Optical enrichment of a racemic chiral crystal by X-ray irradiation
GENERATION of optical activity in a racemic mixture of enantiomers, a process relevant to the origin of life and of naturally chiral compounds1, requires chirality in the interaction responsible for the transformation or the environment in which it occurs. Without violating this strict requirement, we have found a way in which optical enrichment of a racemic mixture can occur without any chiral specificity in the mechanism. A racemic mixture of (1-cyanoethyl)(piperidine)cobaloxime, which contains the chiral 1-cyanoethyl group, crystallizes in a chiral space group (P212121) in which R and S enantiomers occupy crystallographically distinct (diastereoisomeric) positions in the asymmetric unit cell. We have prepared single crystals of the D configuration. Irradiating the crystal with X-rays at 343 K causes racemization of the S en-antiomer while the R enantiomer remains unaltered. This racemization is thought to be due solely to the different volume constraints on the two enantiomers in their different crystal environments. The result is that the number of R molecules is increased at the expense of S, and so the overall composition of the crystal changes from racemic to enriched in the R enantiomer.
“Absolute” Asymmetric Polymerization within Crystalline Architectures: Relevance to the Origin of Homochirality
The sections in this article are 
 
 
Introduction 
“Through-Space” Asymmetric Polymerization in Inclusion Complexes and Liquid Crystals 
Isotactic Oligomers Generated within Monolayers at the Air–Water Interface 
“Absolute” Asymmetric Polymerization in 3-D Crystals 
The Planning and Materialization of an “Absolute” Asymmetric Synthesis of Polymers 
Attempted Amplification of Homochirality 
 
 
Generation of Isotactic Oligopeptides via Polymerization in Racemic Crystals 
(RS)–PheNCA 
(RS)-ValNCA and (RS)-LeuNCA 
 
 
Isotactic Oligopeptides from the Polymerization of Racemic ValNCA or LeuNCA in Aqueous Solution 
Racemic β-Sheets in the Polymerization of α-Amino-Acids in Aqueous Solutions: Homochiral Oligopeptides and Copeptides via the “Ehler–Orgel” Reaction 
Isotactic Oligopeptides from Racemic Thioesters of DL-Leu and DL-Val 
Conclusions 
 
 
Keywords: 
 
asymmetric polymerization; 
crystal engineering; 
solid-state chemistry; 
inclusion complexes; 
racemic β-sheets; 
origins of homochirality
Electron spin resonance study of the anaerobic photolysis of alkylcobaloximes; photochemical electron transfer
Abstract The anaerobic irradiation of alkyl(pyridinato)cobaloximes gives in the first instance the photochemical electron transfer reaction from the solvent or from the equatorial ligands for the methyl- and benzyl-(pyridinato)cobaloximes and homolytic cleavage for the isopropyl, isobutyl, n-pentyl and cyclohexyl compounds.