Research

Soft matter and biomolecular assemblies

By modern x-ray diffraction and x-ray imaging we investigate the structure, dynamics, and self-assembly of biological matter, from model systems, to cells and tissues. In the course of different projects we have for example studied the structure and collective dynamics of membranes, including interactions and the non-equilibrium dynamics of membranes after optical excitation [1]. At present, our research in membrane biophysics concentrates on the problem of membrane fusion. We probe intermediate structures in model membranes in 3D by high resolution x-ray diffraction [2] and study the interaction of model membranes with native synaptic vesicles [3], see Fig.1. Beyond model systems, we extend x-ray diffraction methods to entire biological cells, which we scan by nano-focused x-ray beams, recording and analyzing the local diffraction pattern. In this way, we investigate protein networks in biological cells, myelin structure in nerve fibers, and the packing of DNA in bacterial nucleoids [4]. For such experiments, we use different optics to focus or confine x-ray radiation.

X-ray optics

Our group advances x-ray optics, from design and fabrication of optical components to phase retrieval algorithms, focusing schemes and experimental schemes. A particular new x-ray microscopy approach which we have developed is based on x-ray waveguide optics, providing a highly coherent a quasi-point source for holographic imaging [5]. X-rays can be guided through small channels of low electron density material embedded in a cladding material with higher electron density, similar to the way that visible light is guided through an optical fibre. X-ray waveguides can deliver nano-meter sized x-ray beams of defined shape and coherence properties. We achieve spot sizes down to 10nm, and can now use such x-ray nano-beams for spatially resolved diffraction, and as quasi point sources for holographic imaging and tomography. Recently, we have shown that one can even guide nano-sized x-ray beams “around the corner” [6], see figure on the left side, enabling new functionalities (beam splitters, time delays, reference beam).

Holographic phase contrast x-ray tomography

We perform substantial development in coherent x-ray imaging, holography, and tomography [7]. Experiments are carried out both at in-house x-ray sources (cw and pulsed), as well as at synchrotron, and free electron laser (FEL) facilities. We also closely interact with mathematicians, mainly on phase retrieval and tomographic reconstruction methods. In the end, all the enjoyment we take from advancing x-ray optics has to serve challenging applications, such as the example shown in Fig.3, where the interior structure of peripheral nerves with thousands of axons and connected neurons is reconstructed by phase contrast tomography [8], yielding 3D data without cutting of the tissue, going far beyond the standard histological cross sections

Synchrotron radiation

Our group operates an endstation of nano-imaging at the coherence beamline P10 at the storage ring PETRAIII at DESY, known as the Göttingen Instrument for Nano-Imaging with X-rays (GINIX). Together with collaborating groups at HASYLAB / DESY and other partner groups, we operate and develop this endstation. We are also engaged in a virtual institute on In-Situ Nano-Imaging of Biological and Chemical Processes funded by the Helmholtz Association, and in the partnership for soft-condensed matter (PCSM) with the European Synchrotron Radiation Facility (ESRF), Grenoble.

Funding

Our research is funded by SFB 755 Nanoscale Photonic Imaging, SFB 937 Collective Behavior of Soft and Biological Matter, SFB 803 Functionality Controlled by Organization in and Between Membranes, SFB 1286 Quantitative Synaptology, BMBF (Verbundforschung), the Roentgen-Angstrom Cluster, and the neuroscience cluster of excellence EXC171 Nanoscale Microscopy and Molecular Physiology of the Brain (CNMPB).

References

[1] T. Reusch, D. D. Mai, M. Osterhoff, D. Khakhulin, M. Wulff, T. Salditt
Nonequilibrium Collective Dynamics in Photoexcited Lipid Multilayers by Time Resolved Diffuse X-Ray Scattering, Physical Review Letters 111, 268101 (2013).

[2] S. Aeffner, T. Reusch, B. Weinhausen and T. Salditt, Energetics of stalk intermediates in membrane fusion are controlled by lipid composition , Proc.Natl.Ac.Sci. 109, 1609-1618, (2012).

[3] S. Castorph, D. Riedel, L. Arleth, M. Sztucki, R. Jahn, M. Holt, T. Salditt
Structure Parameters of Synaptic Vesicles Quantified by Small-Angle X-Ray Scattering
Biophysical Journal 98, 1200 (2010)

[4] K. Giewekemeyer, P. Thibault, S. Kalbfleisch, A. Beerlink, C. M. Kewish, M. Dierolf, F. Pfeiffer and T. Salditt , Quantitative biological imaging by ptychographic x-ray diffraction microscopy , Proc.Natl.Ac.Sci. 107, 529-534 (2010).

[5] M. Bartels, M. Krenkel, J. Haber, R. N. Wilke, and T. Salditt , X-Ray Holographic Imaging of Hydrated Biological Cells in Solution , Phyical. Review Letters, 114, 048103 (2015).

[6] T. Salditt, S. Hoffmann, M. Vassholz, J. Haber, M. Osterhoff, J. Hilhorst , X-Ray Optics on a Chip: Guiding X Rays in Curved Channels , Physical Review Letters 115, 203902 (2015).

[7] M. Vassholz, B. Koberstein-Schwarz, A. Ruhlandt, M. Krenkel, T. Salditt
New X-Ray Tomography Method Based on the 3D Radon Transform Compatible with Anisotropic Sources , Physical Review Letters 116, 088101 (2016).

[8] M. Bartels, M. Krenkel, P. Cloetens, W. Möbius, T. Salditt
Myelinated mouse nerves studied by X-ray phase contrast zoom tomography
J. Structural Biology 3, 561-568 (2015).