Asymmetric adsorption on quartz crystals

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. 

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... 

Therefore it was sufficient to resolve this complex into enantiomers to determine its spatial structure. This complex does not form diastereomers and the only method of its resolution was asymmetric adsorption on crystals of quartz - a very inert material. In this maimer, a solution of samples of the complex was vigorously shaken with quartz powder, the solution was filtered and its optical rotation measured. On /-quartz the optical rotation of the remaining solutions reached from + 0.05 to + 0.075 and on <7-quartz they reached from -0.075 to -0.13 . Thus the configuration of the complex proved to correspond to the czx-structure. The z/-quartz preferentially adsorbs the (+)-complex and the /-quartz the (-)-complex. Brazilian twin quartz gave negative results. 

It is well known that silica plays an important role in living matter. This was shown by the selective adsorption of some sugars on quartz crystals . Consequently, Schwab and Wahl tried to determine the asymmetric adsorption of silicic acid specimens recovered from the mushroom Euspongia and from the Japanese sponge. However, they did not find any asymmetric adsorption. It turned out that the stmcture of this silicic acid is similar to the structure of silica. It possesses no helical structure and cannot play any role in the formation of optically active compounds in organisms. 

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... 

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... 

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... 

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