As mentioned above, Max von Laue demonstrated that X-rays are electromagnetic waves, having the same nature as visible light or radio waves. The only difference of course is the very short wavelength of around 1-1.5 Å (Ångström, which is 10-10 meter) that is characteristic for X-rays. For comparison, the wavelength of visible light is between 400 to 700 nm (one nm is 10 Å).
X-ray diffraction is caused by the interaction of electromagnetic waves with the atoms inside the crystals, and particularly with the electrons. The waves get scattered by the electrons, each electron becomes a small X-ray source of its own. Scattered waves from all the electrons within each atom are added to each other, giving diffracted waves from each atom, etc. When the scattered waves are added, they may either get stronger or cancel each other. Those which get stronger are registered by the X-ray detector, as in the figure above. Interestingly, we do not necessarily need X-rays to observe interference, we can, for example go to a lake nearby, through two stones into the water and then observe how the waves from the two stones either reinforce each other or become weaker. There are many demonstrations of wave addition on the web, one of them can be found here
.X-ray data collection, electron density calculation and model building
X-rays may be generated using various laboratory X-ray sources or at synchrotrons, where very high intensity and highly focused X-rays can be generated. There are several synchrotrons around the world with station adapted for collecting X-ray data from protein crystals. Depending on the type of the crystal (crystals may have different cell dimensions and different symmetry groups), different strategies for data collection are followed and a different amount of data is required. Usually the crystal is rotated in the X-ray beam one degree a time, and exposed to X-rays for a short period (seconds to minutes) until a full data set is collected. Depending on the intensity of the X-ray source, the size of the crystal and how well it diffracts, the total time required for data collection may be very different. The data are processed using specific computer programs
Each spot on the image is a diffracted X-ray beam, which emerged from the crystal and was registered by the X-ray detector. Thousands of diffraction spots need to be collected from a protein crystal in order to get a complete data set. The X-ray data are processed and the intensities of the diffraction spots are extracted and used to calculate an electron density map
of the molecules inside the crystal. The electron density, in turn, will tell us where the atoms are located, information which can be used to build a model of the molecule in the crystal.
This is a very short summary of protein X-ray crystallography. I hope to provide more details on this in the near future.