39 research outputs found
Structural dynamics probed by X-ray pulses from synchrotrons and XFELs
This review focuses on how short X-ray pulses from synchrotrons and XFELs can be used to track light-induced structural changes in molecular complexes and proteins via the pump–probe method. The upgrade of the European Synchrotron Radiation Facility to a diffraction-limited storage ring, based on the seven-bend achromat lattice, and how it might boost future pump–probe experiments are described. We discuss some of the first X-ray experiments to achieve 100 ps time resolution, including the dissociation and in-cage recombination of diatomic molecules, as probed by wide-angle X-ray scattering, and the 3D filming of ligand transport in myoglobin, as probed by Laue diffraction. Finally, the use of femtosecond XFEL pulses to investigate primary chemical reactions, bond breakage and bond formation, isomerisation and electron transfer are discussed
Femtosecond X-ray emission study of the spin cross-over dynamics in haem proteins
In haemoglobin (consisting of four globular myoglobin-like subunits), the
change from the low-spin (LS) hexacoordinated haem to the high spin (HS)
pentacoordinated domed form upon ligand detachment and the reverse process upon
ligand binding, represent the transition states that ultimately drive the
respiratory function. Visible-ultraviolet light has long been used to mimic the
ligand release from the haem by photodissociation, while its recombination was
monitored using time-resolved infrared to ultraviolet spectroscopic tools.
However, these are neither element- nor spin-sensitive. Here we investigate the
transition state in the case of Myoglobin-NO (MbNO) using femtosecond Fe Kalpha
and Kbeta non-resonant X-ray emission spectroscopy (XES) at an X-ray
free-electron laser upon photolysis of the Fe-NO bond. We find that the
photoinduced change from the LS (S = 1/2) MbNO to the HS (S = 2)
deoxy-myoglobin (deoxyMb) haem occurs in ca. 800 fs, and that it proceeds via
an intermediate (S = 1) spin state. The XES observables also show that upon NO
recombination to deoxyMb, the return to the planar MbNO ground state is an
electronic relaxation from HS to LS taking place in ca. 30 ps. Thus, the entire
ligand dissociation-recombination cycle in MbNO is a spin cross-over followed
by a reverse spin cross-over process
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Structural photoactivation of a full-length bacterial phytochrome
Phytochromes are light sensor proteins found in plants, bacteria, and fungi. They function by converting a photon absorption event into a conformational signal that propagates from the chromophore through the entire protein. However, the structure of the photoactivated state and the conformational changes that lead to it are not known. We report time-resolved x-ray scattering of the full-length phytochrome from Deinococcus radiodurans on micro- and millisecond time scales. We identify a twist of the histidine kinase output domains with respect to the chromophore-binding domains as the dominant change between the photoactivated and resting states. The time-resolved data further show that the structural changes up to the microsecond time scales are small and localized in the chromophore-binding domains. The global structural change occurs within a few milliseconds, coinciding with the formation of the spectroscopic meta-Rc state. Our findings establish key elements of the signaling mechanism of full-length bacterial phytochromes
Structural pathways for ultrafast melting of optically excited thin polycrystalline Palladium films
Due to its extremely short timescale, the non-equilibrium melting of metals
is exceptionally difficult to probe experimentally. The knowledge of melting
mechanisms is thus based mainly on the results of theoretical predictions. This
work reports on the investigation of ultrafast melting of thin polycrystalline
Pd films studied by optical laser pump - X-ray free-electron laser probe
experiments and molecular-dynamics simulations. By acquiring X-ray diffraction
snapshots with sub-picosecond resolution, we capture the sample's atomic
structure during its transition from the crystalline to the liquid state.
Bridging the timescales of experiments and simulations allows us to formulate a
realistic microscopic picture of melting. We demonstrate that the existing
models of strongly non-equilibrium melting, developed for systems with
relatively weak electron-phonon coupling, remain valid even for ultrafast
heating rates achieved in femtosecond laser-excited Pd. Furthermore, we
highlight the role of pre-existing and transiently generated crystal defects in
the transition to the liquid state.Comment: main manuscript 33 pages, 9 figures; supplemental material 19 pages,
13 figures - all in one fil
Probing Transient Valence Orbital Changes with Picosecond Valence-to-Core X-ray Emission Spectroscopy
We probe the dynamics of valence electrons in photoexcited [Fe(terpy)<sub>2</sub>]<sup>2+</sup> in solution to gain deeper insight into the Fe–ligand bond changes. We use hard X-ray emission spectroscopy (XES), which combines element specificity and high penetration with sensitivity to orbital structure, making it a powerful technique for molecular studies in a wide variety of environments. A picosecond-time-resolved measurement of the complete 1s X-ray emission spectrum captures the transient photoinduced changes and includes the weak valence-to-core (vtc) emission lines that correspond to transitions from occupied valence orbitals to the nascent core-hole. Vtc-XES offers particular insight into the molecular orbitals directly involved in the light-driven dynamics; a change in the metal–ligand orbital overlap results in an intensity reduction and a blue energy shift in agreement with our theoretical calculations and more subtle features at the highest energies reflect changes in the frontier orbital populations
Tuning and Tracking of Coherent Shear Waves in Molecular Films
We have determined the time-dependent
displacement fields in molecular sub-micrometer thin films as response
to femtosecond and picosecond laser pulse heating by time-resolved
X-ray diffraction. This method allows a direct absolute determination
of the molecular displacements induced by electron–phonon interactions,
which are crucial for, for example, charge transport in organic electronic
devices. We demonstrate that two different modes of coherent shear
motion can be photoexcited in a thin film of organic molecules by
careful tuning of the laser penetration depth relative to the thickness
of the film. The measured response of the organic film to impulse
heating is explained by a thermoelastic model and reveals the spatially
resolved displacement in the film. Thereby, information about the
profile of the energy deposition in the film as well as about the
mechanical interaction with the substrate material is obtained
Introducing a standard method for experimental determination of the solvent response in laser pump, x-ray probe time-resolved wide-angle x-ray scattering experiments on systems in solution
WOS:000323520600021International audienceIn time-resolved laser pump, X-ray probe wide-angle X-ray scattering experiments on systems in solution the structural response of the system is accompanied by a solvent response. The solvent response is caused by reorganization of the bulk solvent following the laser pump event, and in order to extract the structural information of the solute, the solvent response has to be treated. Methodologies capable of doing so include both theoretical modelling and experimental determination of the solvent response. In the work presented here, we have investigated how to obtain a reproducible solvent response-the solvent term-experimentally when applying laser pump, X-ray probe time-resolved wide-angle X-ray scattering. The solvent term describes difference scattering arising from the structural response of the solvent to changes in the hydrodynamic parameters: pressure, temperature and density. We present results based on NIR and dye mediated solvent heating, and demonstrate that the solvent response is independent of the heating method. The NIR heating is shown to be rendered unusable by higher order effects under certain experimental conditions, while the dye mediated solvent heating is demonstrated to exhibit first order behaviour with respect to the amount of energy deposited in the solution. We introduce a standardized method for recording solvent responses in laser pump, X-ray probe time-resolved X-ray wide-angle scattering experiments by using dye mediated solvent heating. Furthermore, we have generated a library of solvent terms, which can be used to describe the solvent term in any TRWAXS experiment, and made it available online