Title: Data Collection and S-SAD Phasing of Protein Crystals Staff: Kazuya Hasegawa¹, Seiki Baba¹, Go Ueno² (1 SPring-8/JASRI, 2 RIKEN SPring-8 Center) Abstract: Sulfur SAD (S-SAD) phasing has been got attention because it does not need heavy atom derivative and is expected to improve the throughput of structure determination. However, accurate measurement of diffraction intensity is crucial for the success of S-SAD because of the small anomalous signal of sulfur atom. In this exercise, participants will collect diffraction data from a protein crystal and determine the structure by using S-SAD phasing.
Contents:
Brief Introduction of BL26B1 l. 1.1.  Specification of the X-ray Beam

1.2.
Optical Components
Simple optics which consists of SPring-8 standard double crystal monochromator and 2-dimensional focusing bend cylindrical mirror is adopted. This facilitates stable beam suitable for automated and high-throughput data collection.
1.3.
Experimental Station
BL26B1 & B2 have been developed as RIKEN Structural Genomics Beamline I & II for high throughput protein crystallography (HTPX). The function of the beamline is to collect diffraction data of a large number of protein crystals for structural analysis. In the experimental hutch, an automatic sample changer developed at BL26 is installed to improve the throughput of the experiment. Cooperating with the sample changer, the beamline scheduling software (BSS) executes the successive data collections without any user intervention.
Control Software BSS
1.4.
BSS (Beamline scheduling software) can deal with all of the beamline operation as follows: 1) Control the optical and experimental components such as monochromator, goniometer, and detector. 2) Data collection of crystal diffraction, X-ray fluorescence etc. The important feature of BSS GUI is a schedule list which facilitate automatic data
1.5.
Sample Changer SPACE
SPACE (SPring-8 precise automatic cryo-sample exchanger) was developed at RIKEN SPring-8 Center to promote structural genomics research, and was first implemented at BL26B1 and BL26B2 in 2003. SPACE adopted specially designed plastic pins equipped with screw threads to assure the reproducibility of the crystal position on the goniometer. Because of this feature, once samples have been manually centered and the translation coordinates have been recorded during the crystal screening process, the sample can be automatically centered in the beam before data collection. Now, SPACE can also handle the conventional sample pins with metal bases for use with magnetic goniometer heads, and installed at all MX beamlines at SPring-8 including micro-focus beamline. Fig. 4: Sample changer SPACE 1.6. Specification of beamline equipment . CCD detector (RIGAKU/ Saturn A200) Detector area : 203 × 203 mm² Pixel size : 50 × 50 μm² (unbin) No. of pixels : 4096 × 4096 . Horizontal axis goniometer having motor-driven x, y, z stage . Cryostat : temperature control range 80  ~  350 K . Gas-flow type ionization chamber . Si PIN photodiode detector for fluorescence measurement Multi-channel analyzer of fluorescence measurement . . Control Software BSS Sample Automated Changer SPACE .
2.
Beam Alignment
Every morning, our beamline staff conducts beam alignment to ensure that the incident X-ray illuminates the center cross of the microscope. Following is the beam alignment protocol. 2.1. Confirm and define the rotational center of the goniometer head at the window of the sample monitor software using a sample mount Lithographic loop. If the position is not located at the center of the display, the position should be aligned by the micrometer of the sample monitor CCD camera. The cross of the center position can also be aligned by the software definition of the video capture software. 2.2. BSS Run Option is set to "Admin" mode. 2.3. Set the spindle axis to 0 degree. 2.4. Put the fluorescent plate on the goniometer head. 2.5. Focus the microscope on the surface of the fluorescent plate by moving stl gonio 1 x axis from Axis Tools window of BSS. 2.6. Turn off the sample light. 2.7. Set wavelength 1 Å and open X-ray shutter. 2.8. Record the X-ray intensity at 1A (Electric current of ionization chamber). 2.9. Display the monochromator alignment GUI on BL-Work Station. Record the position of Δθ 1 axis. 2.10. The size of two slits (ST1/ST2) is set to 3 mm by Axis Tools. 2.11. Tune Δθ 1 to maximize intensity by using the Energy tab of Axis Tools. Record the I 0 value, the position of the beam, and the shift of Δθ 1 . 2.12. Beam position should be aligned to the rotational center of the goniometer by the translating the experimental stage. In Z axis direction (height), beam position should be placed on the spindle axis. In Y axis direction (horizontal), the position should be the central cross of the video monitor. 2.13. Change the slit aperture size to the original values. The aperture size of ST1 and ST2 are 0.5/0.3mm and 0.4/0.4mm, respectively. Record the I o value. 2.14.  Tune Δθ₁ again. 2.15.  BSS Run Option is set to "None" mode.
3.
Theoretical background of SAD phasing
3.1.  Anomalous effect
Thomson scattering occurs the following scheme: 1) X-ray photon strikes a free electron, 2) the electromagnetic field of the incident oscillates the electron. 3) the oscillated electron scatters X-ray with the same wavelength as the incident X-ray. However, the electrons of atoms are bound tightly and its oscillation has a certain resonance frequency. Therefore, the scattering factor is depends on the incident wavelength. It can be expressed as the following formula.
f = f。+ f'+if"
In the case of sulfur atom, the wavelength dependent f' and f'' terms are illustrated as following graphs.
3.2.
Harker diagram and phase
Crystallographic F(h) values (structure factors) are given by Fourier transformation of electron density ρ( x ). F( h ) value is a complex and composed of absolute |F( h )| and phase α(h). Inverse Fourier transformation of F(h) gives electron density of crystal, however |F(h)| is only obtained from diffraction experiments. The phase α(h) can be recovered from several experimental methods. Here we describe the scheme of SAD method. Harker diagram (phase diagram) is a picture for understanding phase determination.
Fig 6: Harker diagram of SAD method.
Anomalous effect breaks down Friedel's Law where a reflection F(h) and its centrosymmetric reflection F(-h) have same value of |F|. ΔF, the difference of F(h) and F(-h) is a good approximation of the structure factor amplitude of anomalous scatterers. Therefore, Patterson method or direct phasing can extract the position of the anomalous scatterers. Once the positions of anomalous scatterers are determined, their structure factor (F"H) is calculated. The vector of F " H and its negative can be drawn in the diagram. Next, we can draw two circles: one with radius of | F (+) P | centered on the end of F " H. another with radius |F(-) P | centered on the end of − F ′′ H . We can see the two intersections F P and F P − of the two circles. One of the two points is correct answer. Although the two possibilities cannot be distinguished without any information, we can obtain good phases by the treatment of electron density shape (density modification; solvent flattening etc.) in some cases.
4.
Data Collection
4.1.
Sample mounting on goniometer 1) Pick up a desired crystal by a cryoloop device: Firstly, a desired crystal is scooped with a cryoloop under a microscope. Protein crystal is mostly small and its typical dimension of 0.1~0.2 mm. Protein crystal is obtained from water solution, therefore it is easy to dry up under atmospheric condition and also very fragile. Scooping process should be 2) Soaking the crystal into cryoprotectant: The scooped crystal is transferred into the solution containing cryoprotectant (typically it contains 30% glycerol) which prevents ice formation during sample cooling. 3) Picking the crystal again: To mount the crystal onto a goniometer head, the crystal is picked again. Since glycerol containing solutions are viscous, the excess of the solvent is sometime scooped onto the cryoloop. The excess causes unfavorable scattering which degrades the statistics of diffraction data. Since the anomalous signal for S-SAD phasing is weak, the solvent should be removed as far as possible. 4) Mounting and flush cooling the crystal: Immediately, the picked crystal should be put on the goniometer head with magnetic support. To cool the sample during X-ray irradiation and prevent the radiation damage of the sample, cold nitrogen gas stream is blowing at the crystal position. Before mounting the crystal, the blow is blocked out by the shutter system. And after mounting, the shutter should be immediately opened. This is ‘flush cooling technique’ for preventing ice formation in solvent and crystal.
Collection of diffraction images
1) Set up the data collection software: To take diffraction images, the program BSS is used at this beamline. Following two figures are the windows of data collection GUI.
4.2.
2) Centering the sample to beam position: Diffraction images are taken by oscillation method. In the method, crystal rotates and oscillates with the spindle axis of goniometer. To keep the crystal position during the rotation against X-ray beam, the crystal is aligned to the center of the spindle axis, because X-ray beam is already aligned to the spindle axis. This operation can be achieved in the program BSS using video camera and monitor. 3) Closing experimental hutch and opening down stream shutter of the beamline. 4) Take a few images and check the diffraction: To estimate the quality of the mounted crystal, one or two images should be taken before taking data set. 5) Take a full data set: When the crystal passes through the quality check, the consecutive series of diffraction images are taken. In most cases, 180 images will be taken if you do not have any information of the crystal as space group, cell constant etc. In the case of SAD, highly redundant data collection is required, because it makes the statistics better. 5.
Data processing and structure determination
5.1.
Data processing
Diffraction images are processed by a special computer program. Against the raw data of each image, the following process is performed: 1) the prediction of reflections (diffraction spots) through the determination of lattice constants and crystal orientation and the parameter-refinement of the camera-distance etc [indexing and prediction]. 2) the determination of reflection index and the estimation of its intensity [integration] After obtaining the index and intensity of each reflection in all images, the intensity should be merged into reflections which are equivalent in crystallographic symmetry [scaling]. Finally we obtain F-data (containing hkl : Mirror index and F( hkl ): structure factor amplitude). In the case of SAD, Friedel mates F(+) and F(-) should be recorded independently. Phase Calculation by SAD
5.2.
There are several program packages for this purpose. In this practice, HKL2MAP will be used. This program is a GUI software for SHELX crystallographic analysis package. This analysis is performed as following procedure: 1) SHELXC: the preparation of ∆F data from experimental F data. 2) SHELXD: finding the positions of sufur atoms as anomalous scatterers. 3) SHELXE: phase calculation and phase improvement by density modification. Finally, the calculated phases are checked by electron density maps using the program COOT. Its details will be introduced at the BL practice. Only several pictures