Group—subgroup relations as an aid for structure determination

Abstract

If X-ray diffraction data do not allow a unique assignment of the space group, but the structure of a similar compound is known, group-subgroup relations can help to decide which space group is correct. If the crystal structure of a protein is known and the space group of crystals of another protein is a subgroup or supergroup thereof, the relation can be used to solve the crystal structure. If superstructure reflections are present, the correct space group is a klassengleiche subgroup of the space group without the superstructure reflections. Suspicious parameters of ‘thermal motion’ indicate that the chosen space group may be wrong and a subgroup should be chosen. The initially wrong choice often is a consequence of a twinned crystal. Often, the correct space group then is a subgroup of the initially chosen space group, but it may also be a supergroup.