X-Ray Diffraction: Understanding Reflection Symmetry

In summary, protein crystals do not exhibit reflection symmetry because proteins themselves are chiral. However, the diffraction pattern can have mirror symmetry due to the symmetry of the Bravais lattice, without consideration of the basis. The basis, in this case the chiral proteins, may affect the intensity of the spots but not the position, resulting in symmetry in the diffraction pattern.
  • #1
alona
1
0
hi,

i'm having trouble with a problem regarding X-ray crystallography. the question is as follows:

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In general, crystals can have various symmetry elements: rotation, translation, reflection, etc. however, you never find reflection (mirror) symmetry in protein crystals. why?

On the other hand, you can get mirror symmetry in the diffraction pattern. Why?

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i know the answer to the first part of the question - it's because proteins are made of L-amino acids only, which are not symmetrical, so can't reflect.

i have no idea why you can get mirror symmetry in the diffraction pattern. how can the diffraction image have a form of symmetry that the original crystal doesn't? i'd appreciate if someone could help me with this. thanks.
 
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  • #2
The key here is that the symmetry of the diffraction pattern is determined by the symmetry of the Bravais lattice without reference to the basis. In other words, while the proteins themselves are chiral, the way the proteins are distributed in space can still have reflection symmetry. The full crystal is determined by the Bravais lattice decorated with a basis, in this case chiral proteins; however, only the Bravais lattice determines the diffraction pattern. Note that the basis can affect the intensity of the spots or even cause some spots to vanish, so what "the diffraction pattern is symmetric" means is that the position of the spots is symmetric.
 

1. What is X-Ray Diffraction?

X-ray diffraction is a technique used to study the structure of materials at the atomic or molecular level. It involves exposing a sample to a beam of X-rays and analyzing the resulting diffraction pattern to determine the arrangement of atoms or molecules in the sample.

2. How does X-Ray Diffraction work?

X-ray diffraction works by exposing a sample to a beam of X-rays, which causes the atoms or molecules in the sample to scatter the X-rays in different directions. The scattered X-rays then interfere with each other, creating a diffraction pattern that can be analyzed to determine the arrangement of atoms or molecules in the sample.

3. What is Reflection Symmetry in X-Ray Diffraction?

Reflection symmetry in X-ray diffraction refers to the symmetry of the diffraction pattern produced by a sample. It is the symmetry that occurs when an object is reflected along a plane, resulting in a mirror image that is identical to the original object. In X-ray diffraction, reflection symmetry is used to determine the arrangement of atoms or molecules in a sample.

4. How is X-Ray Diffraction used in scientific research?

X-ray diffraction is a widely used technique in scientific research, particularly in the fields of materials science, chemistry, and biology. It is used to determine the atomic or molecular structure of various materials, including crystals, proteins, and polymers. This information is crucial for understanding the properties and behavior of these materials, and can also aid in the development of new materials and drugs.

5. What are some limitations of X-Ray Diffraction?

Although X-ray diffraction is a powerful tool for studying the structure of materials, it does have some limitations. One limitation is that the sample must be in a crystalline form, meaning that the atoms or molecules are arranged in a regular, repeating pattern. X-ray diffraction also cannot provide information about the chemical composition of a sample, and it may not be sensitive enough to detect small changes in a sample's structure.

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