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MolDStruct: modelling the dynamics and structure of matter exposed to ultrafast X-ray lasers with hybrid collisional-radiative/molecular dynamics
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Chemical and Bio-Molecular Physics. European XFEL, Holzkoppel 4, DE-22869 Schenefeld, Germany.ORCID iD: 0000-0002-2926-5702
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Chemical and Bio-Molecular Physics.ORCID iD: 0000-0003-0707-1832
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Chemical and Bio-Molecular Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - BMC, Biochemistry.ORCID iD: 0000-0001-5029-7429
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2024 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 160, no 18Article in journal (Refereed) Published
Abstract [en]

We describe a method to compute photon–matter interaction and atomic dynamics with x-ray lasers using a hybrid code based on classical molecular dynamics and collisional-radiative calculations. The forces between the atoms are dynamically determined based on changes to their electronic occupations and the formation of a free electron cloud created from the irradiation of photons in the x-ray spectrum. The rapid transition from neutral solid matter to dense plasma phase allows the use of screened potentials, reducing the number of non-bonded interactions. In combination with parallelization through domain decomposition, the hybrid code handles large-scale molecular dynamics and ionization. This method is applicable for large enough samples (solids, liquids, proteins, viruses, atomic clusters, and crystals) that, when exposed to an x-ray laser pulse, turn into a plasma in the first few femtoseconds of the interaction. We present four examples demonstrating the applicability of the method. We investigate the non-thermal heating and scattering of bulk water and damage-induced dynamics of a protein crystal using an x-ray pump–probe scheme. In both cases, we compare to the experimental data. For single particle imaging, we simulate the ultrafast dynamics of a methane cluster exposed to a femtosecond x-ray laser. In the context of coherent diffractive imaging, we study the fragmentation as given by an x-ray pump–probe setup to understand the evolution of radiation damage in the time range of hundreds of femtoseconds.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2024. Vol. 160, no 18
National Category
Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:uu:diva-519450DOI: 10.1063/5.0197225ISI: 001222371200003OAI: oai:DiVA.org:uu-519450DiVA, id: diva2:1824823
Part of project
FIXED: Fluorescent Incoherent X-ray Emission and Diffraction to determine protein structures, Swedish Research CouncilSolving the orientation problem in Single Particle Imaging using XFEL, Swedish Research Council
Funder
Swedish Research Council, 2018- 00740Swedish Research Council, 2019-03935Available from: 2024-01-08 Created: 2024-01-08 Last updated: 2024-06-18Bibliographically approved
In thesis
1. Simulations of ultrafast photon-matter interactions for molecular imaging with X-ray lasers
Open this publication in new window or tab >>Simulations of ultrafast photon-matter interactions for molecular imaging with X-ray lasers
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Biological structure determination has had new avenues of investigation opened due to the introduction of X-ray free-electron lasers (XFELs). These X-ray lasers provide an extreme amount of photons on ultrafast timescales used to probe matter, and in particular biomolecules. The high intensity of the X-rays destroys the sample, though not before structural information has been acquired. The unique properties of the probe provide the unprecedented opportunity to study the un-crystallized form of biological macromolecules, small crystals of biomolecules and their dynamics. 

In this work, we study processes in XFEL imaging experiments that could affect the achievable resolution of the protein structure in a diffraction experiment. Elastic scattering is the process which provides structural information and leaves the sample unperturbed. This interaction occurs far less often compared to damage inducing processes, such as photoabsorption, which leads to rapid ionization of the studied sample. By using density functional theory, we study the effect of ultrahigh charge states in small systems, such as amino acids and peptides, on the subsequent bond breaking and charge dynamics. Reproducible fragmentation patterns are studied in order to find features that could be understood in larger systems, such as proteins. 

Biomolecules are dynamical systems, and the currently used pulse duration is not short enough to outrun the movement of the atoms. The diffraction patterns acquired in an experiment are therefore an incoherent sum of slightly different conformations of the same system. Water can help to reduce these structural variations, but the water molecules themselves will then be a source of noise. Using classical molecular dynamics, we study the optimal amount of water that should be used to achieve the highest resolution. 

To simulate ultrafast molecular dynamics of larger systems such as proteins, we develop a hybrid Monte Carlo/molecular dynamics model. We utilize it to simulate the fragmentation dynamics of small proteins and investigate the possibility to extract structural information from the fragmentation patterns. For larger systems exposed to X-ray lasers, such as viruses and crystals, we develop a hybrid collisional-radiative and classical molecular dynamics approach. The method is used in several projects, both in theoretical studies and to support experiments conducted at XFEL facilities. In particular, we simulate the interaction of hexagonal ice with an X-ray laser, and show the structure makes a phase transition from the native crystal state to a plasma, while still partly retaining structural order. Furthermore, we note that the structural changes occur in an anisotropic manner, where different local structural configurations in ice decay on different time-scales. 

Preliminary experimental results show this anisotropic dynamics in an X-ray pump-probe serial femtosecond X-ray crystallography experiment performed on  I3C crystals. The real space dynamics as a function of probe delay given by our theoretical model and the experiment both show good agreement, where the iodine atoms exhibit correlated motion. The model is also used to calculate the expected atomic displacement and ionization in a hemoglobin crystal, revealing the time and length scales of the dynamics in the protein during the experiment. 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2024. p. 95
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 2353
Keywords
X-ray free-electron laser, molecular dynamics, radiation damage, plasma simulations, density functional theory¸ coherent diffractive imaging, protein structure, X-ray crystallography, single particle imaging
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-519472 (URN)978-91-513-2005-2 (ISBN)
Public defence
2024-02-29, Häggsalen, Ångström, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
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Available from: 2024-02-08 Created: 2024-01-09 Last updated: 2024-02-08

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Dawod, IbrahimCardoch, SebastianAndré, TomasDe Santis, EmilianoCaleman, CarlTimneanu, Nicusor

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