Tag Archives: electron transfer

One Way Traffic: Base‐to‐Backbone Hole Transfer in Nucleoside Phosphorodithioate

by Renata Kaczmarek, Samuel Ward, Dipra Debnath,Taisiya Jacobs, Alexander D. Stark, Dariusz Korczyński, Anil Kumar, Michael D. Sevilla, Sergey A. Denisov, Viacheslav Shcherbakov, Pascal Pernot, Mehran Mostafavi, Roman Dembinski, Amitava Adhikary

Chem. Eur. J. 2020, 26, 9495-9505; https://doi.org/10.1002/chem.202000247

The directionality of the hole‐transfer processes between DNA backbone and base was investigated by using phosphorodithioate [P(S)=S] components. ESR spectroscopy in homogeneous frozen aqueous solutions and 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 the μs timescale. In parallel, ESR spectroscopy, pulse radiolysis, and density functional theory (DFT) calculations confirmed P‐2S. formation in an 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−1]. However, [‐P‐2S 2S‐P‐] formation was not observed by pulse radiolysis [ΔG° (298 K, DFT)=−1.4 kcal mol−1]. Neither P‐2S. nor [‐P‐2S 2S‐P‐] oxidized guanine base; only base‐to‐backbone hole transfer occurs in phosphorodithioate.

Ultra-fast charge migration competes with proton transfer in the early chemistry of H2+

by Furong Wang, Uli Schmidhammer, Aurélien de La Landea and Mehran Mostafavi

Phys. Chem. Chem. Phys., 2017, 19, 2894-2899; doi.org/10.1039/C6CP07013B

Oxidation by the ultra-short lived radical cation of water, H2+, can potentially take place at the interface of water and numerous heterogeneous systems involved in radiation therapy, energy and environmental industries. The oxidation processes induced by H2+ can be mimicked in highly concentrated solutions where the nearest neighbors of H2+ may be molecules other than water. The reactivity of H2+ and D2+ is probed in hydrogenated and deuterated sulfuric acid solutions of various concentrations. The oxidized solute, sulfate radical, is observed at 7 ps and remarkably higher yields are found in deuterated solutions. The isotopic effects reveal the competition between two ultrafast reactions: proton transfer toward H2O (D2O) and electron transfer from HSO4 to H2+ (D2+). Density functional theory simulations decipher the electron transfer mechanism: it proceeds via sub-femtosecond charge migration and is not affected by isotopic substitution. This work definitively demonstrates why direct oxidation triggered by H2+ can be competitive with proton transfer.