Formation of S–Cl Phosphorothioate Adduct Radicals in dsDNA S-Oligomers: Hole Transfer to Guanine vs Disulfide Anion Radical Formation

Abstract

In phosphorothioate containing dsDNA-oligomers (S-oligomers), one of the two non-bridging oxygen atoms in the phosphate moiety of sugar-phosphate backbone is replaced by sulphur. In this work, electron spin resonance (ESR) studies of one-electron oxidation of several S-oligos by Cl2•− at low temperatures are investigated. Electrophilic addition of Cl2•− to phosphorothioate with elimination of Cl− leads to the formation of a 2-center three-electron σ2σ*1 bonded adduct radical (-P-S∸Cl). In AT S-oligomers with mutiple phosphorothioates, i.e., d[ATATAsTsAsT]2, -P-S∸Cl reacts with a neighboring phosphorothioate to form the σ2σ*1 bonded disulphide anion radical ([-P-S∸S-P-]−). With AT S-oligomers with a single phosphorothioate, i.e., d[ATTTAsAAT]2, reduced levels of conversion of -P-S∸Cl dsDNA [-P-S∸S-P-]− are found. For guanine containing S-oligomers containing one phosphorothioate, -P-S∸Cl results in one-electron oxidation of guanine base but not of A, C, or T thereby leading to selective hole transfer to G. The redox potential of -P-S∸Cl is thus higher than that of G but is lower than those of A, C, and T. Spectral assignments to -P-S∸Cl and [-P-S∸S-P-]− are based on reaction of Cl2•− with the model compound diisopropyl phosphorothioate. The results found for d[TGCGsCsGCGCA]2 suggest that [-P-S∸S-P-]− undergoes electron transfer to the one-electron oxidized G healing the base but producing a cyclic disulfide bonded backbone with a substantial bond strength (50 kcal/mol). Formation of -P-S∸Cl and its conversion to [-P-S∸S-P-]− is found to be unaffected by O2 and this is supported by the theoretically calculated electron affinities and reduction potentials of [-P-S-S-P-] and O2.

One-Electron Oxidation of Gemcitabine and Analogs: Mechanism of Formation of C3′ and C2′ Sugar Radicals

“…As expected from the one-electron redox potentials of the bases and the backbone, 27−30,57−59 one-electron oxidation of 2′-dC and 2′-F-dC leads to C• + formation as evidenced by the ca. 16 G doublet that is characteristic of C• + .…”

The influence of the phosphorothioate diester bond on the DNA oxidation process

“…Furthermore, this observation was confirmed by Wu, who investigated the desulfurization process induced by a hydroxyl radical during a negative electrospray ionization mass spectroscopy [39]. It is important to mention that the one-electron oxidation of DNA containing the PT moiety was investigated for the first time by Sevilla et al [19]. In their work, it was postulated that the backbone-to-base hole-transfer mechanism was induced by the Cl 2…”

“…In the presented results the desulfurization process via hole migration is less expected due to the fact that it can be initiated theoretically by a solvated electron or hydroxyl radical [19,[37][38][39]. In the anthraquinone strategy, the ejection of an electron is far enough from the PT bond, therefore it can be trapped by O 2 before it reaches the phosphorothioate moiety.…”

“…This problem is supported by the observation that spermine disulphide can protect ds-DNA from the one-electron oxidative/charge migration process that can lead to its damage [18]. In 2013, the one-electron oxidation of DNA containing the PT moiety was investigated for the first time by Sevilla et al [19]. In their work, the authors postulated that the backbone-to-base hole-transfer mechanism was induced by the Cl 2…”

Insight Into the Inhibition of Ribonucleotide Reductases by 2'-chloro-2'-deoxynucleotides and 2'-azido-2'-deoxynucleotides: Biomimetic Studies with Model Substrates

“…Following our works on DNA and RNA-radicals, 79,141,142,148,152,[158][159][160][161][162][163][164][165][166] the anisotropic simulations of the experimentally recorded ESR spectra were carried out by employing the Bruker programs (WIN-EPR and SimFonia). The ESR parameters (e.g., hyperfine coupling constant (HFCC) values, linwidth, etc.)…”