Independent Generation and Reactivity of Thymidine Radical Cations

Abstract

Thymidine radical cation (1) is produced by ionizing radiation and has been invoked as an intermediate in electron transfer in DNA. Previous studies on its structure and reactivity have utilized thymidine as a precursor, which limits quantitative product analysis because thymidine is readily reformed from 1. In this investigation, radical cation 1 is independently generated via β-heterolysis of a pyrimidine radical generated photochemically from an aryl sulfide. Thymidine is the major product (33%) from 1 at pH 7.2. Diastereomeric mixtures of thymidine glycol and the corresponding 5-hydroxperoxides resulting from water trapping of 1 are formed. Significantly lower yields of products such as 5-formyl-2′-deoxyuridine that are ascribable to deprotonation from the C5-methyl group of 1 are observed. Independent generation of the N3-methyl analogue of 1 (NMe-1) produces considerably higher yields of products derived from water trapping, and these products are formed in much higher yields than those attributable to the C5-methyl group deprotonation in NMe-1. N3-Methyl-thymidine is, however, the major product and is produced in as high as 70% yield when the radical cation is produced in the presence of excess thiol. The effects of exogenous reagents on product distributions are consistent with the formation of diffusively free radical cations (1, NMe-1). This method should be compatible with producing radical cations at defined positions within DNA.

Degradation of Cytosine Radical Cations in 2′-Deoxycytidine and in i-Motif DNA: Hydrogen-Bonding Guided Pathways

“…Until now, there has been increased attention on the radical cations of DNA bases and their degradation reactions, ,− among which the radical cation of guanine (G •+ ) has been of great interest because of the lowest oxidation potential of guanine among the four DNA bases (G < A < C < T). Competitive pathways of hydration and deprotonation subsequent to the formation of the G •+ were revealed.…”

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

“…This is also inconsistent with the previously proposed mechanism, but consistent with product studies on monomeric T•+, which showed that fdU formation is a minor product. 20,22…”

“…DNA hole migration is initiated by one-electron oxidation of DNA, that is, hole injection, and is a primary consequence of the direct effect of ionizing radiation. − Hole injection by chemical and/or photochemical methods takes advantage of the differences in redox potentials to selectively generate the radical cation of 2′-deoxyguanosine (dG) and/or dA. − Short wavelength UV-irradiation (≤254 nm) and other forms of ionizing radiation (e.g., γ-radiolysis, photosensitization) are unselective and generate pyrimidine (e.g., T •+ ) and purine radical cations. − Nucleotide radical cation formation is a hallmark of the direct effect of ionizing radiation. The role of dG •+ in hole migration and subsequent hole trapping by reacting with H 2 O/O 2 is well documented. , The reactivity of T •+ is less well understood than that of dG •+ and dA •+ , due to the lack of methods to selectively produce the pyrimidine radical cation . Most reports on T •+ reactivity have focused on the nucleoside.…”

“…However, recently T•+ was produced from a photochemical precursor ( 4 ) other than thymidine. 20 Irradiation (350 nm) of 4 produces an intermediate radical ( S ) that undergoes β-heterolysis to produce T•+ (Scheme 2). 26–30 Generating monomeric T•+ from 4 revealed that thymidine is the major product under aerobic conditions, even in the absence of exogenous reducing agent.…”

Biphotonic Ionization of DNA: From Model Studies to Cell

“…The degree of deprotonation is sensitive to pH and the p K a of the photosensitizer radical anion. 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 .…”