What is X Ray Crystallography?

21 July 2020

Interview with 

Megan McGregor


The key to the breakthrough of understanding genetic inheritance was understanding the structure of DNA, and, to solve this, Rosalind Franklin used a technique called X-Ray crystallography. This enabled her to figure where the individual atoms are in DNA. But how does it actually work? Materials scientist Megan McGregor explains...

Megan - Rosalind Franklin used x-ray crystallography to create photo 51, a black and white picture showing an X shape made up of spaced apart, black splotches. And it was thanks to this photograph that the structure of DNA as a helix was figured out. This was a picture of crystallised DNA, but what is x-ray crystallography? Lots of materials from ice cubes to steel girders are made up of crystals. Each crystal is composed of a repeating arrangement of atoms with well-defined spacing like soldiers marching in formation. This repeating arrangement gives distinct layers of atoms and the spacing between them can affect many material properties from strength to conductivity. X-ray diffraction is one way to measure the distance between the layers. An X-ray diffraction experiment shines a beam of x-rays onto a material and varies the angle between the beam and the material surface. At certain angles, we can detect a very strong X-ray signal being re-emitted from the surface, like light being reflected off a mirror. Father son team, William and Lawrence Bragg realised that with a bit of maths, these angles could be related to the spacing of the atomic layers in the material. Why? At certain angles, the distance an X-ray travels between the atomic layers matches up perfectly with the wavelength of that X-ray. So if you know the wavelength of the X-rays, and the angle they hit your sample at, you can work out the spacing of the atoms inside the material based on the pattern of reflections you get. Each crystal will generate a unique pattern of strong reflections at certain angles, like a fingerprint that can be used to identify that crystal structure. This is why X-ray diffraction was so useful to those trying to figure out the structure of DNA. Working backwards from the unusual X-ray reflection pattern in photo 51, scientists were able to work out that the atoms of oxygen, hydrogen, carbon, nitrogen, and phosphorus could only be arranged in one way, the famous double helix.


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