Homochirality origin

Chirality at surfaces can be manifested in a number of forms including the intrinsic chirality of the surface structure and even the induction of chirality via the adsorption of achiral molecules onto achiral surfaces. The ability of STM to probe surfaces on a local scale with atomic/molecular resolution has revolutionized the understanding of these phenomena. Surfaces that are globally chiral either due to their intrinsic structure or due to the adsorption of chiral molecules have been shown by STM to establish control over the adsorption behavior of prochiral species. This could have profound consequences for the understanding of the origin of homochirality in life on Earth and in the development of new generations of heterogeneous chiral catalysts that may, finally, make a substantial impact on the pharmaceutical industry. 

In accordance with the autocatalytic process, matrices are again formed. It is surprising that the autocatalysis decreases when only 1 of the 15 building blocks of the peptide has the opposite handedness, e.g., when the N-peptide fragment contains one D-amino acid as well as the 14 L-amino acids. These experimental results show that such a system is able to form homochiral products via self-replication. It can be assumed that similar mechanisms influenced the origin of homochirality on Earth (Saghatelian et al., 2001 Siegel, 2001). 

The question also arises as to where the chiral molecules came from. Were the L-amino acids or the D-sugars selected on the primeval Earth, or are exuaterresuial sources responsible for the homochirality This second possibility is dealt with by hypotheses on the effect of circularly polarised light, of extraterrestrial origin, on chiral molecules in the molecular clouds from which the solar system was formed. One such hypothesis was proposed by Rubenstein et al. (1983) and developed further by others, particularly A. W. Bonner (Bonner and Rubenstein, 1987) both scientists worked at Stanford University. The authors believe that the actual radiation source was synchrotron radiation from supernovae. The excess of one enantiomeric form generated by this irradiation process would have needed to be transported to Earth by comets and meteorites, probably during the bombardment phase around 4.2-3.8 billion years ago. 

Tranter, G. E. 1986. Parity-violating Energy Differences and the Origin of Biomolecular Homochirality. J. Theor. Biol. 119, 467-479. 

Ever since the beginning of life on primitive Earth, biopolymers and biomolecules have essentially comprised optically active constituents because of the natural selection of Z-amino acids and tZ-sugars. Although the origin of this biomolecular handedness is a long debated issue among biologists, chemists, physicists, and astronomers,1 5 it is accepted that our life is a consequence of the chemistry of homochiral biosubstances. Deoxyribonucleic acid (DNA) is a classic example of a chiral biopolymer. Its chirality is essentially characterized... 

On One Hand But Not The Other The Challenge of the Origin and Survival of Homochirality in Prebiotic Chemistry... 

What is the origin of biological chirality and how did it successfully propagate itself to dominance The riddle is at once intriguing, central, and profound. It has been persuasively argued and is now widely accepted that homochirality of biopolymers is essential for the existence of life, and that life would be impossible in its absence. [4] Thus the solution to the riddle of terrestrial homochirality may well be intimately related to how life on Earth originated. 

Greenberg, J. M. Chirality in interstellar dust and in comets Life from dead stars in Physical Origin of Homochirality in Life Cline, D. B., Ed. American Institute of Physics Woodbury, NY, 1996, pp 185-210. 

Avetisov, V. A. Spontaneous mirror symmetry breakingvia enantioselective autocatalysis in Physical Origin of Homochirality in Life,... 

MacDermott, A. J. The weak force and SETH The search for extraterrestrial homochirality in Physicd Origin of Homochirdity... 

For instance, LIF experiments, carried out on the heterochiral complex between F and M = (7 )-2-butanol, indicate the presence of three new bands at —136, — 114, and — 73 cm from the band origin of the bare F. Similar experiments on the homochiral complex between F and M = (5)-2-butanol exhibit two new bands at — 125 and — 69 cm from the band origin of the bare F. HB experiments show that four different isomers coexist in the jet, two of them corresponding to the... 

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. 

The expression homochiral in its original meaning concerns isometric molecules of various kinds with the same configuration, for example, the (i,)-aminoacids. There were propositions to use this word in a narrow sense, as the equivalent of enantiopure. For a discussion, see Ref. 6, p. 215. 

We describe highly enantioselective asymmetric autocatalysis with amplification of chirality and asymmetric autocatalysis initiated by chiral triggers. Asymmetric autocatalysis correlates between the origin of chirality and the homochirality of organic compounds. We also describe spontaneous absolute asymmetric synthesis in combination with asymmetric autocatalysis. 

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 ... 

I have mentioned that there are two main aspects to consider on the origin of homochirality, one being the breaking of symmetry, the other the preservation and amplification of this initial inbalance. The latter will be considered further in the next chapter, which deals with macromolecules. 

These arguments suggest that homochirality was most probably already implemented in the early stages of the origin of life, as it would have been very difficult to reach the high order and selectivity of the first self-reproducing cells if all possible diastereomeric macromolecules had been around. 

Such a capability of an oligonucleotide system deserves special attention in the context of the problem of the origin of biomolecular homochirality breaking molecular mirror symmetry by de-racemization is an intrinsic property of such a system whenever the constitutional complexity of the products of co-oligomerization exceeds a critical level. 

It is worthwhile also mentioning that the origin of homochirality has been viewed in terms of a bifurcation scenario (Kondepudi and Prigogine, 1981 Kondepudi et al, 1985). In this case, the homochirality present on Earth would be a product of contingency. 

Bolli, M., Micura, R., and Eschenmoser, A. (1997a). Ryranosyl-RNA chiroselective self-assembly of base sequences by ligative oligomerization of tetranucleotide-2, 3 -cyclophosphates (with a commentary concerning the origin of biomolecular homochirality). Chem. Biol, 4, 309-20. 

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