Reaction dynamics of small sulfur-containing molecules with a hydrated electron in the gas phase

Ionizing radiation can generate hydrated electrons leading to biological radiation damage. Thiol groups (—SH) contribute to tertiary protein structures through the formation of disulfide linkages (S—S) in cystine residues. Improved understanding of the electron-transfer processes with small model systems provides fundamental insights into their reduction chemistry at the molecular level.

Dissociative electron transfers of CH₃SH and CH₃SSCH₃ with a hydrated electron are examined using DFT molecular dynamics methods performed with CP2K package. Gaussian-planewave basis set is employed for the electronic wavefunctions. Revised PBE functionals with self-interaction correction for the local spin density is used. Dispersion correction for non-covalent interaction is also included.

With hydrated electrons, two reactions with relevance for biological radiation damage were observed, including reduction of the disulfide bond of CH₃SSCH₃ and the thiol group of CH₃SH followed by a hydrogen atom loss. Our theoretically predicted reaction energies are in good agreement with experimental values estimated from nanocalorimetry measured with a FTICR mass spectrometer. The present DFT-MD simulations revealed the dynamics of reductive electron transfer from a water cluster to the σ* orbital of the dissociating bonds, monitored by the evolution of the electron spin density. Such electron transfer happens upon a solvent reorganization with a small activation barrier. For the reaction of CH₃SH, several short-life intermediates involving different hydrogen bonding interactions are observed which are characterized by their reaction energies, bond distances and power spectra.

Dissociative electron transfer of some sulfur-containing functional groups in aqueous media is characterized with gas-phase models using DFT-MD methods.