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This article was originally published as a chapter in the book “Design and Catastrophe: 51 Scientists Explore Evidence in Nature"
You have certainly looked at the mirror countless times, yet have you ever noticed that the image you see is not exactly like you? If you are wearing a shirt with lettering, words will be written backward in the mirror. So, the image you see is not really you, but your mirror image. Although this doesn’t seem like much of a difference, it has major implications in chemistry, especially in biological systems.
When a molecule and its mirror image are not superimposable, due to a somewhat different three-dimensional arrangement, the molecule is said to be chiral—based on the ancient Greek word for hand. Indeed, this reminds us of our hands, one being like the mirror image of the other, yet non-superimposable. The two antipodes—the molecule and its mirror image—are said to be enantiomers and may have their respective chiral configuration indicated with the prefixes D- and L-, which stand for dexter and laevo, respectively, right and left in Latin.
The property of chirality, or handedness, is displayed by a great number of organic molecules presenting at least a chiral center, a chiral axis, or a chiral plane.[1] In fact, chirality is a pervading feature, with nearly 50% of molecules in living organisms being chiral. Moreover, biological systems are homochiral, appearing just in the L or D form through all the biosphere.[2] In other words, in living organisms, only L-amino acids and D-carbohydrates are present, not the corresponding mirror image, D-amino acids and L-carbohydrates.
Owing to homochirality, biological systems are able to discriminate enantiomeric compounds; thus a living organism may have a specific response toward one enantiomer and a dissimilar one toward the other optical isomer.[3] Insect pheromones, plant growth regulators, pesticides, and drugs are among some of these natural or human-made compounds.[4]
For example, L,L-aspartame (see Figure 12-1), is perceived by us as having a very sweet taste, whereas the D,D-isomer tastes bitter. In pharmaceutical drugs, an enantiomeric substance may exhibit highly beneficial therapeutic effects while its mirror image compound can be toxic or even mutagenic. This is due to the precise molecular recognition characteristics of natural binding sites in protein enzymes, receptors, and transport systems.[5] Widely known is the case of thalidomide, where the D form of the drug is a mild analgesic, and the L form is a powerful teratogen.[6]
While it is clear that chirality plays an important role in life, the major question to be answered is about the origin of homochirality in biological systems. As a matter of fact, to have a substance that is enantiopure, that is, that presents only one of the possible optical isomers for a specific compound, either that isomer was selectively formed or the racemate, the 1:1 mixture of optical isomers, needs to be separated through a process called chiral resolution. Interestingly, both situations normally rely on the use of other enantiopure chiral auxiliaries.[7]
I have worked for a number of years with asymmetric catalysis, preparing chiral ligands and catalysts and performing catalytic reactions in order to synthesize enantioenriched and enantiopure compounds. This experience has impressed me with how much thought and research is needed to design and develop efficient asymmetric catalytic systems, even when using chiral enantiopure auxiliaries.
Producing pure optically active substances already presents its challenges to chemists, but what we see in living organisms goes much beyond in terms of complexity. Macromolecules and polymers made up of homochiral units form complex structures, such as the DNA helix—homochiral itself—which in turn interact in multipart and intricate systems necessary for life.
The naturalistic scenario, in which life is not created by a Designer but emerges by chance, is then faced with a question that still lacks a definite answer: If living matter evolved in prebiotic times from chiral molecules formed out of simple achiral precursors, how did this [chiral] resolution appear?[8]
Questions may arise regarding why do we not find homochiral biological systems with D-amino acids and L-carbohydrates, or even with other combinations such as DD or LL. After all, in a blind chiral resolution scenario, would it not be expected that different combinations of homochiral compounds would be formed?
When considering the chemistry of life, we can clearly see the uniqueness of each compound that makes us alive. Homochirality in biological systems further reflects the fine-tuning of the conditions needed for living organisms to exist on Earth, adding to the array of factors that make the probability for life originating without design infinitesimal. To me, looking at the mirror from the perspective of chemistry and biochemistry clearly reveals that there is a Designer behind our existence.
NOTES
[1] RS Cahn, C Ingold, V Prelog. Specification of molecular chirality. Angewandte Chemie International Edition 1966; 5(4):385–415.
[2] LD Barron. Symmetry and molecular chirality. Chemical Society Reviews 1986; 15:189–223.
[3] VA Tverdislov, LV Yakovenko, AA Zhavoronkov. Chirality as a problem of biochemistry. Russian Journal of General Chemistry 2007; 77(11):1994–2005.
[4] Ibid.
[5] Ibid.
[6] Tragically, thousands of babies, whose mothers had taken thalidomide while pregnant, were born with external and internal deformities. See SK Teo, WA Colburn, WG Tracewell, KA Kook, DI Stirling, MS Jaworsky, MA Scheffer, SD Thomas, OL Laskin. Clinical pharmacokinetics of thalidomide. Clinical Pharmacokinets 2004; 43(5):311–327.
[7]. D Ager. Handbook of chiral chemicals. 2nd ed. Boca Raton (FL): Taylor & Francis; 2005.
[8]. Barron, op. cit., p. 219.
Nelson C. Martins is an assistant professor of chemistry at Middle East University, Lebanon. He holds a PhD in Chemistry from the University of the Algarve, having done most of his doctoral work at the University of Liverpool. He also completed two years of postdoctoral research at the State University of Campinas. His research interests are in chiral ligands synthesis and asymmetric catalysis for organic synthesis, subjects on which he has coauthored several peer reviewed papers in scientific journals. He has contributed his expertise as reviewer for some journals of the Royal Chemical Society.