysisof1 in the presence of acetone (2 %v/v) as sensitizer. [a] Reducing agent (mm) dG yield [%] [b] Mass balance [%] [b] Fe 2+ (10) 85 AE 29 0 AE 1 PhSH (10) 89 AE 39 3 AE 1 BME (10) 87 AE 29 1 AE 2 none [c] 55 AE 36 5 AE 1 none [d] 35 AE 24 4 AE 3 Tabelle 2: Effect of acetonitrile on the conversiono f1 (0.1 mm)w ithin 30 min in the presence of acetophenone (1 %v/v).…”