Homochirality in nature

It has already been mentioned that the origin of homochirality in nature can be viewed in terms of the controversy between determinism and contingency. Is the L form of amino acids determined by some physical law of nature or is it a matter of chance in chemical evolution ... 

Spontaneous absolute asymmetric synthesis, i.e., the formation of an optically active compound without the use of chiral materials, has been proposed as one of the origins of biological homochirality in nature [14,15]. Spontaneous... 

AMPLIFICATION OF SMALL CPL-INDUCED ENANTIOMERIC EXCESSES AND CPL AS A SOURCE OF HOMOCHIRALITY IN NATURE... 

So far no system has been found that e.g. amplifies the ees of amino acids created in cpl-photolysis that might explain the homochirality in nature. 

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. 

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... 

Homochirality is the choice in Nature for one handedness or another. Macroscopic homochirality can be observed in the winding of some species of snail shell... 

To this point, all the examples presented have been ones in which the origin of the asymmetric induction has been unimolecular in nature, that is, the molecules adopt homochiral conformations in the solid state that favor the formation of one enantiomer over the other, usually through the close intramolecular approach of reactive centers bimolecular crystal packing effects appear to play little or no role in governing the stereochemical outcome of such reactions. This raises the interesting question of whether the soUd-state ionic chiral auxiUary approach to asymmetric synthesis could be made to work for conformationally unbiased reactants, i.e., those possessing symmetrical, conformationally locked structures. Two such cases are presented and discussed below. 

In this chapter the question of homochirality has also been considered according to Meir Lahav breaking of symmetry is not the problem. I do not know how many scientists would agree with him, but it is certainly true that in the laboratory chiral compounds can be obtained starting from racemic mixtures -and this by simple means, without invoking subtle effects of parity violation. Of course we do not know how homochirality really evolved in nature however, it is comforting to know that there is in principle an experimental solution to the problem. 

In summary, the origin of the chiral amplification is basically the difference in stability of the homochiral and heterochiral dinuclear Zn complexes. These complexes act as catalyst precursors, but differences in their kinetic behavior also affect the degree of the nonlinear effect. This investigation is probably the first example of elucidation of a molecular mechanism of catalytic chiral amplification (41) and may provide a chemical model of one means of propagation of chirality in nature. 

RNA is a homochiral polymer. It occurs in nature only in the d configuration. We investigated how the chiral 49nt catalyst affects the stereochemistry of the reaction by analysis of the products by chiral HPLC (Figure 5.3.6). An enantiomeric excess (ee) of >95% was observed for the catalyzed reaction between anthracene-hexaethyleneglycol and N-pentylmaleimide. 

This synthesis of 1 is noteworthy for its seminal demonstration of the utility of tandem electrophilic hydrazination-nucleophilic cyclisation9 for preparing functionalised homochiral cyclic a hydrazino acids.10 It highlights, yet again, the great utility of functionalised glyceraldehyde synthons for the installation of remote hydroxy stereocentres in natural product target molecules. 

The easiest combination of two chiral guests in the cylindrical capsule comes by the double encapsulation of racemic species with adequate size, shape and polarity to be accommodated in pairs, giving two diastereoisomeric complexes, the homochiral couple (R) - (R)/(S) - (S) and the heterochiral (R) -(S) combination. ( )-trans-1,2-Cyclohcxancdiol has all the above requisites and showed a 1.2 ratio between the two complexes in favor of the heterochiral combination [60]. This observation may be related to the preference in nature for centrosymmetric crystals or, alternatively stated, the higher melting points of racemates vs. enantiopure compounds where the resolution is driven by the less soluble pair [61,62], In the cylindrical capsule a single couple of chiral molecules is extrapolated from the bulk and the interactions between the two is governed by the shape of the cavity and their goodness of fit within the cavity. 

The question of the (nearly) exclusive use of l amino acids and d sugars in living organisms is one of the unsolved enigmas of the chemical sciences. As there are nonracemic amino acids found in meteorites [143,144] and as cpl occurs in outer space [145] and can be produced under natural terrestrial environmental conditions, cpl photolysis is discussed as a source of homochirality (up to scenarios like the synchronization of terrestrial cpl day cycles with the periodicity of tides [146]). The question of amino acid homochirality being due to asymmetric photolysis in space is discussed by Cerf and Jorisson [147], who summarize, In conclusion, we hope to have convinced the reader that the role of extraterrestrial asymmetric photolysis in the origin of the homochirality of natural amino acids on Earth, if at all involved, is far more complicated than is usually apprehended in the astronomical literature. The same authors more recently gave a critical review of the ample literature on asymmetric photochemistry as a possible source of homochirality [148]. 

Asymmetric photochemistry is intensively discussed as one of the possibilities for the origin of the observed homochirality of amino acids and saccharides in biological systems. There is no doubt that also in natural environments cpl exists and may create an enantiomeric excess in a photoreaction, but according to all knowledge, there must be an amplification mechanism, which has not yet been safely detected. 

A characteristic hallmark of life is believed to be its homochirality .36 In general, it is true, although natural products are not always enantiomerically pure.37 The origin of biomolecular homochirality is discussed in depth by MacDermott.36 Those who are interested to see whether the parity-violating weak force is the cosmic dissymmetry that Pasteur was looking for should read her chapter in the book entitled Chirality in Natural and Applied Sciencd. 

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