Short description
(English)
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The electronic structure of the metalloporphyrin in heme proteins (hemoglobine, myoglobine, cytochrome C, Vitamin B12, etc.) is at the core of their chemical reactivity and therefore, biological functions. In particular, the binding of diatomic ligands, such as O2, NO, CO, etc., and also triatomic ones, such as H2O, H2S, N3, etc., to the metal atom (Fe, Ni, Co) is entirely governed by properties such as charge transfer, bonding and back-bonding between porphyrin and the metal atom, as well as spin state of the metalloporphyrin. All these properties can best be visualized by soft X-ray absorption spectroscopy (XAS) at the L2,3 (2p3/2,1/2-3d)-edges of the metal atom, since these interrogate the 3d orbitals of the metal atom, that are involved in the chemical bonding and reactivity. Recently, the first soft X-ray spectra of haemoglobin in physiological solutions were successfully recorded thanks to the use of very thin flow cells equipped with soft X-ray transparent silicon nitride windows. This opens the way to implementing optical pump/soft X-ray probe studies of the ligand dissociation and rebinding with picosecond and femtosecond time resolutions, in order to map out the electronic structure changes in the course of the biological function. Indeed, the optical excitation mimics the process of ligand detachment from the metal atom. In this proposal, we plan to carry out such experiments with 50 ps time resolution at the synchrotron BESSY in Berlin in a first stage. These experiments will prepare the ground for the second stage of the proposal, which consists in carrying out experiments with femtosecond time resolution at the Free Electron Laser facilty FLASH in DESY-Hamburg.
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Partners and International Organizations
(English)
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AT, BE, CH, DE, DK, ES, FI, FR, GR, HR, HU, IE, IT, NO, PL, RO, SE, UK
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Abstract
(English)
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The goal of this project was the implementation of our newly developed high repetition rate system into the soft X-ray regime allowing us to investigate for example the L-edges of transition metals, but also the K-edge of light elements. The L2,3-edges stem from the 2p1/2,3/2 core orbitals and access to the valence d-orbitals, which form the chemical bond with the surrounding atoms. We have successfully implemented the set-up and investigated the charge carrier dynamics in a complex system with different elements, such as in the dye-sensitized solar cells (DSSC). Our high repetition rate experiments are based on a high-power picosecond laser pump/ X-ray probe scheme. The repetition rate of the setup runs at an integer fraction of the storage ring and allows us to record at MHz repetition rates, which yields an order of magnitude increase in the signal-to-noise of time-resolved XAS compared to the previous kHz repetition setup. The better signal-to-noise ratio enables us to measure very dilute systems and therefore come more closely to the sample real conditions (e.g. physiological solutions). Furthermore, the MHz repetition rate system is compact and therefore simple to transfer to different beam lines providing us with hard as well as soft X-ray probe pulses at different energies. During this project we investigated the charge transfer between the different components of dye-sensitized solar cells (DSSC), which are composed of TiO2 nanoparticles and Ruthenium based dye molecules (N719). Our investigation of the charge transfer dynamics between the dye-sensitized solar cell components aimed for a better understanding of DSSC performance using time-resolved XAS and may lead to a future improvement of the photo conversion efficiency of the DSSC.
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