STSM by Raluca Cireasa, CNRS, Orsay (FR) with Francesca Calegari, National Research Council of Italy CNR-IFN (IT)
On April 14th, 2014 (20 days)

RACI-logosUltrafast Dynamics of Biomolecules Induced by XUV Attosecond Pulses

Exposing biomolecules, including those of the human body, to high energy photons (from VUV to X-ray) leads to radiation damage. The objective of this project is to use XUV ultrashort pulses (hundreds of attosecond, 1as=10-18 s) to investigate the modifications induced in biomolecules at the level of the electrons and the first stages of the chemical (nuclear) changes. We have investigated several molecules belonging to different classes: aminoacids (Glycine), nucleobasis (Uracil), radiosensitizers=molecules that increase the effect of the radiotherapies (5-Halouracils).

The charge dynamics (electron and hole motions) launched by the photoionisation process are expected to be extremely important in triggering subsequent nuclear dynamics and the associated chemical change and therefore, it plays an essential role in many chemical and biological processes. We have performed pump-probe experiments to study the ultrafast dynamics occurring in some benchmarks biomolecules upon excitation with XUV attosecond pulses. The ensuing dynamics was detected by recording Time-of-Flight (TOF) spectra of the singly ionised fragments as a function of the delay between the XUV pump (~300 as) and a broad VIS-IR probe pulses (~4 fs) and as a function of the XUV energy spectrum. For all systems, we have recorded ultrafast dynamics (<10 fs) manifested as delayed signal appearances and/or sharp rising/decaying signals. An important objective of the experiments on halogenated uracil (FU and BrU) and uracil (U) was to measure the H/H+ transfer dynamics underlying the formation of some fragments. An ultrafast decay of 30-40 fs was observed on the signal from the fragment 43 (HNCO or FCCH) and the complementary rising behaviour on the same timescale was observed for fragment 44, which can be only formed by H/H+ transfer, most probably via tautomerisation (see Fig.). Similar behaviours, although slower, were also measured for the fragments 31 (FC) and 32. These dynamics may be associated with H or proton transfer processes where the difference in timescale is determined by the initial and final sites of the H/proton transfer and, in particular, to the involvement of the halogen atom. Similar dynamics were observed for the corresponding fragments of BrU and U.

The analysis of the results obtained during this campaign is underway. Some preliminary results were already presented as talks at IPW&RIXS2014 conference and at departmental seminars and when finalised, this work will also be the object of joint publications. The STSM has given us the opportunity of setting up a longer term collaboration for designing a gas-phase biomolecular source for the Milano setup – based on the ISMO source that was used for the STSM experiments, for testing the performances of this source at ISMO using energy resolved spectroscopy techniques (beginning of 2015) and for undertaking other common projects to investigate the ultrafast dynamics induced by XUV attosecond pulses in biomolecules (Spring 2015).

RACI-imgTautomerisation dynamics for 5-FluoroUracil from the di-keto isomer to keto-enol isomer

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