The European XFEL (EuXFEL) is a 3.4-km long X-ray source, which produces femtosecond, ultrabrilliant and spatially coherent X-ray pulses at megahertz (MHz) repetition rates. This X-ray source has been designed to enable the observation of ultrafast processes with near-atomic spatial resolution. Time-resolved crystallographic investigations on biological macromolecules belong to an important class of experiments that explore fundamental and functional structural displacements in these molecules. Due to the unusual MHz X-ray pulse structure at the EuXFEL, these experiments are challenging. Here, we demonstrate how a biological reaction can be followed on ultrafast timescales at the EuXFEL. We investigate the picosecond time range in the photocycle of photoactive yellow protein (PYP) with MHz X-ray pulse rates. We show that difference electron density maps of excellent quality can be obtained. The results connect the previously explored femtosecond PYP dynamics to timescales accessible at synchrotrons. This opens the door to a wide range of time-resolved studies at the EuXFEL.
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Data has been deposited with the Coherent X-ray Imaging Data Bank73 with CXIDB ID 100. This includes: stream files for all data and for data separated into each time delay, MTZ and PDB files for all time delays, including the dark/reference structures. We have deposited data (mtz-files and structures) for the 10 ps, 30 ps and 80 ps time delays, as well as the dark3 (30 ps) and pure dark reference structures, with the Protein Data Bank, with deposition codes 6P4I, 6P5D, 6P5E, 6P5G and 6P5F, respectively.
Linux scripts and Fortran source codes for the calculation of weighted difference maps, extrapolated electron density maps and the integration of negative densities within a spherical volume are included in a demonstration, which is available online as Supplementary Data.
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We acknowledge European XFEL in Schenefeld, Germany, for provision of X-ray free-electron laser beamtime at Scientific Instrument SPB/SFX and thank the instrument group and facility staff for their assistance. This work was supported by NSF Science and Technology Centers (grant no. NSF-1231306; Biology with X-ray Lasers) to H.N.C., P.F., A.O., A.R., P.S. and M.S.). CFEL (H.N.C.) is supported by the Gottfried Wilhelm Leibniz Program of the DFG; the project ‘X-probe’ funded by the European Union’s 2020 Research and Innovation Program under the Marie Sklodowska-Curie grant agreement (no. 637295); the European Research Council, ‘Frontiers in Attosecond X-ray Science: Imaging and Spectroscopy (AXSIS)’ (no. ERC-2013-SyG 609920, together with P.F.); and the Human Frontiers Science Program grant (no. RGP0010 2017). P.F. and A.R. acknowledge the support of funding from the Biodesign Center for Applied Structural Discovery at Arizona State University and NSF award (no. 1565180). Funding from the National Institutes of Health (grant nos. R01GM095583 to P.F. and R01GM117342 to M.F.) is also acknowledged.
The authors declare no competing interests.
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Integrated supplementary information
Supplementary Fig. 1 Setup of a MHz TR-SFX experiment at the EuXFEL (modified from Wiedorn et al.11).
X-ray pulses arrive in 1.13 MHz bursts which repeat every 100 ms. There are 176 X-ray pulses in the burst. The KB-mirror system focuses the X-ray beam to a 2–3 μm focal spot. The fs-laser delivers 376 kHz pulses (λ=420 nm, blue) synchronized to the X-ray pulses. The laser focus is 42 μm Ø in the X-ray interaction region (dotted circle). The microcrystals are mixed with fluorinated oil and injected by a GDVN. The jet produced by the GDVN, the laser beam as well as the X-ray pulses precisely intersect. The time-resolved diffraction patterns are collected by the AGIPD. Diffraction patterns with common time-delays were separated based on the pulse ID (see also Fig. 2b) and combined to datasets.
a, Hit rates (red) and indexing rates (black) with 1.13 MHz X-ray pulse repetition rate. Note, the strong drop of the hit-rate after the first pulse from 2% to 1%. 472,528 total patterns, 41,559 hits and 24,815 indexed patterns were separated on the basis of pulse IDs. From these, hit rates and indexing rates were calculated. b, Hit rates (red) and indexing rates (black) with 564 kHz X-ray pulse repetition. The overall hit rate is about 2%. 52,495,158 total patterns, 304,673 hits and 142,948 indexed patterns were separated on the basis of pulse IDs from which hit rates and indexing rates were calculated. Blue solid line in a and b, X-ray pulse energy (on arbitrary scale). The indexing rate varies only slightly and is about 40% - 60%.
a, Structure of PYP. Some important residues in the chromophore (pCA) binding pocket are marked. The M41-71 moiety (residues 41 to 71) is marked in red. Helix H74-88 is marked. b, Dark state spectra of PYP. Black: measured in solution, red: in the crystal. The wavelength at the absorption maximum is marked. Excitation has been achieved with 240 fs laser pulses with λ=420 nm. c, Solid spheres: root mean square displacements of 31 Cα atoms in M41-71 relative to the dark (reference) structure, red spheres: from data measured at EuXFEL. Dashed line: fit by a function consisting of an exponential, a strongly damped, phase shifted cosine function and a straight line as outlined in the text.
The green line denotes the M41-71 moiety. The scale on top is in Å. a - d, Difference distance matrices derived from structures at 10 ps, 30 ps, 80 ps and 100 ps relative to that at 3 ps, respectively. Difference distances are also shown for helix H74-88.
The DED map at 30 ps is overlaid on the entire PYP and contoured from +/- 2σ to +/- 4σ in steps of 0.5σ. Red: negative DED, green: positive DED. The 3σ level, c, is the best compromise to distinguish the signal, for example on the pCA chromophore, from spurious noise features distributed within the protein volume.
The factor N has been determined to calculate extrapolated, conventional maps from data collected at various X-ray sources. Black spheres: summed absolute negative DED in a sphere of R = 4 Å centered on the PCA chromophore double bond. Red dotted lines: the more horizontal line follows the initial slope of the data; the second line delineates the constant incline with larger Ns. The Next (in brackets) can be estimated from the intersection of the two lines. a, 3ps data from CXI at LCLS collected with fs laser excitation in the absorption maximum (Pande et al. 14). Factor N = 16, PT = 12.5 %, insert: 1 μs data collected with ns laser excitation. N = 4, and PT = 50% (Tenboer et al.4). b, c, and d, Factors N for the 10 ps, 30 ps, and 80 ps data collected at the EXFEL with fs laser excitation outside the absorption maximum. PT is about 7 % throughout. Insert in d, 100 ps data collected at APS (about 6% PT, Jung et al.33). 13,214, 13,542, 13,722, 13,142, 13,014 and 12,889 observed difference amplitudes are used to determine extrapolated maps for the 100ps, 1 μs, 3ps, 10ps, 30ps and 80ps time delays, respectively.
Supplementary Fig. 8 Observed and calculated difference electron densities (DED) near the pCA chromophore.
Left panels: observed difference electron density (blue: 3 σ, red: -3 σ contour levels). Right panels: calculated difference electron density (blue: 4 σ, red: -4 σ contour levels). Yellow model: structure of the dark (reference) state; blue model: structure at a particular time delay. a, 10 ps; b, 30 ps, c, 80 ps. In panel b pairwise difference density features are marked with α (negative) and β (positive). The feature γ shows the signal caused by the Cys-69 sulfur. The marked DED features can be readily detected at the other time delays. 13,142, 13,014 and 12,889 difference amplitudes were used to calculate the observed DED maps for a, b and c, respectively.
Supplementary Figs. 1–8 and Tables 1–7.
demo.tar.zip. Compressed repository containing a demonstration and software for difference map calculation and structure determination. After the TR-SFX experiment datasets of reference (dark) and time-dependent intensities are available. This demonstration guides through the processes of difference map calculation and structure determination from extrapolated electron density maps.
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Pandey, S., Bean, R., Sato, T. et al. Time-resolved serial femtosecond crystallography at the European XFEL. Nat Methods 17, 73–78 (2020). https://doi.org/10.1038/s41592-019-0628-z
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