Section 13.1.4.3. Structure determination

Table 13.1.4.1 distinguishes a number of different situations in which noncrystallographic symmetry can be used to aid structure determination. The most frequent application of molecular-replacement methods is to cases where a structure is partially known, but is not yet susceptible to refinement by standard techniques.

Two types of situation arise in the standard case, where noncrystallographically related subunits exist in the same crystal. Most frequently [type (2)], the noncrystallographic symmetry allows the electron density to be improved at the given resolution. Occasionally, high-order noncrystallographic symmetry may be used to extend the resolution to the point where conventional structural refinement becomes possible (Schevitz et al., 1981; McKenna et al., 1992). In the most favourable case, high-order noncrystallographic symmetry constraints may allow direct structure determination [type (1)], starting from the position of a symmetric particle in the asymmetric unit (Jack, 1973).

In the generalized case, most often, similarities with a known molecular structure can be employed to improve an unknown structure [types (5) and (6)]. Such techniques were first used by Tollin (1969) (before structural refinement was possible) and by Fehlhammer & Bode (1975).

It is also possible that a refinable structure could be generated from intensity data observed from several different crystal forms, using noncrystallographic symmetry constraints, but this is not known to have been done in practice [types (3) and (4)].