Current and future detector developments at the Swiss Light Source

The detector group of the Swiss Light Source (SLS) at the Paul Scherrer Institut (PSI) has a long history of x-ray detector developments for synchrotrons. Initially these detectors were all single photon counting systems. In the last years the focus at PSI was moving towards charge integrating systems mainly driven by the detector needs for the upcoming XFELs. Charge integrating systems however also solve some of the problems of single photon counting systems. Charge integrating systems have an almost infinite linear count rate capability, allow systems with smallest pixel sizes and for low photon energies. In this presentation we give an overview of the detector developments at PSI and focus on Jungfrau, Mönch and Eiger. Eiger is a single photon counting system specifically developed for high frame rates. It has a 75 micron pixel size and can run at frame rates up to 24 kHz. A 9M Eiger detector will be installed in a few months at the cSAXS beamline of the SLS. Jungfrau uses the same sensor as Eiger (about 4cm x 8 cm with a pixel size of 75 microns). It has a charge integrating architecture with dynamic gain switching to achieve a dynamic range of 10^4 photons (at 12 keV). With a frame rate of up to 2 kHz Jungfrau is currently being developed for applications at both XFELs and synchrotrons. 16M Jungfrau detectors are foreseen at the SwissFEL. Mönch is currently a research project. A first prototype with 160x160 pixels and a pixel size of 25 microns was designed and is currently characterised. It offers the smallest pixel size of current hybrid pixel detectors and also has a very low noise allowing hybrid pixel detectors to be used down to about 400eV. We present measurement results for Jungfrau, Mönch and Eiger and give an outlook on future possible systems.

Molecular mechanism of strigolactone perception by DWARF14

The terpenoid, small-compound strigolactones (SLs) are plant hormones that regulate plant shoot branching, which is an important agronomic trait that determines crop yields. An α/β-hydrolase protein, DWARF14 (D14), has been recognized to be an essential component of plant SL signaling. Recently, it has been demonstrated that D14 interacts with a gibberellin (GA)-signaling repressor SLR1 in an SL-dependent manner [1], which suggests that SLR1 mediates crosstalk between the SL and GA signalings in the regulation of plant shoot branching. Although D14 functions in SL perception to promote the interaction with SLR1, its molecular mechanism remains unclear. Here, we report the crystal structure of D14 in the complex with 5-hydroxy-3-methylbutenolide (D-OH), which is a reaction product of SLs. The structure was solved at a 2.10-Å resolution when an SL synthetic analogue, (–)-ent-2'-epi-GR7, was soaked into D14 crystals [1]. In the complex structure, D-OH was located at a site far from the catalytic residues including H297 and appeared to function as a plug for the catalytic cavity to induce an overall hydrophobic surface with a hydrophilic patch between the two α-helices in the cap structure of D14. In the binding site, the indole amine of Trp205 formed a hydrogen bond with the oxygen atom of the C2' hydroxy group, which arose from the catalytic reaction of D14, instead of a water molecule in the structure of apo D14. In addition, the side chain of Phe245 moved 1.3 Å toward D-OH. Mutational analyses of D14 showed that the interaction between D14 and SLR1 required an enzymatic activity of D14 and the residues Trp205 and Phe245 were essential for the SL-dependent SLR1-binding of D14. These results suggest that the D14–D-OH complex mediates the interaction with SLR1 in which the D-OH-induced surface and/or structural change is crucial.

Complex structure of MERS-CoV spike glycoprotein with human receptor DPP4

Recently identified the Middle East Respiratory Syndrome Coronavirus (MERS-CoV) causes severe and fatal acute respiratory illness in human. No prophylactic and therapeutic agents specifically against MERS-CoV are currently available. Entry of MERS-CoV into the target cells depends on binding of receptor-binding domain (RBD) on viral envelope spike glycoprotein to the cellular receptor dipeptidyl peptidase 4 (DPP4). We report the 3.0 angstrom-resolution crystal structure of MERS-CoV RBD bound to the extracellular domain of DPP4. The structure shows that MERS-CoV RBD consists of a core and a receptor binding subdomain. MERS-CoV RBD and related SARS-CoV RBD share a high degree of structural similarity in their core subdomains, but are notably divergent in the receptor binding subdomain. Structural and mutagenesis analyses identified several key residues in the receptor binding subdomain of RBD that are critical for viral binding to DPP4 and entry into the target cell. Two RBD-specific potent human neutralizing monoclonal antibodies were derived from single-chain variable region fragments (scFvs) of nonimmune human antibody library. They inhibited infection of both pseudotyped and live MERS-CoV with IC50 at nanomolar concentration. Biochemical analysis indicated that these two antibodies blocked RBD interaction with DPP4 on the cell surface.