My question is very simple, why in Laue crystallography, we use polychromatic X-rays?
I've done some research on Google but the answer was not clear. There was one website mentioned very briefly in one sentence said that "Polychromatic X-rays allow capturing at faster timescale". This of course does not satisfy me.
To my own understand, I think the reason for using polychromatic X-rays is because in Laue crystallography, the protein sample is not static but undergo reaction in real time. So as it changes its state, the ability to diffract X-ray at one wavelength will change. So by using multiple wavelength at the same time, as the protein changes its conformation, there will always be at least 1 wavelength that can be diffracted by the protein at that particular state => allow us to capture the structure at different states of the protein in a shorter length of time. If you use monochromatic X-rays, you will need to stop the experiment, change the wavelength and when you come back, the state of the protein has already changed, so you will lose information.
Is my understanding correct?
Any help will be appreciated.
Thank you
Laue crystallography or diffraction was invented in Munich more hundred years ago by Max von Laue and his collaborators. They used single crystal ad polycromatic X-ray beam. Why use polycromatic beam? The answer is simple. If you do not know the orientation of the crystal if you use a monocromatic X-ray beam then none of the available hkl planes may satisfy Bragg condition 2d sintheta = n.lambda. But if you have a polycromatic beam then it is very likely that many of the hkl planes of the crystal will simulataneously satisfy Bragg conditions provided the wavelengths have simlar magnitudes of the lattice spacings. So you will observe many Bragg spots albeit of X-rays of different avelengths. This is a very handy method and is much more likely to reveal diffraction effects. It is lucky that von Laue and coworkers used polycromatic X-rays; otherwise they may not have discovered X-ray diffraction.
However the Laue method has some drawbacks: (1) Unless you have good knowledge of spectral intensity distribution for different wavelengths you can determine the relative intensties of reflections originating from different wavelengths, (2)The Bragg spots may arise from higher order wavelengths. (3) The background scattering is usually higher. These are not convinient for structure determination and analysis. That was reason Bragg and his coworkers preferred to use monocromatic X-ray methods. For single crystals monocromatic X-ray investigations need rotation and orientation of the single cryrstal in order to satisfy the Bragg condition. Powders have of course the advantage of all possble oriented crystals in the sample and therefore do not need rotation or orientation. So the Laue method was practically abandoned for many many years. This method was only used for orienting crystals but not for structural anaysis.
Things have changed in recent years due to the availability of strong polycromatic synchrotron X-ray beam and efficient detectors. All other drawbacks of Laue method have been practically removed and the Laue crystallography has become extremely popular. The advantage is obviously the fast data collection absolutely needed for protein crystal with very large nit cells. By using X-ray laser one can now do Laue crystallography from very very small crystals and may be in future one will be able to study single protein molecules.
Tapan: So if you use polychromatic X-rays, you don't need to rotate the crystal? While if you use monochromatic X-rays, rotation is needed?
So far I taiked about X-ray Laue crystallography. But you can as well do neutron Laue crystallography. This is very useful for determination of H (proton) positions in a structure. This cannot be done by X-rays. We use a lot of neutron Laue crystallography in our highest flux neutron source of the Institut Laue-Langevin. There exists also several spallation neutron sources in the world, ISIS (not the fanatic muslims!) in UK is of course one of them. There is a single crytal diffractometer there called SXD which uses the Laue method as well. But you also use Laue crystal at the Diamond X-ray source situated at the Rutherford-Appleton laboratory. All these facilities are rather close to you.
Oh and I have one more thing following your previous answer: If using monochromatic X-ray only requires the crystal to be rotated, then why dont they just rotate it then? Is rotating the crystal in X-ray crystallography that complicated that even leads to avoiding it? Will rotating the crystal cause any significant damage to it?
I did not mean roting the crystal with a high speed. I simply mean you to scan the crystal by rotating step by step with small steps. This called sample rotation or omega scan. You go to X-ray diffraction of the Bristol University and you will find some single crystal diffractometer for sure. Ask the instrument responsible how does he measure the intensities of the reflections.
But my question is: "If the only drawback of using monochromatic X-ray is rotating the crystal, and rotating it doesn't cause any damage, then why dont we just use monochromatic X-ray all the time? Why do we need to develop Laue crystallography then?
Because It takes much more time to collect data with monochromatic X-ray diffraction whereas with Laue method you can collect data much faster; For protein crystals this is an important issue.
Informative and nice reply of Dr. Tapan Chatterji are very useful. I agree with Dr. Chatterji. However, people working on liquid crystal and liquids are also using monochromatic X-ray in Laue system. Instead of Laue spots they are getting Debye-rings from their sample.
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