DEVELOPING PHOTOCHEMICAL PRECURSORS FOR INDEPENDENT GENERATION OF NEUTRAL PURINE RADICALS AND INVESTIGATING THE REACTIVITY OF 2′-DEOXYADENOSIN-N6-YL RADICAL IN DNA
Embargo until
2021-05-01
Date
2018-12-05
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Johns Hopkins University
Abstract
DNA damage is deleterious to cells and can lead to cell death or cancer. The cytotoxicity of DNA damage is exploited by cancer treatments such as radiotherapy. Ionizing radiation damages DNA indirectly by reacting with hydroxyl radicals to generate reactive intermediates, e.g., nucleobase and deoxyribose radicals; or by directly generating nucleobase radicals. Neutral purine radicals (dA•, dG(N1-H)•, and dG(N2-H)•) generated by formal hydrogen atom abstraction of purines are believed to play a prominent role in oxidative DNA damage and DNA hole migration. However, very little is known about their reactivity.
Studying these intermediates via radiolysis is complicated due to the concomitant formation of other reactive species. To overcome the dearth of methods for photochemically generating neutral purine radicals, we developed two general strategies. The first type of precursors utilized photoinduced homolytic cleavage of weak covalent bonds (3, 4 and 5). The second type of precursors (6 and 7) underwent a Norrish Type I photocleavage and β-fragmentation cascade to form neutral purine radicals and acetone. These precursors allowed mechanistic studies on the reactivity of neutral purine radicals as monomers in solutions or within DNA.
Herein, we report that dA• initiates tandem lesion formation in 5'-d(GTA) sequences but does not induce DNA hole migration. We propose that the tandem lesion formation is initiated by dA• abstracting the hydrogen atom from the C5-methyl group of the 5'-adjacent thymidine. dA• is converted to dA in this process, and thus the involvement of dA• in the tandem lesion formation is traceless by typical product analysis. Furthermore, we demonstrate, for the first time, that dA• is protonated at neutral pH when flanked by dA. The formation of dA•+ by this protonation results in DNA damage arising from hole transfer. Finally, we disprove the published mechanism for the formation of strand damage observed during DNA hole migration in poly(dA-T) sequences and provide support for an alternative mechanism, in which the strand damage is tracelessly induced by dA•.
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Keywords
DNA damage, purine radicals, photochemistry