36  Chirality
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When amino acids and sugars, the most important building blocks for life, are produced in a laboratory, an equal number of left-turning and right-turning molecules is produced. However, virtually only left-turning molecules can be used to form living cells. Right-turning molecules have a toxic effect on the cell. Since the genome of the simplest known form of life already consists of approximately a half million building blocks, it is not imaginable that a sufficient number of primordial substances would ever occur under natural circumstances.


The basic building blocks for the genetic material as well as those for the proteins are characterized by the fact that their image cannot be superimposed on their mirror image in the same manner as the right and left hand. This property is known as chirality.

Production of chiral molecules is usually very complicated and always requires the presence of chiral information (e.g., from a chiral catalyst). Left-turning and right-turning molecules occur in equal quantities in all chemical processes producing amino acids and sugars.

Before life originated, a decision must have been made at some time in favour of right-turning or left-turning molecules. At the level of living organisms, it is imaginable that competition between individuals and species could have led to selection processes. A selection process based on competition is not imaginable for a mixture of substances in which both constituents contain the same chemical energy (1).


Amino acids from space?

Chemist Ronald Breslow from Columbia University in New York found an explanation for how exclusively left-turning amino acids could congregate in the model used for the primordial soup theory. At a convention of the American Chemical Society in New Orleans, he reported that amino acids which were brought to Earth by meteorites were subjected to radiation which had a higher tendency to destroy right-turning amino acids. This would result in an excess of left-turning amino acids.

Breslow and his colleagues were able to simulate in the laboratory how left-turning and right-turning amino acids combine with one another during crystallization so that only those amino acids that do not have a partner remain dissolved in the water. If the comet transported more left-rotating amino acids than right-rotating, it would be possible for a solution to develop in such a process, which contained almost exclusively left-turning amino acids (2).  However, it can also be assumed that a slight excess of left-turning or right-turning amino acids would quickly equalise in a geological environment (3).

 
Bacteria from right-turning amino acids:
 
Since there are only few bacteria on Earth which consist of right-turning amino acids, at least two different comets must have fallen on the Earth. This would have to develop all twenty different amino acids essential for formation of a living cell twice.



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References:

(1) Junker und Scherer, Evolution, ein kritisches Lehrbuch, Weyel, 2006, page 108.
(2) Spiegel Online, Chemiker simulieren Siegeszug linksdrehender Moleküle, 7 April, 2008, http://www.spiegel.de/wissenschaft/mensch/0,1518,545766,00.html.
(3) K. Dose, Präbiotische Evolution und der Ursprung des Lebens, Chemie unserer Zeit 21, 1987, pages 177–185.
 
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