Molecular Chirality in Living Systems

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

FIGURE 2.1. Molecular structures of a-pinene enantiomers and d-limonene. 

Biot and Turpentine. The experimental measurement of the optical rotation of a flammable substance such as turpentine in the gas phase presented a formidable challenge for Biot and his co workers. They set up the experiment in the cloisters of an ancient church, and constructed a tube 30 meters long in order to get enough molecules of turpentine in the gas phase to see a measurable effect. Of course, they also had to heat the turpentine with an open fire to produce turpentine vapor. Apparently, soon after they made the first key measurement, one of the wooden beams caught fire, and they had to call in some extra help to put out the flames. The 

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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.). 

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. 

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. 

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. 

Why Nature uses only one enantiomer of most important biochemicals is an easier question to answer than how this asymmetry came about in the first place, or why L-amino acids and D-sugars were the favoured enantiomers, since, for example, proteins made out of racemic samples of amino acids would be complicated by the possibility of enormous numbers of diastereomers. Some have suggested that life arose on the surface of single chiral quartz crystals, which provided the asymmetric environment needed to make life s molecules enantiomerically pure. Or perhaps the asymmetry present in the spin of electrons released as gamma rays acted as a source of molecular asymmetry. Given that enantiomerically pure living systems should be simpler than racemic ones, maybe it was just chance that the L-amino acids and the D-sugars won out. 

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