Quartz chiral crystals

Fig. 1 Quartz minerals consisting of covalently linked silicon-oxide tetrahedra that are corner-connected and twisted in a helical fashion around three- and six-fold screw axes, rendering individual quartz crystals chiral...

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 the first half of the nineteenth century, it was known that certain minerals, the prime example being quartz, formed chiral crystals. Often, it was seen that rocks could be composed of a physical mixture of small but macroscopic right-handed and left-handed crystals. This kind of mixture, composed of macroscopic chiral domains (crystals) occurring in both enantiomeric forms, was termed a conglomerate. 

In recent years, stereochemistry, dealing with the three-dimensional behavior of chiral molecules, has become a significant area of research in modern organic chemistry. The development of stereochemistry can, however, be traced as far back as the nineteenth century. In 1801, the French mineralogist Haiiy noticed that quartz crystals exhibited hemihedral phenomena, which implied that certain facets of the crystals were disposed as nonsuperimposable species showing a typical relationship between an object and its mirror image. In 1809, the French physicist Malus, who also studied quartz crystals, observed that they could induce the polarization of light. 

The observed distribution of chirality among quartz crystals within the Earth is extremely dose to 50 50, as would be expeded from a randomly sdeded sample of... 

Figure 2-36. Illustrations of chiral pairs, (a) Decorations (in Bern, Switzerland, photograph by the authors) whose motifs of fourfold rotational symmetry are each other s mirror images (b) Quartz crystals (c) J. S. Bach, Die Kunst der Fuge, Contrapunctus XVIII, detail (d) Legs (detail of Kay Worden s sculpture, Wave, in Newport, Rhode Island), (photograph by the authors) (e) A molecule and its mirror image in which a carbon atom is surrounded by four different atoms, for example, CHFClBr.

An assembly of molecules may be achiral for one of two reasons. Either all the molecules present are achiral, or the two kinds of enantiomorphs are present in equal amounts. Chemical reactions between achiral molecules lead to achiral products. Either all product molecules will be achiral or the two kinds of chiral molecules will be produced in equal amounts. Chiral crystals may sometimes be obtained from achiral solutions. When this happens, the two enantiomorphs will be obtained in (roughly) equal numbers, as was observed by Pasteur. Quartz crystals are an inorganic example of chirality (Figure 2-36b). Roughly equal numbers of left-handed and right-handed crystals are obtained from the achiral silica melt. 

Generally, optical activity is observed when electrons are displaced along chiral paths by an applied electric field. This does not necessarily require the involvement of dissymmetric molecules. Quartz crystals, in which the first observation < of optical activity was achieved, have no chiral molecules or ions, and their optical activity arises simply from the helical placement of atoms in the crystal. Likewise, a crystal composed of achiral untwistable molecules may become optically active if there are strong helical interactions between neighboring molecules. 

The symmetry of the model of a molecule or of a molecular ensemble depends on the conditions of the relevant physical (or chemical) measurement, and may vary for the same system according to time scale of observation and instrumental sensitivity. Whether the model of a chemical system is chiral or achiral may therefore depend on the conditions of observation. There is no ambiguity when chirality properties are observed the hemihedrality of quartz crystals, the optical rotation of hexahelicene, and the enantiospecificity of hog-kidney acylase, for example, are all unmistakable manifestations of an underlying structural chirality. On the other hand, achirality is not so simply implied by the absence of such observations. 

Chapter 1 considers the possible relationships of earthly clays and other minerals to the origin of chirality in organic molecules. Attempts to establish experimental evidence of asymmetric adsorption on clays were unsuccessfiil, but die search for chirality did find naturally occurring enantiomorphic crystals like quartz. Asymmetric adsorption of organic molecules on quartz crystals such as separation of racemic mixtures, like Co or Cr complexes, alcohols and other compounds, allowed for the conclusion that quartz crystals can serve as possible sources of chirality but not of homochirality. This latter conclusion results fi om the finding that all studied locations of quartz crystals contain equal amounts of d- and /-forms. The preparations of synthetic adsorbents such as imprinting silica gels are also considered. More than 130 references are analyzed. 

There are many natural minerals and salts that posses optical activity in their crystalline state owing to their chiral lattices, sueh as quartz, einnabar, mica, chlorates, bromates, and iodates. Crystal chirality of other minerals, like aluminosilicates, such as zeolites, were not investigated, but these minerals are considered by many investigators as possible sourees of chirality and the origin of homochirality in our biosphere The optieal activities of clays have not revealed reliable evidence of chirality and therefore they do not play any positive role in our understanding of the origin or of the amplification of homochirality in nature. 

Unlike the clays, owing to its chiral lattice, quartz must display certain selectivities for the adsorption of mainly neutral organic compounds possessing molecular or crystal-like chirality. Glucose, galactose and arabinose selectively adsorb on quartz crystals and quartz dissolves in solutions of these monosaccharides Racemic acids seem not to be resolved on quartz however, it has been shown that d- or /-quartz exert an orienting influence on the epitaxy of hemihedral crystals of Glu, Ala, and Gly. For example, the influence is seen for Glu, Ala, and Gly adsorbed on the surface of quartz crystals 1010, and of Ala on /-quartz 1011. These crystals are like the asymmetric epitaxy of crystals of (+) camphor on biotite and calcite... 

This chapter summarizes data about the application of chiral metal catalysts supported on optically active quartz crystals in hydrogenation and other reactions. Despite the low enantioselective efficiency of these catalysts, recent result show that almost 100% enantioselectivity results when they are involved in autocatalytic processes. 

The asymmetric adsorption of some organic and complex compounds on quartz crystals was described in Chapter 1.2. In 1932 Schwab and cowoikers were first to show that chiral quartz crystals can be used as chiral supports for metal catalysts Seven years later Stankiewicz, in his dissertation... 

Schwab and Rudolph " prepared chiral catalysts by supporting the metals on the surfaces of ferreted fine powdered optically active quartz crystals, which proved to be active during as3nnmetric dehydrogenation and dehydration of racemic butan-2-ol. The dehydration-dehydrogenation reactions of butan-2-ol (Scheme 2.1.) were carried out in the vapor phase at... 

Another complicating factor is that the surface of quartz is the only asymmetric factor in the metal-quartz catalysis and its specific areas in most cases were very small (only 44 cm /g " ). This coupled with the fact that the amount of metal deposited on quartz was rather high, so the extent of racemization of butanol during reaction would be high, which detracts from the effectiveness of the catalyst. Thus, quartz appeared not to be an effective chiral carrier for catalysis or adsorption in asymmetric experiments. Nevertheless, in general the data using quartz crystals are of interest and received positive evaluations in several publications... 

The mechanism of conversion of an achiral molecule into a chiral one was found by Kuhn in circular dichroism spectra. The type of natural quartz used, amethyst crystals, contained colloidal inclusions of Fe. The circular dichroism spectra revealed bonds involving Fe. It would be of importance from that point of view if the films of metals removed from surfaces of quartz crystal would show circular dichroism spectra, too. 

Recently it has been shown that optically active quartz crystals as asymmetric inductors become very effective in autocatalytic enantioselective reactions. Soai et al. have shown that in asymmetric autocatalysis, the action of small amounts of chiral reaction products (involved in the reaction cycle) may enhance the enantioselective excess by a factor of 94 after introduction of an intermediate into the reaction. Optically active synthetic quartz crystals were used in this reaction with ratios of 1 1.9 quartz to aldehyde and 1 2.2 quartz to diisopropyl-zinc. 

Thiemann showed that this calculation of PVED for quartz crystal seems to lack a soimd physical basis because in all examined quartz locations the amoimts of d- and /-quartz crystals are equal. Hence the quartz crystals are erroneously considered as a possible source of one handedness in nature, although local formation of optically active molecules, for example, via autocatalytic processes, appears quite possible in revealing chirality rather than homochirality (Klabunovskii ). 

An accidental external chiral influence of a one-time evolutionary step selects in a preferred manner one enantiomer. Pasteur and later van t Hoff considered such possibilities, and since that time there have been innumerable different proposals of this type. As an example, we mention the start of an evolution on a random chiral matrix, for example, a left-quartz (L-quartz) crystal [75]. When a favored enantiomer is formed, it could propagate itself and then remain dominant [76]. A currently popular possibility is the generation of an excess of one enantiomer in an interstellar gas cloud through polarized light. This excess could be then carried by meteorites to the early Earth and would provide favorable starting conditions for one type of enantiomer. The observation of an excess of enantiomers of chiral biological precursor molecules in meteorites has persuaded many to favor this hypothesis [77]. 

Why Nature uses only one enantiomer of most important biochemicals is an easier question to answer than how this asymmetry came about in the first place, or why L-amino acids and D-sugars were the favoured enantiomers, since, for example, proteins made out of racemic samples of amino acids would be complicated by the possibility of enormous numbers of diastereomers. Some have suggested that life arose on the surface of single chiral quartz crystals, which provided the asymmetric environment needed to make life s molecules enantiomerically pure. Or perhaps the asymmetry present in the spin of electrons released as gamma rays acted as a source of molecular asymmetry. Given that enantiomerically pure living systems should be simpler than racemic ones, maybe it was just chance that the L-amino acids and the D-sugars won out. 

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