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Proton transfer (PT) reactions are fundamental to numerous chemical and biological processes. While sub-picosecond PT involving electronically excited states has been extensively studied, little is known about ultrafast PT triggered by photoionization. Here, we employ femtosecond optical pump-probe spectroscopy and quantum dynamics calculations to investigate the ultrafast proton transfer dynamics of the aqueous phenol radical cation (PhOH˙+). Analysis of the vibrational wave packet dynamics reveals unusually short dephasing times of 0.18 ± 0.02 ps and 0.16 ± 0.02 ps for the PhOH˙+ O-H wag and bend frequencies, respectively, suggestive of ultrafast PT occurring on the ∼0.1 ps timescale. The reduced potential energy surface obtained from ab initio calculations shows that PT is barrierless when it is coupled to the intermolecular hindered translation between PhOH˙+ and the proton-acceptor water molecule. Quantum dynamics calculations yield a lifetime of 193 fs for PhOH˙+, in good agreement with the experimental results and consistent with the PT reaction being mediated by the intermolecular O⋯O stretch. These results suggest that photoionization can be harnessed to produce photoacids that undergo ultrafast PT. In addition, they also show that PT can serve as an ultrafast deactivation channel for limiting the oxidative damage potential of radical cations. Ministry of Education (MOE) Nanyang Technological University Published version We acknowledge financial support from the Ministry of Education, Singapore (grant no. RG1/20, RG105/17 and MOE2014-T2- 2-052). M. S. B.M. Y. is supported by the Nanyang President’s Graduate Scholarship. H. S. and M. Y. are supported by the National Natural Science Foundation of China (grant no. 21973109 to H. S., and 21773297, 21973108 and 21921004 to M.Y.).

The phenylalanine radical (Phe˙) has been proposed to mediate biological electron transport (ET) and exhibit long-lived electronic coherences following attosecond photoionization. However, the coupling of ultrafast structural reorganization to the oxidation/ionization of biomolecules such as phenylalanine remains unexplored. Moreover, studies of ET involving Phe˙ are hindered by its hitherto unobserved electronic spectrum. Here, we report the spectroscopic observation and coherent vibrational dynamics of aqueous Phe˙, prepared by sub-6 fs photodetachment of phenylalaninate anions. Sub-picosecond transient absorption spectroscopy reveals the ultraviolet absorption signature of Phe˙. Ultrafast structural reorganization drives coherent vibrational motion involving nine fundamental frequencies and one overtone. DFT calculations rationalize the absence of the decarboxylation reaction, a photodegradation pathway previously identified for Phe˙. Our findings guide the interpretation of future attosecond experiments aimed at elucidating coherent electron motion in photoionized aqueous biomolecules and pave way for the spectroscopic identification of Phe˙ in studies of biological ET. Accepted version

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. Ministry of Education (MOE) Nanyang Technological University Published version The authors acknowledge financial support from the Ministry of Education, Singapore (Grant Nos. RG1/20, RG105/17, and MOE2014-T2-2-052). M.S.B.M.Y. was supported by the Nanyang President’s Graduate Scholarship.

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., The interaction of biomolecules with ionizing radiation induces structural changes which are still largely unknown. The authors use femtosecond wave packet spectroscopy to observe ultrafast structural dynamics that follow the photodetachment of phenoxide in aqueous solution.

The ultrafast dynamics triggered by the photodetachment of the tyrosinate dianion in aqueous environment shed light on the elementary processes that accompany the interaction of ionizing radiation with biological matter. Photodetachment of the tryosinate dianion yields the tyrosyl radical anion, an important intermediate in biological redox reactions, although the study of its ultrafast dynamics is limited. Here, we utilize femtosecond optical pump-probe spectroscopy to investigate the ultrafast structural reorganization dynamics that follow the photodetachment of the tyrosinate dianion in aqueous solution. Photodetachment of the tyrosinate dianion leads to vibrational wave packet motion along seven vibrational modes that are coupled to the photodetachment process. The vibrational modes are assigned with the aid of density functional theory (DFT) calculations. Our results offer a glimpse of the elementary dynamics of ionized biomolecules and suggest the possibility of extending this approach to investigate the ionization-induced structural rearrangement of other aromatic amino acids and larger biomolecules. Ministry of Education (MOE) Accepted version We acknowledge financial support from the Ministry of Education, Singapore (RG1/20, RG105/17, and MOE2014-T2-2-052). M. S. B. M. Y. is supported by the Nanyang President’s Graduate Scholarship.

The ionisation of liquid water is a phenomenon that accompanies high-energy radiation with biological systems and aqueous solutions. The study of this process is of fundamental importance to radiation biology and radiation chemistry.[1-3] Upon photoionisation, a water radical cation, H2O●+, is produced upon an ejection of an electron which subsequently relaxes into the localized hydrated electron, ehyd-. The H2O●+ radical cation undergoes a proton transfer reaction to yield a hydroxyl radical, ●OH and a hydronium ion, H3O+. Previous studies have measured the timescale of decay of the H2O●+ radical cation to be 46 fs.[4] To build on that discovery, this study focuses on detecting the formation time of the proton transfer product, the ●OH radical, by monitoring its deep-ultraviolet (DUV) absorption. Herein, this study presents the successful generation of ultrashort DUV pulses, and the ultrafast pump-probe spectroscopic measurements of the absorption of the ●OH radical. Published version

The ultrafast dynamics triggered by the photodetachment of the tyrosinate dianion in aqueous environment shed light on the elementary processes that accompany the interaction of ionizing radiation with biological matter. Photodetachment of the tryosinate dianion yields the tyrosyl radical anion, an important intermediate in biological redox reactions, although the study of its ultrafast dynamics is limited. Here, we utilize femtosecond optical pump–probe spectroscopy to investigate the ultrafast structural reorganization dynamics that follow the photodetachment of the tyrosinate dianion in aqueous solution. Photodetachment of the tyrosinate dianion leads to vibrational wave packet motion along seven vibrational modes that are coupled to the photodetachment process. The vibrational modes are assigned with the aid of density functional theory (DFT) calculations. Our results offer a glimpse of the elementary dynamics of ionized biomolecules and suggest the possibility of extending this approach to investigate the ionization-induced structural rearrangement of other aromatic amino acids and larger biomolecules.