a b 



Fig. 13. Crystals of [a) laevo- and 

 {h) dex<ro-quartz. 



190 ABIOGENIC ORGANIC-CHEMICAL EVOLUTION 



the thumb Avill be on the right of the left hand, etc. Thus 

 each hand is the mirror image of the other (Fig. 13). 



In the ordinary labora- 

 tory synthesis of organic 

 substances we always obtain 

 mixtures of equal parts of 

 both forms of dissymmetric 

 molecules (a so-called race- 

 mate). This is easy to under- 

 stand, because the formation 

 of one form or the other 

 (the dextro or laevo anti- 

 pode) in a chemical reaction 

 depends on which of two 

 atoms, placed on one side or 

 the other of the plane of symmetry, will be replaced by a new 

 group of atoms. But the very concept of symmetry implies 

 that both of the atoms in question are subject to identical 

 forces. The probability that one antipode or the other will 

 be formed is therefore exactly the same. Such large numbers 

 of molecules take part in these chemical reactions that statisti- 

 cal laws apply to them and it is very unlikely that an excess of 

 one or other antipode will arise. Indeed we do not usually 

 observe such an excess under natural conditions in the absence 

 of life, or in laboratory syntheses."* In Miller's experiments, 

 for example, when he used silent electric discharges, alanine 

 and the other amino acids always appeared in the racemic 

 form. 



In living organisms, on the other hand, the amino acids 

 of which the natural proteins are formed are exclusively in 

 the L configuration. The d forms of amino acids are to be 

 found for certain only in some specific bacterial or fungal 

 products, particularly in antibiotics (e.g. D-leucine in grami- 

 cidin"^ and D-phenylalanine in tyrocidine"®). In such cases, 

 however, the l forms of these acids usually are absent. 



As a general rule, if a substance having dissymmetric 

 molecules is elaborated by a particular organism, that organ- 

 ism will only produce one of its two forms. The antipode of 

 that substance is either not to be found in living things, or 

 else it is produced by some other organism. This rule applies 



