Acta Cryst. (2000). B56, 715–719 Dalhus and Go ¨ rbitz  Non-centrosymmetric racemates 715 research papers Acta Crystallographica Section B Structural Science ISSN 0108-7681 Non-centrosymmetric racemates: space-group frequencies and conformational similarities between crystallographically independent mole- cules Bjørn Dalhus* and Carl Henrik Go ¨rbitz Department of Chemistry, University of Oslo, PO Box 1033, Blindern, N-0315 Oslo, Norway Correspondence e-mail: bjornda@kjemi.uio.no # 2000 International Union of Crystallography Printed in Great Britain – all rights reserved dl-Allylglycine (dl-2-amino-4-pentenoic acid, C 5 H 9 NO 2 ) yields crystals with Pca2 1 symmetry and two crystallographi- cally independent yet pseudo-inversion-related enantiomers. The distribution among the common space groups of other crystalline racemates with more than one molecule in the asymmetric unit has been established. The conformational similarities between crystallographically independent enan- tiomers in 114 non-centrosymmetric racemates were quanti- fied using the r.m.s. deviation for a molecular superposition. The analysis shows that in the majority of crystals the conformations of the crystallographically independent mole- cules are very similar with mean r.m.s. deviation = 0.190 A ˚ . In almost 80% of the structures the mean r.m.s. deviations is in the interval 0–0.2 A ˚ . It is estimated that racemates constitute 23% of the centrosymmetric organic structures in the Cambridge Structural Database. Received 8 November 1999 Accepted 7 February 2000 1. Introduction The structure of dl-allylglycine (dl-2-amino-4-pentenoic acid) was determined as part of an ongoing programme to provide accurate H-atom positions in crystal structures of hydrophobic amino acids (Dalhus & Go ¨ rbitz, 1999, and references therein). Intuitively, we had expected this racemate to crystallize in a space group with a center of symmetry, but surprisingly the space group proved to be non-centrosym- metric (Pca2 1 ). To our knowledge, no systematic investigation dealing with this phenomenon has been presented previously. We thus decided to retrieve the relevant data from the Cambridge Structural Database (CSD; Allen & Kennard, 1993) to establish the frequency with which racemates crys- tallize in non-centrosymmetric space groups, and also the degree of similarity between the two enantiomeric molecules in the asymmetric unit. 2. Experimental Racemic dl-allylglycine was obtained from Sigma, and an aqueous solution was mixed with tetramethoxysilane in the ratio 10:1 (volume/volume) and left for some minutes to polymerize. Relatively large crystals appeared as ethanol diffused into the gel at room temperature. The structure was solved and refined with SHELXTL (Sheldrick, 1994). All non-H atoms were refined aniso- tropically. Amino H atoms were refined isotropically, while other H atoms were placed geometrically and refined with constraints to keep all C—H distances and all C—C—H angles on one C atom the same. Further, isotropic displacement parameters for the H atoms were fixed at 1.2U eq of the bonded