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Chirality

One of the most significant obstacles to an evolutionary explanation of the development of life molecules such as RNA, DNA and proteins - chirality - refers to the spatial arrangement of molecules and their constituent atoms. Many molecules, including the amino acids critical to the life of cells, are chiral molecules. A molecule is chiral when its molecular-structural mirror 'image' does not properly superimpose onto it (as with the two wings of a butterfly). Such molecules have one or more chiral centres - atoms in the molecule that do co-incide when the molecule and its mirror image are superimposed. The science of determining the chirality of molecules in the laboratory is called stereochemistry¹.

Some vexing challenges arise for proposed biochemical chemical evolutionary theories by chirality from the fact that mirror images of chiral molecules do actually exist in nature. The existence of such molecules imposes stringent requirements for functional molecular binding and reactions. Amino acids and nucleotides, like other classes of chiral molecules, each have two forms - a right handed or dextrorotory form (also referred to as dextroform) and a left handed or levorotory form (also referred to as levoform)². These mirror-asymmetric forms of each molecule are known as steroisomers, stereochemical enantiomers or chiral molecules. They are identified experimentally as stereochemical enantiomers by measuring the direction in which they rotate the plane of polarized light³. All such molecules exist in nature in a racemic (random) mixture, with equal proportions of levoform and dextroform enantiomers^{4 & 5}. However, in cells, all amino acids need to be levoform (left handed) enantiomers, whilst all nucleotides need to be dextroform, otherwise protein synthesis as programmed by DNA will fail, and DNA itself could not be formed⁶. Very precise sequencing of amino acids is required to produce functional proteins via RNA polymerase transcription and ribosomal translation, and such sequencing, without pre-existing DNA, is extraordinarily mathematically improbable - beyond the point of mathematical impossibility even with conservative estimates for the universal upper probability bound⁷. Levoform molecules only bind or react effectively with the levoform enantiomers of counterpart molecules, likewise for dextroform molecules⁸. The estimated minimum number of amino acids for protein production is about 100 000, and this corresponds to the minimum number of DNA base pairs for biosynthesis and life⁹. Smaller numbers are thought to be extremely unlikely to be able to support cellular life. Because all amino acids to form the cell must be levoform, and all nucleotides must be dextroform, then all of the minimum (approximate) 100 000 of each would have to appear simultaneously and instantaneously (lesser numbers would not be self sustaining)¹⁰. For such an event to occur in nature, where enantiomers occur in racemic (random) equal proportion, by chance - is astronomically mathematically improbable - even given very generous estimates of the amount of time and space available in the universe since its birth will allow.

Various theories to answer the question of how nature might have come up with a non-racemic mixture of chiral life molecules have been proposed, including the effect of circularly polarised light or ultra-violet radiation from the sun or other celestial x-ray sources, gamma burst radiation from supernovas and even panspermial delivery of batches of levorotory amino acids from distant nebulae via asteroids or comets¹¹. However, attempts at reproducing such conditions and effects in the laboratory has been, largely, spectacularly unsuccessful to date.

Editor(s): B. Long

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