One Way Traffic: Base‐to‐backbone Hole Transfer in Nucleoside Phosphorodithioates
Résumé
Directionality of the hole transfer processes between DNA-backbone and -base was investigated using phosphorodithioate [P(S−)=S] components. ESR spectroscopy in homogeneous frozen aqueous solutions, pulse radiolysis in aqueous solution at ambient temperature confirmed initial
formation of G•+-P(S−)=S. The ionization potential of G-P(S−)=S was calculated to be slightly lower than that of guanine in 5’-dGMP. Subsequent thermally-activated hole transfer from G•+ to P(S−)=S led to dithiyl radical (P-2S•) formation on μs timescale. In parallel, ESR spectroscopy and
pulse radiolysis, and density functional theory (DFT) calculations confirmed P-2S• formation in abasic phosphorodithioate model compound. ESR investigations at low temperatures and higher G-P(S−)=S concentrations showed a bimolecular conversion of P-2S• to the σ2-σ*1-bonded dimer
anion radical [-P-2S∸2S-P-]− [ΔG (150 K, DFT) = −7.2 kcal/mol]. However, [-P-2S∸2S-P-]−formation was not observed by pulse radiolysis [ΔG° (298 K, DFT) = −1.4 kcal/mol]. Neither P-2S• nor [-P-2S∸2S-P-]− oxidized guanine base; base-to-backbone hole transfer occurs in phosphorodithioate.
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