Sep 012015
 
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Among the building blocks of life there are molecules that behave like mirror images to one another. They are called enantiomers, which means that the atoms are connected in the same way, but in three dimensions they have another arrangement – just as the right hand differs from the left hand. To distinguish between two of such so-called chiral molecules we add L- or D- to their names. Though many properties of two enantiomers are similar, in biological systems they can show a rather different behavior. One example is the odor of carvone: one enantiomer smells like spearmint while the other one smells like caraway. [1] Furthermore, in nature we almost only find L-amino acids whilst sugars appear in their D-form – a phenomenon we call homochirality. [2]

Despite intensive research on this topic, we still do not know why nature chose to favor the corresponding configuration. There are several hypothesis on the origin of homochirality. Some state that it is a result of necessity, others explain it on a “by chance”-basis. In each case, an initially small excess of one enantiomer could have been amplified until only the D- or the L-form dominated.

One possibility is that asymmetric photochemistry led to an enantiomer enrichment in space that meteorites could have brought down to earth. Currently, the Rosetta mission investigates the question on enantiomer excesses on comets. [2] In some cases also the crystallization conditions can lead to a symmetry breaking. Furthermore, there is a really small energy difference due to parity violation (calculated to be on the order of 10-12 – 10-15 J/mol) between two enantiomers and by now we cannot exclude that this also could be the origin of homochirality. [2, 3]

Either way, to understand where homochirality stems from would also improve our knowledge of the origin of life itself.

Nicola Reusch

References
[1] Theodore J. Leitereg, Dante G. Guadagni, Jean. Harris, Thomas R. Mon, Roy., J. Agric. Food Chem., 1971, 19 (4), 785–787.
[2] Iuliia Myrgorodska, Cornelia Meinert, Zita Martins, Louis Le Sergeant d’Hendecourt, Uwe J. Meierhenrich, Angew. Chem., 2015, 127: 1420–1430.
[3] Martin Quack, Angew. Chem., 2012, 114: 4812–4825.

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