Observation of intra- and intermolecular vibrational coherences of the aqueous tryptophan radical induced by photodetachment

Abstract

The study of the photodetachment of amino acids in aqueous solution is pertinent to the understanding of elementary processes that follow the interaction of ionizing radiation with biological matter. In the case of tryptophan, the tryptophan radical that is produced by electron ejection also plays an important role in numerous redox reactions in biology, although studies of its ultrafast molecular dynamics are limited. Here, we employ femtosecond optical pump-probe spectroscopy to elucidate the ultrafast structural rearrangement dynamics that accompany the photodetachment of the aqueous tryptophan anion by intense, ∼5-fs laser pulses. The observed vibrational wave packet dynamics, in conjunction with density functional theory calculations, identify the vibrational modes of the tryptophan radical, which participate in structural rearrangement upon photodetachment. Aside from intramolecular vibrational modes, our results also point to the involvement of intermolecular modes that drive solvent reorganization about the N-H moiety of the indole sidechain. Our study offers new insight into the ultrafast molecular dynamics of ionized biomolecules and suggests that the present experimental approach can be extended to investigate the photoionization-or photodetachment-induced structural dynamics of larger biomolecules.

Ultrafast proton transfer of the aqueous phenol radical cation

“…The detection of vibrational coherences from transient species whose differential absorption signatures are not apparent is remarkable but not unprecedented -we recently demonstrated the possibility of resolving the vibrational wave packet dynamics of the tryptophan radical, prepared by the strong-field photodetachment of the aqueous tryptophan anion, even though its absorption signature in the visible region is obscured by the strong absorption band of the hydrated electron byproduct. 87 We note that the linewidths of FFT spectral features that are assigned to the O-H and O-D modes appear narrower than expected -a dephasing time T 2 is expected to yield a FWHM linewidth of 1= ffiffi ffi 2 p pcT 2 for the associated peak in the FFT power spectrum. We tentatively attribute this discrepancy to possible interference from nearby frequency components, which result in the narrowing of the lineshape.…”

“…Our recent studies of the vibrational wave packet dynamics of radical species prepared by strong-field photoionization or photodetachment show that the displacive mechanism for the generation of vibrational coherences can be extended from photoexcitation to photoionization 85,86 or photodetachment. 82,87,88 According to this mechanism, wave packet motion is launched along vibrational modes for which the equilibrium geometries of the species before and after electron ejection are displaced. Extending this analysis to the present work would require us to consider the equilibrium geometries of neutral PhOH and the PhOH + radical cation.…”

“…89,90 Second, vibrational wave packet motion could be launched in PhOH + following the displacive mechanism uncovered in our recent work. 82,[85][86][87] Subsequent vibrational coherence transfer to the PhO product is possible for vibrational modes that are weakly coupled to proton transfer and that exhibit minimal vibrational coordinate transformation between the PhOH + reactant and the PhO product. The transfer of vibrational coherence between different electronic states was recently observed in transition metal complexes after ultrafast intersystem crossing.…”

Ultrafast Vibrational Wave Packet Dynamics of the Aqueous Guanine Radical Anion Induced by Photodetachment

“…The ultrafast structural rearrangement of radical species impacts key biological processes such as long-range electron transfer in proteins − and DNA. − Thus, further research is needed to extend our understanding of the ultrafast structural rearrangement involving aqueous biomolecules. To this end, previous work has elucidated the ultrafast structural rearrangement dynamics of various aqueous amino acids. − …”