20 Can Science Explain Life? 



laborators in Germany, and molecules containing 

 as many as nineteen amino acid groups have been 

 prepared. 



There is an abundance of evidence that the 

 peptide linking — CO — NH — occurs in the pro- 

 tein materials of all living cells. For example, 

 the same pink or violet color (the biuret reaction), 

 which is produced from this linking in any of the 

 higher polypeptides by the addition of an excess 

 of sodium hydroxide and a trace of copper sul- 

 phate is also produced upon the addition of these 

 reagents to proteins. Polypeptides and proteins 

 also produce the same amino acids when heated 

 with mineral acids or alkalies, so that they must 

 contain the same structural units. That the amino 

 acid groups in proteins are connected by means of 

 peptide linkings is also evidenced by the fact that 

 all proteins which have not been previously treated 

 with mineral acids or alkalies form neutral solu- 

 tions, so that the number of free carboxyl groups 

 must be equal to the number of free amino groups. 

 The fact that only a small fraction (usually less 

 than ten percent), of the total protein nitrogen 

 can be separated as free amino nitrogen shows 

 that most of the amino groups of the original 

 amino acid molecules must have become modified 

 or occupied in some manner at the time when 



