Minerals chirality

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... 

It should be possible to use the special properties of chiral structures for particular separation problems. According to Belinski and Tencer, one possible way in which nature solved the ribose problem could have involved an enantioselective and diastereoselective purification process acting on a mixture of biomolecules, which left ribose as the only molecule available for further reactions. The authors propose a theoretical mechanism in which a type of chromatographic process occurs at chiral mineral surfaces. This paper is likely to stimulate new experiments as well as the quest for as yet unknown surfaces which can separate racemic carbohydrate mixtures. The question arises, however, as to whether there were minerals present on the young Earth which are now unknown, as they no longer exist on the Earth of today (Belinski and Tencer, 2007). 

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. 

This sort of analysis could be extended to any metal-catalyzed chemistry in which a large runaway chiral excess is induced in the product by way of a small chiral excess of the molecules that serve as ligands to the metal. It is only necessary that the D,L-metal center be kinetically slower and thermodynamically more stable than the d,d- or L,L-complexes in order that any small e.e. of a chiral ligand be translated into chiral dominance of the reaction product. That the initial e.e. resulting in chiral takeover within a reacting system can be induced by asymmetric mineral surfaces indicates that a general chemical route to the asymmetry of life may exist. 

These schemes expressly induded the idea that clays and porous minerals adsorbed, absorbed, and ultimately concentrated any extant organics from a dilute oceanic broth on the early Earth. This idea is critical because it is difficult to imagine that the kind of polymerization and chiral amplification processes discussed above could or would have occurred in a water solution not much more than millimolar in organics. [131]... 

These possibilities rectify the proposed subsequent appearance and amplification of chiral autocatalytic molecules and hypercydes. [190] Any autocatalytic systems would propagate [191] throughout an extensive adjoining hydrated porous network already rich in layered amphiphiles, lipids, polymeric materials, amino acids, thiols, and so forth. In addition, amphiphiles are known to be organized into lipid membranes by interaction with the inner surfaces of porous minerals. [136] It is a small organizational jump from these membranes to frilly formed lipid vesides. 

Artificial hydrothermal vents might be constructed and supplied with plausible concentrations of simple reactants such as CO, H2, NH3, and H2S. Appropriate levels of amino adds induding a small chiral excess, along with the sorts of amphiphilic molecules described above, can be rationalized by the findings from the Murchison meteorite. Organic molecules such as found in irradiated interstellar ice models, including HMT, can also be induded. The system should indude weathered feldspars, which can be modified to indude the reduced transition-metal minerals that they are known to contain. [134] Such minerals as Fe,Ni sulfides are likely to have been both present and stable in the environment of early Earth and are known [153, 155] to catalyze formation of organic molecules from simpler precursors. 

Several methods are available for removal of the 2-pyrrolidinemethanol unit from the products of Birch reduction-alkylation28,29. The most straightforward consists of refluxing with aqueous mineral acid and the chiral auxiliary can be recovered from the aqueous phase. However, under these conditions, one of the remaining double bonds of the alkylation product can undergo protiolactonization. 

When an aldehyde is allowed to react with an optically active amine and hydrocyanic acid, one of the two diastereomeric amino nitriles, (124a) or (124b), may be formed in excess. To prepare the chiral amino acids (125a) or (125b), the nitriles (124a) and (124b), respectively, are hydrolyzed with mineral acids, whereupon R is split off. However, this asymmetric synthesis of amino acids has no industrial significance. 

When a mineral or Lewis acid replaces the carboxylic component in the Passerini reaction, the final products are usually a-hydroxyamides. Also in this case, when chiral carbonyl compounds or isocyanides are employed, the asymmetric induction is, with very few exceptions, scarce [18, 19]. For example, the pyridinium trifluoroacetate-mediated reaction of racemic cyclic ketone 14 with t-butyl isocyanide is reported to afford a single isomer [19] (Scheme 1.7). This example, together with those reported in Schemes 1.3 and 1.4, suggests that high induction may be obtained only by using rigid cyclic or polycyclic substrates. 

The method relies on the p and n salts having different solubilities, and they must not form solid solutions or double salts. The more insoluble salt is filtered and the purified acid recovered by adding mineral acid. This method of chiral resolution is well established, and lists of resolving agents for many classes of racemic compounds are available [22], Inclusion chemistry may be employed for the same purpose by preparing host-guest compounds with a chiral host ... 

Hazen RM (2004) Chiral Crystal Faces of Common Rock-Forming Minerals In Pa-lyi G, Zucchi C (eds) Progress in Biological Chirality chap 9. Elsevier, Oxford, pp 137-151... 

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...

Experiments of this kind suggest that similarly kinked chiral sites present at surfaces of minerals might operate as catalytic centers for asymmetric synthesis. 

The first example of preparation of chiral catalytic sites anchored onto a mineral surface and optically active in hydrogenation reaction was reported by Nagel et al in 1986.15 The supported catalysts were prepared through the covalent fixation of rhodium complex of 3,4-(/ ,/ )-bis(diphenylphosphino)pyrrolidine onto silica surface with different linkers. (Scheme 1). 

Only few reports deal with the use of mineral supports for immobilising chiral auxiliaries able to. activate dialkylzinc addition to aldehyde (Scheme 10). 

Work is in progress in order to understand the mechanism of the mineral surface activity and to improve the enantioselective efficiency of the hybrid chiral materials. 

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