# X-ray diffraction

1. May 1, 2014

### tahaha

I'm working on a physical chemistry project that is a report on a paper that uses X-ray crystallography to characterize a protein. So I am studying X-ray crystallography more in depth than the book introduces, and I have a few questions that I couldn't really find answers to online.

1. How does measuring more diffraction peaks improve resolution? Does that mean a LARGER crystal improve resolution? (I don't think it's that simple...)

2. How is the resolution defined? People online seem to say that the limit is wavelength/2, but I'm having trouble deriving it from the Bragg Law (I think a larger diffraction angle means higher resolution?)

3. Regarding the unit cell, do you just "choose" the shape and dimensions of the unit cell (square, triangular, oblique, etc.) as long as it contains at least one of each atom of your molecule? How does this choice affect the resolution?

4. This is pretty much a technical question: How do detergents interrupt crystallization?

Thank you!

2. May 2, 2014

### UltrafastPED

Resolution is related to the instrument being used, as well as the sample. Larger, purer crystals give better data; if you are using powder diffraction then the purity is important, as well as the total quantity of material. See
http://www.proteinstructures.com/Experimental/Experimental/electron-density.html

Improved separation of the diffraction peaks requires pure materials, or a binder that is "transparent" to the x-rays. Analysis is improved with more peaks and accurate intensity measurements - just imagine if the peaks were blurred together ...

The choice of unit cell arbitrary - as long you can "tile" the entire crystal with it; see http://www.doitpoms.ac.uk/tlplib/crystallography3/unit_cell.php

This site has introductory tutorials on x-ray diffraction:
http://www.doitpoms.ac.uk/tlplib/xray-diffraction/intro.php

Detergents are a complicated question - see http://www.piercenet.com/method/detergents-cell-lysis-protein-extraction