The Chirality of Living Systems

Mirror-Image Asymmetry An Introduction to the Origin and Consequences of Chirality by James P. Riehl 

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Almost 140 years ago Pasteur showed how a racemic mixture could be separated into its chiral constituents. Ever since, theories such as the three possibilities above have been proposed to explain an abiotic origin for molecular chirality in living systems. At the present time, however, no agreement exists about which explanation is best. In each ofthese scenarios, we can imagine production of some initial enantiomeric excess (e.e.). 

The recognition of one molecule out of a crowd of many other molecules requires distinction of certain molecular attributes, such as size, polarity, hydrogen bond pattern, chirality, or other physicochemical properties. If several attributes can be checked simultaneously, recognition becomes more selective. Recognition between an enzyme and a substrate was described first by Emil Fischer as the well-known lock and key principle [2], Molecular recognition between complementary DNA strands [3] or protein ligand interactions [4] is very important for the molecular function of living systems. 

Cytosine is an essentially flat molecule. The three-dimensional structure of cytosine crystals revealed helices. It is involved in the Genetic Code of 17 amino acids and controls essential features of living systems. Cytosine can form under prebiotic conditions. Nonchiral cytosine spontaneously forms highly enantioenriched crystals upon stirring during crystallization. Furthermore, chiral crystals of cytosine act as chiral initiator for asymmetric autocatalysis with amplification of chirality to provide for a virtually enantiopure compound (Fig. 3.5). 

Except for inorganic salts and a relatively few low-molecular-weight organic substances, the molecules in living systems, both plant and animal, are chiral. Although these molecules can exist as a number of stereoisomers, almost invariably only one stereoisomer is found in nature. Of course, instances do occur in which more than one stereoisomer is found, but these rarely exist together in the same biological system. 

There are many speculations on the origin of chirality of biosystems. Most interesting for the self assembly of reproducing catalytic systems are theories on the amplification of enantiomeric excess. Frankl proposed a general mechanism for spontaneous asymmetric synthesis. He showed that if the production of living molecules of life is rare and, hence, slow compared with their rate of multiplication, the whole Earth is likely to be extensively populated with the progeny of the first event before another appears. A living entity is defined as one able to reproduce its own kind. Frank showed that a simple and sufficient life model is a chemical substance which is a catalyst for its own production (hence, autocatalytic) and an anticatalyst for the production of its optical enantiomers. 

Despite a continuing debate about the origins of molecular chirality, its presence in living systems is ubiqnitous. The efficiency of biological processes serves as a continual challenge to chemists in terms of the design of artificial systems that possess a similar capacity in terms of stereoselectivity and catalytic efficiency, and the aspect of chiralily remains a key feature for such artificial systems. 

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