Introduction to Protein Crystallography: X-ray Laboratory Instrumentation

Laboratory instrumentation for protein crystallography
For (collecting) X-ray diffraction data, specialized instrumentation for X-ray generation is used. Below you may see an example of a laboratory X-ray generator, which uses a so called rotating anode for the generation of X-rays. The intensity of X-rays obtained from this source is of course many times higher than the intensity used in medical X-ray machines. Usually the higher the intensity, the better quality data can be obtained from the crystal and smaller crystals can be used for data collection.

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X-ray instrumentation for protein crystallography at synchrotrons
For obtaining high quality diffraction data is an advantage to have access to a synchrotron. Synchrotrons have revolutionized protein crystallography and substantially contributed to the near-exponential growth of the number of available protein structures. X-rays produced at synchrotron beamlines have high intensity and may also have very fine focus, which allows for the use of very small crystals, down to 30-40 micrometers or even smaller. Normally, protein crystals have to be frozen to cryo-temperatures prior to the insertion into the X-ray beam. This will reduce the radiation damage caused by the very high intensity of the X-rays emitted at the synchrotron. However, prior to freezing, the crystals need to be transferred to a solution with a cryoprotectant. The cryoprotectant will ensure that no ice crystals are formed in the drop around the crystal, since ice will give rise to its own diffraction, which will interfere with protein diffraction. Examples of cryoprotectants used for freezing crystals can be found here. In practice, the crystals are frozen directly while placed in position at the beamline. Only after that the beam shutter is opened and a monochromatic X-ray beam is allowed to hit the crystal.

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On top of the base unit, which contains the generator, there is another important part of the setup, the X-ray detector, which is better seen on the figure below (the box with the black window on the right). In this case it is an image-plate based X-ray detector. The detector registers the intensity of the diffracted X-rays and the data are subsequently transferred to a computer, where they are processed by the experimenter. Essentially, the data (intensities of the diffracted rays) are corrected for inevitable instrumental and experimental errors and then scaled together (in order to compare their relative intensities). Also the space group symmetry of the crystal is determined, which is an essential characteristics of the crystal and is required for all subsequent operations which include calculation of the electron density map of the molecule (or molecules) in the crystal and building the structural model.

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