Amino Acids and the Asymmetry of Life

TLDR: In this article, a review of the role of chirality in the origin of life on Earth is presented. But the authors focus on the left-handedness of amino acid enantiomers.

Abstract: ‘How did life start on Earth?’ and ‘Why were left-handed amino acids selected for the architecture of proteins?’ A new attempt to answer these questions of high public and interdisciplinary scientific interest will be provided by this review. It will describe most recent experimental data on how the basic and molecular building blocks of life, amino acids, formed in a prebiotic setting. Most amino acids are chiral, that is that they cannot be superimposed with their mirror image molecules (enantiomers). In processes triggering the origin of life on Earth, the equal occurrence, i.e. the parity between left-handed amino acids and their right-handed mirror images, was violated. In the case of amino acids, the balance was tipped to the left – as a result of which life's proteins today exclusively implement the left-handed form of amino acids, called l-amino acid enantiomers. Neither plants, nor animals, including humans, make use of d-amino acids for the molecular architecture of their proteins (enzymes). This review addresses the molecular asymmetry of amino acids in living organisms, namely the preference for left-handedness. What was the cause for the violation of molecular parity of amino acids in the emergence of life on Earth? All the fascinating models proposed by physicists, chemists, and biologists will be vividly presented including the scientific conflicts. Special emphasis will be given to amino acid enantiomers that were subjected to chiral photons. The interaction between racemic molecules and chiral photons was shown to produce an enantiomeric enrichment that will be discussed in the context of absolute asymmetric synthesis. The concluding paragraphs will describe the attempt to verify any of those models with the chirality-module of the Rosetta mission. This European space mission contains probe Philae that was launched on board the Rosetta spacecraft with the aim of landing on the icy surface of comet 67P/Churyumov-Gerasimenko and analysing whether chiral organic compounds are present that could have been brought to the Earth by comet impacts.