Editing X-ray crystallography (section)

==== Recording the reflections ====
[[File:X-ray diffraction pattern 3clpro.jpg|thumb|An X-ray diffraction pattern of a crystallized enzyme. The pattern of spots (''reflections'') and the relative strength of each spot (''intensities'') can be used to determine the structure of the enzyme.]]
The relative intensities of the reflections provides information to determine the arrangement of molecules within the crystal in atomic detail. The intensities of these reflections may be recorded with [[photographic film]], an area detector (such as a [[hybrid pixel detector|pixel detector]]) or with a [[charge-coupled device]] (CCD) image sensor. The peaks at small angles correspond to low-resolution data, whereas those at high angles represent high-resolution data; thus, an upper limit on the eventual resolution of the structure can be determined from the first few images. Some measures of diffraction quality can be determined at this point, such as the [[mosaicity]] of the crystal and its overall disorder, as observed in the peak widths. Some pathologies of the crystal that would render it unfit for solving the structure can also be diagnosed quickly at this point.{{cn|date=July 2024}}

One set of spots is insufficient to reconstruct the whole crystal; it represents only a small slice of the full three dimensional set. To collect all the necessary information, the crystal must be rotated step-by-step through 180°, with an image recorded at every step; actually, slightly more than 180° is required to cover [[reciprocal space]], due to the curvature of the [[Ewald sphere]]. However, if the crystal has a higher symmetry, a smaller angular range such as 90° or 45° may be recorded. The rotation axis should be changed at least once, to avoid developing a "blind spot" in reciprocal space close to the rotation axis. It is customary to rock the crystal slightly (by 0.5–2°) to catch a broader region of reciprocal space.{{cn|date=July 2024}}

Multiple data sets may be necessary for certain [[Phase problem|phasing]] methods. For example, [[multi-wavelength anomalous dispersion]] phasing requires that the scattering be recorded at least three (and usually four, for redundancy) wavelengths of the incoming X-ray radiation. A single crystal may degrade too much during the collection of one data set, owing to radiation damage; in such cases, data sets on multiple crystals must be taken.<ref>{{cite journal |vauthors=Ravelli RB, Garman EF |date=October 2006 |title=Radiation damage in macromolecular cryocrystallography |journal=Current Opinion in Structural Biology |volume=16 |issue=5 |pages=624–629 |doi=10.1016/j.sbi.2006.08.001 |pmid=16938450}}</ref>