Two-Dimensional Crystallization of Biological Macromolecules - Molecules: Nucleation, Aggregation and Crystallization - page 103

(B)

Fig. 2. ( Continued )

Objects in space forming 3D crystals or in a pure plane without any
extension in the third dimension can be arranged with 230 space-group
and 17 projection-group symmetries, respectively. Biological macromol-
ecules in a 2D crystal on the other hand will form one of 17 two-sided
plane groups (Holser, 1958). Unlike pure 2D objects they may have sym-
metry operations in the plane of the membrane in such a way that one
molecule may be facing upwards while its neighbor is in the opposite
direction. In fact, this is very common for reconstituted membrane pro-
teins. Obviously, it does not form a viable system for a membrane protein
having vectorial transport as its main function but for 2D crystallization it
may be an advantage since the layer will be symmetrical and less prone to
bending. Flatness is often a problem for membrane proteins crystallized in
their native membrane.

Size

The size of the coherent crystalline area will directly influence the signal-
to-noise ratio in diffraction data either as calculated from images or as
acquired directly in electron diffraction patterns. In fact, diffraction spots