The Chemical Basis of Life 21 



these amino acids were assimilated by the proto- 

 plasmic materials, and it is inconceivable how the 

 removal from the field of action of exactly equal 

 numbers of amino and carboxyl groups could be 

 explained in any other manner than by the forma- 

 tion of peptide linkings. 



From considerations such as these the conclu- 

 sion has generally been reached that the proto- 

 plasmic materials of living cells grow by the for- 

 mation of peptide linkings with amino acid mole- 

 cules, so as to produce either the chain structure 

 of polypeptides or the ring structure of diketo- 

 piperazines. It was formerly thought that the 

 structure thus produced was entirely of poly- 

 peptide nature, but more recent investigations 

 have shown that diketopiperazines are always 

 present among the hydrolytic cleavage products 

 of proteins, even under conditions which would 

 render their synthetic formation from the initially 

 produced polypeptides highly improbable. How- 

 ever, the chemical structures of the solid or col- 

 loidal protoplasmic materials which have been 

 subjected to hydrolysis may not be identical with 

 the molecular structures of synthetically prepared 

 polypeptides, even though they may both contain 

 peptide linkings, and with our present scanty 

 knowledge of protein structure we would not be 



