(2) Prebiotic molecules necessary for the appearance of life could have been brought to the primitive Earth by meteoritic impacts, or even cometary impacts. Chirality

There is another important property of organic molecules which enables us to distinguish organic molecules of biological origin from abiotic molecules: This is chirality. What is a chiral molecule? It is a molecule that has two isomers, i.e., two species with the same chemical formula but which have a different structural arrangement, depending on whether this is right-handed or left-handed. The isomer of any chiral molecule appears identical to its mirror-image (Fig. 9.10). With the exception of glycine, all the amino acids used in the construction of proteins are chiral molecules.

The special feature of living organisms on Earth is to use just one form of chi-rality, left-handed chirality. All proteins are built from amino acids with left-handed chirality. Only the left-handed form of amino acids can stabilize the a-helix structure in proteins, because of the spatial position of side chains of amino acids in the polymer chains. Right-handed form of amino acids would lead to a configuration that prevents proteins from retain its helical structure, and would lead to a disruption of the molecule. With abiotic organic molecules, however, we observe equal number of both types of chirality. A mixture of amino acids of both types would lead to the formation of proteins that would not ensure their biological functions. So terrestrial life, consisting of left-handed chiral molecules, could develop subsequently only with that type of chirality.

What is the chirality of the amino acids found in the Murchison meteorite and other carbonaceous chondrites? There again, we find an excess of left-handed chi-rality, which tends to reinforce the theory that prebiotic molecules arrived on Earth from outside. It is purely chance that has favoured left-handed chirality in the pre-biotic molecules that have arrived from space? One possible explanation is that it could be the effect of circularly polarized, ultraviolet stellar radiation which tended to preferentially destroy right-handed chiral molecules (whether these were abiotic or not), and thus create an excess of the left-handed form. This phenomenon may be explained by the fact that in circularly polarized light, the electric field direction rotates along the beam. It is thus a chiral phenomenon. Left- and right-handed molecules have different absorption intensities for left- and right-circularly

Glycine Alamine


Fig. 9.10 Examples of a non-chiral molecule (glycine, left) and a chiral molecule (alanine, right) (After Gilmour and Sephton, 2003)

polarized light. The resulting photolysis efficiency therefore depends both on the light and on the types of molecules.

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