The Origin of Chirality in Nature

The molecules of life are for the most part chiral, and in living systems they are almost always enantiomerically pure. In addition, groups of biomolecules are generally homochiral —all amino acids have the same sense of chirality and all sugars have the same sense of chirality. As already discussed, the chirality of the amino acids leads to chiral enzymes, which in turn produce chiral natural products. All the chiral compounds found in nature that are readily accessible to synthetic chemists for the construction of more complex molecules are referred to as the chiral pool. 

What is the origin of the chirality of the molecules of life, and the reason for the homochirality We cannot distinguish enantiomers unless we have a chiral environment. Further, in a reaction that forms a stereocenter, we cannot create an excess of one enantiomer over another without some chirality to start with. In the laboratory today, all enantiomeric excesses that we exploit ultimately derive from natural materials. Whether it is the interaction with an enantiomerically pure amino acid from a natural source, or an individual manually separating enantiomorphous crystals (first achieved by Pasteur), the source of enantiomeric excess in the modern chemistry laboratory is always a living system. But how was this achieved in the absence of life This is a fascinating, complex, and controversial topic that we can touch on only briefly here. This question is often phrased as the quest for the origin of chirality in nature, but more correctly it is the origin of enantiomeric excess and homochirality we seek. 

The alternative type of model emphasizes the possible role of an inherently chiral bias of 

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Abstract Understanding the origin of chirality in nature has been an active area of research since the time of Pasteur. In this chapter we examine one possible route by which this asymmetry could have arisen, namely chiral-specific chemistry induced by spin-polarized electrons. The various sources of spin-polarized electrons (parity violation, photoemission, and secondary processes) are discussed. Experiments aimed at exploring these interactions are reviewed starting with those based on the Vester-Ulbricht hypothesis through recent studies of spin polarized secondary electrons from a magnetic substrate. We will conclude with a discussion of possible new avenues of research that could impact this area. 

What is the origin of chirality in nature Most biomolecules can be synthesized in mirror-image shapes. Yet in organisms, amino acids are always left-handed, and sugars are always right handed. The origins of this preference remain a mystery. 

Another aspect of NLE is asymmetric autocatalysis as an event following symmetry breaking in nature. On the origin of chirality in nature, two major mechanisms have been proposed [28]. (1) Chance mechanism To generate an optically... 

Terminology aside, the reaction of a chemical sample composed of only achiral molecules (such as 2-butanone) cannot give rise to products with any chiral bias (i.e., any enantiomeric excess) without the intervention of an external chiral influence. This observation has significant implications for discussions of such topics as the origin of chirality in natural systems (see Section 6.8.3). 

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. 

Youatt J.B. and Brown R.D. (1981) Origin of chirality in Nature. A reassessment of the postdated role of bentonite, Sci., 212, 1145-1146. Bormer W.A. (1991) The origin and amplification of biomolecular chirality. 

Ireland and Scheuer s experiments demonstrated that the in vivo photochemical transformation of 9,10-deoxytiidachione (2) into photodeoxytridachione (3) was non-enzymatic [17], suggesting that the origin of chirality in sacoglossan natural products, derived from achiral polyene precursors, does not lie in enzymatic assistance. 

In place of the above-mentioned chiral organic compounds, chiral inorganic substrates have been examined as chiral initiators. Quartz (Si02) exhibits both dextrorotatory (d) and levorotatory (Z) enantiomorphs that exist in nature. Quartz is considered as one of the origins of chirality of organic compounds186. 

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. 

Olefin polymerization using heterogeneous catalysts is a very important reaction and stereochemical aspects have been studied extensively. For a review on this topic see Pino et al. [9], Briefly, the origin of stereoregularity in polyolefins (47) is explained by the chiral nature of the acdve site during polymerization. If the absolute configuration of the first intermediate can be controlled by chiral premodification then we should obtain a non-racemic mixture of R - and "S"-chains. This has indeed been observed e.g. with catalyst M4 for the polymerization (partial kinetic resolution) of racemic 3,7-dimethyl-l-octene (ee 37%) and also for the racemic monomer 46 using Cd-tartate M5. 

The reaction of methallyltri-n-butylstannane 117 with achiral aldehydes is also effectively promoted by the binol-Ti complex [89 c]. In all but one case (cyclo-hexanecarboxaldehyde), the yields and enantioselectivities observed with the methallylstannane are identical or higher than those obtained in the reactions with allyltributylstannane with only 10 mol% of the binol-Ti complex (Scheme 10-50). Insight into the nature of the titanium catalyst is provided by the observation of asymmetric amplification [89 b] and chiral poisoning [89 g]. An intruiging hypothesis on the origin of enantioselection in allylation and related reactions [89 h]. 

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. 

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. 

These results have a threefold significance first, as a practical method of amplifying small chiral effects in organic reactions, second, as a possible way to discover and assign the chirality of substances, such as natural minerals, that can be used as chiral catalysts (or carriers), and third, as a way to search for the possibility of the origin of chirality on Earth, where enantiomorphic minerals like quartz can exist (see Klabunovskii... 

Tranter G.E. (1985) Parity violation energy difference of chiral minerals and the origin of biomolecular homochirality. Nature 318, 1712-1713. McDermott A.J. (1996) The weak force and SETT the search for extra-terres-trial homochirality. Physical origin of homochirality in life, Amer. Inst. Phys. Conf, Santa Monica CA, Febr. 1995, Proceedings, Cline D.B. (ed.). Press Woodbury, N.Y., p. 241-254. 

The origin of optical activity in molecules often reduces to the question of how the molecule acquires the electronic properties expected of a chiral object when it is formed from an achiral object. Most often an achiral molecule becomes chiral by chemical substitution. In coordination compounds, chirality commonly arises by the assembly of achiral units. So it is natural to develop ideas on the origins of chiral spectroscopic properties from the interactions of chirally disposed, but intrinsically achiral, units. Where this approach, an example of the independent systems model, can be used, it has obvious economic benefits. Exceptions will occur with strongly interacting subunits, e.g., twisted metal-metal-bonded systems, and in these cases the system must be treated as a whole—as an intrinsically chiral chromophore. ... 

From the atomic to the macroscopic level chirality is a characteristic feature of biological systems and plays an important role in the interplay of structure and function. Originating from small chiral precursors complex macromolecules such as proteins or DNA have developed during evolution. On a supramolecular level chirality is expressed in molecular organization, e.g. in the secondary and tertiary structure of proteins, in membranes, cells or tissues. On a macroscopic level, it appears in the chirality of our hands or in the asymmetric arrangement of our organs, or in the helicity of snail shells. Nature usually displays a preference for one sense of chirality over the other. This leads to specific interactions called chiral recognition. 

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