AbstractsBiology & Animal Science

Time-resolved photoelectron spectroscopy of DNA molecules in solution

by Franziska Buchner

Institution: Freie Universität Berlin
Department: FB Physik
Degree: PhD
Year: 2015
Record ID: 1117282
Full text PDF: http://edocs.fu-berlin.de/diss/receive/FUDISS_thesis_000000099382


In this thesis, I have advanced the recently-developed method of time-resolved liquid jet photoelectron spectroscopy to study the excited-state dynamics of solvated molecules applying femtosecond pulses in the ultraviolet range. The focus of this work was on the molecules that probably are the most relevant for life on earth: the DNA bases. Despite extensive studies with other ultrafast techniques, there are still important open questions. Time-resolved photoelectron spectroscopy provides a complementary and important view on the excited-state relaxation of these molecules and therefore helps to answer these questions. When photoreactions in water are triggered (in particular by ultrashort pulses), the hydrated electron is an omnipresent intermediate. In order to disentangle signal of the hydrated electron from that of the molecule under investigation, I have first investigated the appearance of this species from photoexcitation to formation of free hydrated electrons in time-resolved photoelectron spectroscopy. As precursor iodide is used which is known to efficiently generate hydrated electrons after excitation in the deep ultraviolet range. The hydrated electron is bound by 3.4 eV. Surprisingly and due to an enhanced surface sensitivity of photoelectron spectroscopy, I have found a new decay channel, in which the hydrated electrons that are generated in the vicinity of the surface recombine with the geminate iodine radical on a sub-ps timescale, while in bulk this recombination takes 22 ps. In this work, time-resolved photoelectron spectroscopy was for the first time applied to study the excited-state dynamics of DNA bases and nucleosides in diluted solution. For adenine, my results reproduce results from the literature: The excited state decays with two different lifetimes: 90 fs and 8 ps, and the two different decays are assigned to the two tautomers of adenine present in aqueous solution. For guanosine, I observe two spectral components that are associated with two different decay times: ~250 fs and ~2 ps. The faster decay is assigned to propagation of the excited-state wave packet out of the steep Franck-Condon region, while the slower decay corresponds to the internal conversion at the conical intersection between the excited state and the ground state. Previous studies were contradictory. The new results from photoelectron spectroscopy support the interpretation from fluorescence up-conversion experiments and therefore contribute with important new information that will guide to a more complete understanding of the photodynamics of guanine. For the pyrimidine bases (thymine and cytosine) and their nucleosides, photoelectron spectroscopy shows spectral contributions from two channels associated with two different decay times in the range of ~100 fs and ~400 fs for thymine, and ~170 fs and ~1.4 ps for cytosine. As suggested by molecular dynamics simulations, I have interpreted the data by two different relaxation paths on a single excited-state potential energy surface. The involvement of the nπ* state,…