Independent Generation and Investigation of the C3'-deoxy-3'-thymidinyl Radical: A Proposed Intermediate in DNA-LEE Interactions.

摘要

Ionizing radiation (IR) can damage biological systems by targeting the genetic material of the cells producing lethal DNA lesions. When not repaired these lesions contribute to cell death. Oxidative damage to DNA through IR is partially produced by the secondary particles created along the ionization track. Secondary low-energy electrons (LEEs, 1-30 eV) are the most abundant secondary species produced by IR. It was proposed that LEE can added to DNA constituents resulting in the formation of transient molecular anion intermediates that eventually lead to bond dissociation. It is believed that carbon-centered radicals on the sugar moiety are a major reactive intermediate formed in this process. One of these is the C3'-deoxy-3'-thymidinyl radical (76). Accordingly, the goal of this project is to elucidate the pathways involved in the fate of this radical in DNA to answer mechanistic questions related to DNA damage by LEEs.;Site-selective generation of the C3'-deoxy-3'-thymidinyl radical (76) from photolabile precursors should facilitate mechanistic studies as allows generation of one intermediate. The synthesis of the C3'-deoxy-3'-(selenophenyl)thymidine (73 and 75) as precursors of this radical has been completed and their efficiencies in radical generation have been established. Photolysis of 73 under anaerobic conditions resulted in the formation of the 2',3'-dideoxythymidine (77), 2',3'-didehydro-2',3'-dideoxythymidine (78) and 3',4'-didehydro-2',3'-dideoxythymidine (79) and the isomeric product 75. We believe that their formation is mainly through secondary reactions by the phenylselenyl radical (27). Photolysis experiments in the presence of bis(tributyltin) (80) suggest a disproportionation mechanism.;To investigate the fate of the C3'-deoxy-3'-thymidinyl radical in DNA, the modified nucleosides were incorporated into oligonucleotides using “reverse” DNA synthesis. Photolysis of the selenated oligomers under physiological conditions resulted in formation of damage products related to products identified in monomer experiments. Phenylated oligomers (140) and thymidine-containing oligomers (141) were also identified. The outcomes of these experiments will greatly add to our understanding of the mechanistic pathways of oxidative damage to DNA as it relates to LEEs.