Space-group frequencies of proteins and of organic compounds with more than one formula unit in the asymmetric unit

An analysis of the distribution of organic crystal structures with more than one formula unit in the asymmetric unit among the 230 space groups has been carried out for the compounds listed in the Cambridge Structural Database. 8.3% of the total number of compounds (51 611) in the database have more than one formula unit in the asymmetric unit; 81% of these are reported in only five space groups: P2₁/c (27.8%), P (23.5%), P2₁ (13.8%), P1 (8.5%) and P2₁2₁2₁(7.8%). When all compounds are considered, the first five most populous space groups are: P2₁/c (36.6%), P(16.9%), P2₁2₁2₁ (11.0%), C2/c (7.0%) and P2₁ (6.4%). The distribution of compounds among the seven crystal systems is also reported. The frequencies indicate a preferential occurrence of lower-symmetry space groups for structures with more than one formula unit in the asymmetric unit. Space-group frequencies for protein crystal structures from the Protein Data Bank, Brookhaven, have been calculated and are compared with those of chiral small molecules.