Dear Chen, Twinning can be thought of as of two or more macro-crystals glued or grown together. The reason that the reflections often overlap is that they share one common plane from which they grow in different directions. Many twinning tests are based on the fact that the two (or more) macro crystals do not interfere, which changes the intensity distributions. Since there is no interference, twinning cannot make spots disappear. Moreover, translational operations between twin domains would be equivalent to move the crystal a little in the beam, as with centering, which will not have any influence on the diffraction pattern (except for weak diffraction because of missing the beam). Disorder can have many causes, but is caused by different orientations of residues/molecules/whatever in different asymmetric units. It is close range, so there will be interference. However, since it is usually randomly distributed over the crystal, it will not cause disappearance of spots.
The X-ray coherent length is depending on the crystal, not the synchrotron and my gut feeling is that it is at least several hundred unit cells, but here other experts may correct me. Disappearance of spots can occur due to a wrong space group assignment (e.g. screw axis have been overlooked) or translational non-crystallographic symmetry. In this case, I would first run a modern MR program to see if you get a solution and otherwise you will have to analyze very careful your space group, unit cell etc. to find out what is going on. My 2 cents, Herman Von: CCP4 bulletin board [mailto:[email protected]] Im Auftrag von Chen Zhao Gesendet: Donnerstag, 24. April 2014 22:13 An: [email protected] Betreff: [ccp4bb] Twinning VS. Disorder Dear all, Hello! I am kinda confused and am thinking about the definition of twinning and disorder. I am just a starting student and might make some fundamental mistakes. 1) Twinning is a macroscopic phenomenon and the result is the addition of the intensity from different lattices; disorder is a microscopic phenomenon and the result is the addition of structure factors from different crystal "domains". Is this statement valid? 2) I am now very confused about how to define the macroscopic versus the microscopic level when I think of the systematic absences introduced by translational operation. Or in other words, can the translational operation between the twin domains create systematic absences? My answer is probably no because the distance between the two domains are too far away compared to the coherent length of the X-ray, i.e. the addition of the intensity alone cannot make some spots disappear. Is it true? If yes, what is the x-ray coherence length at the synchrotron in general? 3) If the statement in 2) is valid, then if a "twinning operation" can introduce systematic absences, this should be a disorder instead of a twin based on the definitions in 1). Is this right? Your answers will be greatly appreciated! Sincerely, Chen
