Probing thionucleobase dynamics at the sulfur L-edge

• Applicants: Professor Dr. Markus Gühr; Professor Dr. Peter Saalfrank
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry ; Theoretical Chemistry: Electronic Structure, Dynamics, Simulation
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 445713302

This joint experimental-theoretical proposal aims at further insight into the UV-induced photorelaxation of isolated thiolated nucleobases. We suggest ultrafast and x-ray probing at the sulfur L-edge to utilize the x-ray typical element and site-selectivity.Thionucleobases are an important class of modified nucleobases, playing an important role as medication as well as in a basic and applied research context. The UV-photoexcitation of these molecules leads to long living triplet states, responsible for cross linking and the creation of reactive singlet oxygen in organisms. These properties create on one hand a higher skin cancer risk for patients treated with thionucleobases, on the other hand they might open the path for targeted photoinduced tumor therapy.We want to monitor the relaxation path from the initially excited singlet ππ* state over the singlet nπ* into the triplet states for different thiouracils as well as thiothymines. While the literature shows strong evidence for the singlet nπ* state acting as a doorway to the lower lying triplet states, the details of the processes are actively debated. We have been using time-resolved resonant x-ray absorption spectroscopy as well as time-resolved Auger spectroscopy on canonical nucleobases before to establish these types of probe schemes in the soft x-ray domain for gas phase targets. We now want to transfer the knowledge gained to the thionucleobases. We will first establish the x-ray absorption, photoelectron and Auger spectra at the L1 and L2,3 edges in a joint experimental-theory effort. Based on these, we will perform time resolved nonresonant Auger spectroscopy that will allow us to observe a predicted C-S bond-elongation. We have already performed preliminary experimental studies on the special case of UV-excited 2-thiouracil at the FLASH free electron laser, showing promising time-dependent features at the S L-edge. We will work on the prediction of S 2s and 2p core-level absorption spectra for thionucleobases in different electronic states and important molecular geometries, with and without UV pre-excitation. Selected geometries will be identified by stationary quantum chemical mthods and also ‚on the fly‘ by non-adiabatic dynamics simulations including spin-orbit coupling. The calculation of (pump-probe) x-ray absorption spectra will be done with a newly developed, efficient transition potential method based on density functional theory. Preliminary studies on UV excited 2-thiouracil predict a pre-edge feature, similar to those observed for thymine in previous experiments. These simulations will help the interpretation and guide experimental efforts using the intensity, spectral position and polarization dependence of the pre-edge feature as well as the whole near-edge spectrum to learn about the molecular photorelaxation. Our final goal will be a joint theory-experimental study in which an experiment is accompanied by a time-dependent simulation of the x-ray absorption.

• DFG Programme: Research Grants