BL18U1

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Beamline Introduction
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The Protein Micro-crystallography beamline BL18U1, together with the crystallographic beamline BL19U1 for protein complex structures and the crystallographic beamline BL17B1 for high-throughput proteins, is a strategic component of the structural biology platform in the large research infrastructures of the Chinese Academy of Sciences (CAS) in Pudong New District, Shanghai. The BL18U1 beamline was built from 2012 to 2014, commissioned in late 2014 and officially open for user service in March 2015. Since then, the beamline has been in continuous operation.
The BL18U1 Protein Micro-crystallography beamline uses U25 undulator as the light source, which has high brightness and beam collimation. The energy range is 5-18 keV, covering the the absorption edges of common heavy elements, such as the K absorption edge of Se, Br, Zn, Cu, Fe, the L absorption edge of Hg, Pt, and the wavelength of S anomalous scattering. 
The spot size at the sample point can reach 10μm in both horizontal and vertical directions. When the beam energy is 12 keV, the vertical divergence angle of the beam is 0.25 mrad and the horizontal divergence angle is 0.7 mrad, which can fully meet the requirements of various high-resolution experiments. For situations where a smaller beam divergence angle is required, further beam confinement can be achieved by tuning slits. 
The BL18U1 protein Micro-crystallography beamline can meet the data collection requirements for small crystals, and is also suitable for the collection and structural determination of various protein crystal data. The available experimental methods for crystal structure determination include multiple wavelength anomalous scattering (MAD), single wavelength anomalous scattering (SAD), molecular displacement (MR), isomorphic displacement (MIR) methods.
Beamline Scientific Goal
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The main goal of BL18U1 is to achieve high brightness with a small focused beam size, so that the diffraction data of crystals with size of 5-10 microns in each dimension can be effectively collected, and at the same time, the anomalous diffraction signal of elements such as S, P and Ca can be effectively collected.
Beamline Optical Layout
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The optical layout of the Beamline is illustrated in the following Figure 1


Figure 1 The Optical Layout of BL18U1 Beamline

To generate a high-brilliance synchrotron beam in the energy range of 5-18 keV at an electron energy of 3.5 GeV, the U25 undulator with a period of 25 mm and 80 periods was selected as the light source for the BL18U1 protein micro-crystallographic beamline. The U25 undulator covers the energy range from 5 keV to 18 keV and is capable of detecting anomalous diffraction of S for the S-SAD phase method. The energy range also covers the K-absorption edges of Cr and Mn and the L-edges of I, Xe, Cs and Ba and has higher brightness and relatively lower heat generation.

Beamline Specifications
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The specification of the BL18U1 beamline is listed in Table 1. The energy of 5-18 keV is tunable with an energy resolution of less than 1.8x10-4 at 12.4 keV. The beam size at the sample point was tested to be 9.86 µm (H) x 4.9 µm (V) after careful tuning of the optical elements. The photon flux of the focused beam is 5.81x1011 ph/s at a current of 300 mA, and the divergence of the focused beam is 0.27 (H) x 0.17 (V) mrad2. The divergence does not change with energy and could be further minimized by the slit in front of the focused beam. However, the photon flux also decreases with the minimized divergence.
  Source type  Undulator, U25
 25 mm x 80 periods, 6–12 mm gap
  Toroidal mirror  800x70x35 mm3, Rh-coated, 3.5 mrad
  Cylindrical mirror  1000x85x55 mm3, Rh-coated, 3.5 mrad 
  Monochromator  1000x85x55 mm3, Rh-coated, 3.5 mrad 
  Energy range (keV)  5–18 
  Energy resolution (DE/E) (@12.4 keV)  2x10-4 
  Focused beam size (FWHM) (µm2) (Hx V) (@12.4 keV)  <10x7, best 9.86 x 4.9 
  Divergence (mrad2) (HxV) (@12.4 keV)  <0.27x0.17 
  Flux (at 12.4 keV, 300 mA) (photons/s)  5.81x 1011 
Table 1 The Specifications of BL18U1 Beamline
Endstation
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The experiment station is located at the end of the crystallography beamline. In the experimental station there are several important devices such as a detector, a diffractometer, a sample changer, a fluorescence detector, a cold current, an ion chamber, attenuators and more, as shown in Figure 2. 
Figure 2 The The overview of BL18U1 experimental hutch
A highly integrated Maatel MD2 diffractometer is equipped in the BL18U1 experimental hutch, the sphere of confusion(SOC) of the installed MD2 diffractometer is smaller than 1.5 microns. The incident beam passes through an aperture with three pinholes of 10, 20 and 50 microns in diameter. Next to the aperture is a capillary beamstop, which consists of a capillary and a beamstop. The capillary has a diameter of 150 micrometers and a length of 3.0 cm. It effectively shields the scattered beams coming from above before they hit the crystal in the sample field. The beamstop prevents direct beam from hitting the detector.
For a better view of the crystal, a pneumatically operated backlight and two front lights are also integrated into the MD2 system. A motorized scintillator/photodiode pole is also integrated into the MD2 system for convenient visualization of the beam position and monitoring of the intensity.
The BL18U1 beamline is equipped with a high-performance Pilatus 3 6M detector. The detector has dimensions of about 420 mm x 434 mm, with 0.172 x 0.172 square microns in each pixel, and has a full frame rate of 100 frames per second, with a 0.9 ms readout time for each frame. The detector has been in constant operation for more than nine years without any breakdown. An automatic sample changer, the Rigaku Actor Sample Changer, is used for fast and non-intervention sample transfer. Its dewar accommodates 5 unipucks, each of which holds 16 samples, and thus the diffraction of 80 total samples can be tested or collected for each batch of experiment.
The crystal is cryo-cooled by the Oxford Cryostream’s Oxford 800 series. The cryosystem can maintain the temperature in the range of 100 to 400 Kelvin of the sample,
To protect the sample from ice-forming, careful maintenance of temperature and humidity in the experimental hutch is critical. Therefore, an air-conditioning system was installed in the hutch, and the temperature could be controlled at 25 ± 1℃. An industrial dehumidifier was also installed to maintain the humidity in the hutch within the range of 46%~52%
Data Collection and Processing
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The MXCuBE3 has been introduced as the user interface for data collection tasks. This new interface has proven to be more user-friendly, reducing the likelihood of errors and enhancing the efficiency of automatically organizing user data through the IsPyB database. Users collect diffraction data using MXCUBE3 interface which function as upper layer that controls the major devices such as MD2 diffractometer, sample changer, Pilatus3 6M detector, and fast shutter. 
The data processing software implemented in BL18U1 endstation computers includes HKL3000, XDS, autoPROC, and autoPX. The autoPX is the first domestically developed high performance data processing software with full intellectual properties. Crystallographic packages of CCP4 suite, PHENIX package, and SHELX are also implemented in the endstation computers. These on-site software and packages provide a convenient option for immediate data processing, structure solution, and refinement to check the structure.
An automatic data processing pipeline featuring xia2-XDS, autoProc, xia2-DIALS, autoPX was implemented in BL18U1, and the pipeline has been running continuously since its implementation.