The Origin of Chirality in Living Systems

Three obvious questions follow from the observations presented in Chapter 2  

Does life have to be chiral, or could life develop that uses, for example, a racemic mixture of amino acids  

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

Could life develop using o-amino adds and L-sugars  

If the answer to question 2 is yes, then why, indeed, does life on earth use L-amino adds and D-sugars, and not D-amino adds and L-sugars  

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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. 

In the previous three chapters we have tried to provide a careful definition of what we mean by chirality, briefly discussed the scientific breakthroughs that form the basis of our current understanding of chirality in living systems, and outlined the various possible origins of chirahty on earth. In the rest of this book we will try to connect our knowledge of molecular handedness to our human experiences in a chiral world. As we will soon see, there are in fact very few things in our lives that are not affected by chirality. In the remaining chapters we will present some examples to illustrate this point. 

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.). 

It is estimated that approximately one-half of all drugs worldwide exist as stereoisomers. However, only one-half of stereoisomeric drugs are marketed as the individual stereoisomer and most of the latter are of natural or semi-synthetic origin. There is increasing awareness of the clinical importance of drug stereoselectivity because differences in the behavior of isomers in the chiral living system can result in significant differences in clinical outcomes. Table 1 presents a number of examples of these difference. 

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. 

Life processes operating at the molecular level involve chiral synthetic reactions performed within a chiral environment presumably this has been so since the origin of the first living entity (i-5). However, in prebiotic times, before homochiral (the existence of one enantiomer) biochemistry, the probable product of a chemical reaction would have been the extracellular formation of an equal mixture of two enantiomers, one of which was sequestered selectively by a protocell. The question that has intrigued stereochemists since the time of Pasteur is what force designed optical purity in natural products originally and whether the same force continues to operate in living systems in one form or other. 

Should we be surprised to find chiral structures in nature Surely we should not, because aU living things are made up of chiral amino acids, sugars, and other components. One major questions is whether the fact that the building blocks of life are chiral controls the overall chirality of the macroscopic system, or whether this macroscopic chirality has a more macroscopic origin. We have already discussed the many... 

The design has been well proved in quality assurance and origin control of flavours and fragrances. A double-oven system is shown in the Fig. 17.3, with two independent temperature controls and two detectors (DM 1, DM 2). A live switching coupling piece is used to switch the effluent flow to either the first detector or the chiral column. With optimum pneumatic adjustment of the MDGC system, certain fractions are selectively transferred onto the chiral main column as they are eluted from the precolumn (heart-cutting technique) [15]. 

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