Enantiomorphous crystal

Keywords 2D self-assembly Chiral surfaces Crystallization Enantiomorphism Molecular monolayers... 

Left-hand and Right hand crystals Enantiomorphism... 

Polymer Solutions. Perhaps the most extensively studied macromolecular Hquid crystals are the synthetic polypeptides, such as poly( y-benzyl L-glutamate) [25513-40-0] (PBLG). PBLG is a homopolymer of the L-enantiomorph of a single amino acid with the foUowiag repeat unit. 

Two mols, for example, 270 grams, of racemic a-methylphenethylamine base are reacted with one mol (150 grams) of d-tartaric acid, thereby forming dl-a-methylphenethylamine d-tartrate, a neutral salt. The neutral salt thus obtained is fully dissolved by the addition of sufficient, say about 1 liter, of absolute ethanol, and heating to about the boiling point. The solution is then allowed to cool to room temperature with occasional stirring to effect crystallization. The crystals are filtered off and will be found to contain a preponderance of the levo enantiomorph. 

In literature, SOHNCKE space-group types are often termed chiral space groups , which is not correct. Most chiral molecular compounds do not crystallize in a chiral (enantiomorphic) space group. For details see [86]. 

Another hypothesis on homochirality involves interaction of biomolecules with minerals, either at rock surfaces or at the sea bottom thus, adsorption processes of biomolecules at chiral mineral surfaces have been studied. Klabunovskii and Thiemann (2000) used a large selection of analytical data, provided by other authors, to study whether natural, optically active quartz could have played a role in the emergence of optical activity on the primeval Earth. Some researchers consider it possible that enantioselective adsorption by one of the quartz species (L or D) could have led to the homochirality of biomolecules. Asymmetric adsorption at enantiomor-phic quartz crystals has been detected L-quartz preferentially adsorbs L-alanine. Asymmetrical hydrogenation using d- or L-quartz as active catalysts is also possible. However, if the information in a large number of publications is averaged out, as Klabunovskii and Thiemann could show, there is no clear preference in nature for one of the two enantiomorphic quartz structures. It is possible that rhomobohedral... 

In 1822, the British astronomer Sir John Herschel observed that there was a correlation between hemihedralism and optical rotation. He found that all quartz crystals having the odd faces inclined in one direction rotated the plane of polarized light in one direction, while the enantiomorphous crystals rotate the polarized light in the opposite direction. 

Pasteur thus made the important deduction that the rotation of polarized light caused by different tartaric acid salt crystals was the property of chiral molecules. The (+)- and ( )-tartaric acids were thought to be related as an object to its mirror image in three dimensions. These tartaric acid salts were dissymmetric and enantiomorphous at the molecular level. It was this dissymmetry that provided the power to rotate the polarized light. 

The manual separation of the enantiomorphous crystals of sodium ammonium tartrate tetrahydrate (Figure 1) by Pasteur in 1848 (1) is historically significant, because it laid the foundations of modem stereochemistry. This experiment demonstrated for the first time that certain classes of molecules display enan-tiomorphism even when dissolved in solvent. These observations eventually paved the way for the inspired suggestion, made more than two decades later, by van t Hoff (2) and Le Bel (3), of a tetrahedral arrangement of bonds around the carbon atom. 

Figure I. Enantiomorphous crystals of sodium ammonium tartrate -4H20.

Figure 2. Two enantiomorphous sets of hands arranged in a lattice (a) right hands forming an R crystal (b) left hands forming an S crystal.

Crystals composed of the R and S enantiomers of the same racemic mixture must be related by mirror symmetry in terms of both their internal structure and external shape. Enantiomorphous crystals may be sorted visually only if the crystals develop recognizable hemihedral faces. [Opposite (hid) and (hkl) crystal faces are hemihedral if their surface structures are not related to each other by symmetry other than translation, in which case the crystal structure is polar along a vector joining the two faces. Under such circumstances the hemihedral (hkl) and (hkl) faces may not be morphologically equivalent.] It is well known that Pasteur s discovery of enantiomorphism through die asymmetric shape of die crystals of racemic sodium ammonium tartrate was due in part to a confluence of favorable circumstances. In the cold climate of Paris, Pasteur obtained crystals in the form of conglomerates. These crystals were large and exhibited easily seen hemihedral faces. In contrast, at temperatures above 27°C sodium ammonium tartrate forms a racemic compound. 

The etching of an enantiomorphous pair of asn crystals in the presence of (/ )-aspartic acid and () )-A-methylasparagine is illustrated in Figure 12. Under these conditions, only the R crystals are etched (Figure 12a), whereas the S crystals dissolve smoothly (Figure 12b). The dramatic differences in the surfaces of the two enantiomorphous crystals after dissolution again make it possible to perform a manual Pasteur-type sorting of the R and S crystals with a quantitative enantiomeric yield. 

We illustrated in Section II why conventional X-ray diffraction cannot distinguish between enantiomorphous crystal structures. It has not been generally appreciated that, in contrast to the situation for chiral crystals, the orientations of the constituent molecules in centrosymmetric crystals may be unambiguously assigned with respect to the crystal axes. Thus, in principle, absolute configuration can be assigned to chiral molecules in centrosymmetric crystals. The problem, however, is how to use this information which is lost once the crystal is dissolved. 

S)-Serine forms tabular crystals with point symmetry 2hn (Figure 22a) the crystals affected by either (/ )- or (S)-thr exhibit reduced morphological symmetry 2 (the mirror plane is lost) and are enantiomorphous (Figure 22b, c). When (R,S)-thr is used as the additive, the morphological symmetry 2/m is left unchanged because the effects induced by each additive separately combine. The crystals turn into rhombs, with a clear increase in the areas of the 011 side faces relative to those of the pure crystals (Figure 22d) (45, 78). 

Single crystals of ferroelectric TGS grown from solution generally contain domains of the two enantiomorphous phases (110). This juxtaposition of enantiomorphous domains may be explained in terms of the minor deviations of the crystal structure from a centrosymmetric arrangement. The relative concentrations of the two enantiomorphous phases in a single crystal may be determined by means of anomalous dispersion of X-rays (111,112). 

Lock (113) found that TGS crystals grown in the presence of resolved alanine are unipoled that is, they contain only one enantiomorphous phase. Thus it should be possible to assign the absolute configuration of the occluded additive by measuring the electric dipole of the crystal, provided that one can predict which enantiomorph of TGS is induced by the resolved additive. 

A solution or melt of a racemic mixture of enantiomers may crystallize either as a racemic phase or as a mixture of two resolved enantiomorphic phases. The molecules in these two enantiomorphic phases will be exact mirror images of one another. However, a given enantiomer, say R, will have different environments in the racemate and in the resolved crystal and will be conformationally different. Correspondingly, the R molecule in the resolved crystal and the S molecule in the racemate will not be exact mirror images. 

Finally, one should be cautioned that, occasionally, substances form chiral single crystals of nearly racemic composition. For example, hexahelicene crystals grown from racemic solutions apparently undergo spontaneous resolution, displaying the enantiomorphic space group P2[2,2, however, the e.e. in the crystal is only —2%. This material (and probably others as well) has a lamellar, twinned structure in which alternating layers, 20 p,m thick, of optically pure (/ )-( + )-and (M)-( — )-hexahelicene are perfectly aligned to build up the observed crystal (266). 

---