Abstract
The elementary processes that accompany the interaction of ionizing radiation with biologically relevant molecules are of fundamental importance. However, the ultrafast structural rearrangement dynamics induced by the ionization of biomolecules in aqueous solution remain hitherto unknown. Here, we employ femtosecond optical pump-probe spectroscopy to elucidate the vibrational wave packet dynamics that follow the photodetachment of phenoxide, a structural mimic of tyrosine, in aqueous solution. Photodetachment of phenoxide leads to wave packet dynamics of the phenoxyl radical along 12 different vibrational modes. Eight of the modes are totally symmetric and support structural rearrangement upon electron ejection. Comparison to a previous photodetachment study of phenoxide in the gas phase reveals the important role played by the solvent environment in driving ultrafast structural reorganization induced by ionizing radiation. This work provides insight into the ultrafast molecular dynamics that follow the interaction of ionizing radiation with molecules in aqueous solution.
Highlights
The elementary processes that accompany the interaction of ionizing radiation with biologically relevant molecules are of fundamental importance
We propose to employ optical pump-probe spectroscopy with few-cycle laser pulses to observe vibrational wave packet motion launched by impulsive ionization or detachment of solutes in aqueous solution
We rule out the formation of PhO∙ via the dense manifold of electronic excited states[33] of PhO– because the pump-power dependence measurement shows that photodetachment occurs directly via a four-photon process
Summary
The elementary processes that accompany the interaction of ionizing radiation with biologically relevant molecules are of fundamental importance. We employ femtosecond optical pump-probe spectroscopy to elucidate the vibrational wave packet dynamics that follow the photodetachment of phenoxide, a structural mimic of tyrosine, in aqueous solution. Photodetachment of phenoxide leads to wave packet dynamics of the phenoxyl radical along 12 different vibrational modes. Comparison to a previous photodetachment study of phenoxide in the gas phase reveals the important role played by the solvent environment in driving ultrafast structural reorganization induced by ionizing radiation. In the absence of time-domain measurements, analysis of X-ray emission spectra, Auger electron spectra, and ab initio simulations reveal ultrafast X-ray-induced proton transfer dynamics in liquid water[12,13,14,15] and small biomolecules in aqueous solutions[16] occurring within the 4-fs lifetime of the oxygen 1 s core level lifetime. The conjugate acid of phenoxide, phenol (PhOH), is the chromophore in the redox-active amino acid tyrosine, which in turn suggests that the phenoxyl radical can serve as a model for the tyrosyl radical, a key intermediate in numerous biochemical redox[8,28] and proton-coupled electron transfer reactions[29,30,31]
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