X Ray Crystalography

8/14/2019 X Ray Crystalography

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X-rays are electromagnetic radiation with wavelengthsbetween about 0.02 and 100 (1 = 10-10 meters).


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X-ray tube. consists of anevacuated chamber with a tungstenfilament at one end of the tube,called the cathode, and a metaltarget at the other end, called ananode. Electrical current is runthrough the tungsten filament,causing it to glow and emit

electrons. A large voltage difference(measured in kilovolts) is placedbetween the cathode and the anode,causing the electrons to move athigh velocity from the filament to theanode target. Upon striking the

atoms in the target, the electronsdislodge inner shell electronsresulting in outer shell electronshaving to jump to a lower energyshell to replace the dislodgedelectrons. These electronic

transitions results in the generationof X-rays.


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optical spectrum forhydrogen. The distinct lines near 435 nm, 487nm, and 655 nm show transitions from the 5th, 4th and 3rd energyshells, respectively, into the 2nd energy shell.

Even the most energetic line hydrogen emits (when an electron dropsdown from the second shell to the first) has only enough energy to bean ultraviolet photon.
http://imagine.gsfc.nasa.gov/docs/dict_ei.htmlhttp://imagine.gsfc.nasa.gov/docs/dict_qz.htmlhttp://imagine.gsfc.nasa.gov/docs/dict_qz.htmlhttp://imagine.gsfc.nasa.gov/docs/dict_ei.html


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Cartoon of a hypothetical crystallinelattice


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Crystallisation

Diagram of hanging drop method. Reservoir solution (blue) usually containsbuffer and precipitant. Protein solution (red) contains the same compounds, butin lower concentrations. The protein solution may also contain trace metals orions necessary for precipitation of particular proteins. For instance, insulin isknown to require trace amounts of zinc for crystallization (McRee, 1993).


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Diagram of sitting drop method. In this method, the proteindrop sits on a pedestal above the reservoir solution, asopposed to hanging.


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Panel 1 Region of exceptional experimental electron density map created from multi-wavelength data collected at the ALS.Only solvent flattening and density modification have been performed. No phase combination with any model phases has

taken place. Panel 2 : .to show the excellent correspondence with actual protein residues, a previously determinedmolecular replacement model of Arg has been placed into the electron density. The model can be unambiguously and easilybuilt into the electron density.

Panel 3 : View is a cross section of the apoE four helix bundle. The empty space in the centre of helix one and two is clearlyvisible. Panel 4: : to show the excellent correspondence of experimental electron density with actual protein residues, modelhelices have been placed into the electron density. Experimental phases had an average figure of merit of 0.86. There is verylittle noise and excellent connectivity in the electron density.
http://www-als.lbl.gov/http://www-als.lbl.gov/


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Panel 1 : experimental electron density map created from multi-wavelength data collectedOnly solvent flattening and density modification have been performed. No phase combinationwith any model phases has taken place. Panel 2 : tyrosine residue fitted into experimentalelectron density. Panel 3 electron density of combined MAD/MIR map in tetanus C fragment.


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X Ray Crystalography

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