Real-Time Probing of a Three-Electron Bonded Radical: Ultrafast One-Electron Reduction of a Disulfide Biomolecule

Abstract

The primary steps of a one-electron reduction of a disulfide molecule (cystamine) by the infrared p-like state of an excited hydrated electron have been investigated by femtosecond laser spectroscopy. The initial electron photodetachment from chloride ion or phenothiazine is triggered by a two-photon ultraviolet excitation. In homogeneous aqueous cystamine solution, a subpicosecond univalent reduction of the disulfide molecule by an infrared prehydrated electron competes with the nonadiabatic relaxation of trapped electrons (electron solvation process). This presolvation one-electron reduction occurs with a characteristic time of 160 ± 20 fs at 294 K. Within the electron solvation regime, this elementary redox process is totally achieved in less than 1 × 10-12 s and exhibits a probability 9 times higher than the radiationless relaxation of an infrared excited electron. In aqueous organized assemblies (cationic micelles) the partitioning of reactants does not influence the frequency rate of a prehydration reaction but modifies the early branching between reactive and nonreactive IR electronic channels. The real-time UV probing of a nascent sulfur-centered radical anion (RS∴SR-)aq is discussed in the framework of a two-center-three-electron bond.