BIOCHEMICALLY IMPORTANT COMPOUNDS 211 



sequence is very thoroughly adapted to the performance of 

 the physiological functions which they carry out in the living 

 cell. Such a sequence is hardly likely to have arisen merely 

 from the action of the simple laws which we have hitherto 

 been discussing. 



What has been said about the formation of nucleic acids 

 applies also to the primary synthesis of proteins. 



The possibility that amino acids might have been formed 

 on the surface of the Earth before the appearance of life 

 received its theoretical foundation and experimental con- 

 firmation in such experiments as those of Miller. The ques- 

 tion of the polymerisation of amino acids to form polypeptides 

 is more complicated. 



Under laboratory conditions this reaction may be carried 

 out by comparatively simple and extremely diverse methods. 

 For example, a-aminocaproic acid may be polymerised simply 

 by heating it.^" Polymerised amino acids are obtained by 

 the decarboxylation of :^-carboxy anhydrides in the presence 

 of a small amount of water^^^ and in other ways. All these 

 reactions take place in media containing only traces of water. 

 It follows that they could not take place under the conditions 

 prevailing in the primaeval atmosphere and hydrosphere of 

 the Earth. Simply allowing aqueous solutions of amino acids 

 to stand does not lead to any appreciable polymerisation, in 

 contrast to what happens when sugars are synthesised from 

 formaldehyde by Butlerov's method. This has a simple 

 theoretical explanation, in that amino acids cannot poly- 

 merise to form polypeptides without taking up free energy. 

 Calculations show that the formation of a single peptide 

 bond requires, on the average, about 3,000 cal/mole.^^* 

 Thus, in a homogeneous medium containing a suitable 

 catalyst the equilibrium constant for the synthesis of alanyl- 

 glycine, for example, from alanine and glycine, will only 

 be 001. 



It has, however, been suggested comparatively recently 

 by K. Linderstr0m-Lang,^^^ that in the synthesis of large 

 peptides from amino acids and other peptides, the change 

 of free energy Af may be considerably less than 3,000 cal. 

 This suggestion has been confirmed experimentally by A. 

 Dobry, J. S. Fruton and J. M. Sturtevant.^^" 



