Abstract
‘Homochirality’ or one-handedness is a characteristic hallmark of terrestrial life, which is based on nucleic acids made of all-D sugars coding for proteins made of all-L amino acids. But why is life made of L-amino acids and not their ‘unnatural’ D mirror images? In this article we consider at what stage in the origin of life homochirality first arose, and whether life's handedness was selected by chance or by a chiral influence. We discuss in detail the two most popular chiral influences – circularly polarized radiation, and parity-violation by the weak force – together with mechanisms to amplify small enantiomeric excesses to homochirality. We focus in particular on the ‘parity-violating energy difference’ or ‘PVED’ between enantiomers, and review calculations of the PVED, which show that in most cases the natural enantiomers are indeed of slightly lower energy than their ‘unnatural’ mirror images. We also review proposed methods to measure the PVED. We conclude with a review of the new field of ‘exochirality’ (chirality outside Earth), and discuss proposed instruments to search for homochirality as a signature of life both in situ on solar system bodies and remotely on extra-solar planets.
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