Structural relationships in crystals accommodating different stereoisomers of 2-amino-3-methylpentanoic acid.
A reinvestigation of the crystal structure of the 1:1 mixture of the two racemates DL-isoleucine and DL-allo-isoleucine, with a detailed analysis of interatomic distances between alternative side-chain positions, reveals a systematic distribution of the four stereoisomers in this crystal. Two different molecular chains exist in the crystal and each such chain accommodates a single diastereomeric pair only (L-isoleucine:D-allo-isoleucine or D-isoleucine:L-allo-isoleucine). The crystal is built up by a stacking of such chains in two dimensions and three different packing modes for the two types of chains are discussed. Crystallization experiments of the two individual racemates in the 1:1 mixture of DL-isoleucine:DL-allo-isoleucine have been undertaken. The structure of the racemate DL-isoleucine is presented. The molecular arrangements in this racemate and the 1:1 DL-isoleucine:DL-allo-isoleucine mixture are closely related. Furthermore, the spontaneous resolution of enantiomers upon crystallization of the other racemate, DL-allo-isoleucine, is rationalized on the basis of the aforementioned analysis of interatomic distances in the 1:1 DL-isoleucine:DL-allo-isoleucine complex. Structural data for a new L-isoleucine: D-allo-isoleucine complex are also given.
Dalhus
B
B
Department of Chemistry, University of Oslo, PO Box 1033, Blindern, N-0315 Oslo, Norway. bjornda@kjemi.uio.no
Gorbitz
C H
CH
D003460:Crystallization; D018360:Crystallography, X-Ray; D007532:Isoleucine / Q000031:analogs & derivatives* / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D013237:Stereoisomerism
C516826:2-amino-3-methylpentanoic acid; D007532:Isoleucine