96 A. I. RINGER 



In this reaction we see the possibility of a molecule of aspartic acid 

 uniting with one molecule of glycocoll, one of alanin, and one of aspartic 

 acid ; the resultant tetra-peptid has one free NH 2 and three COOH 

 radicals, which means it can further form compounds along two branch 

 lines outside of the original line. The different possibilities can be best 

 illustrated graphically. 



From the above consideration one can readily see the difficulties that 

 confront the investigator of the chemistry of the proteins, and when one 

 also realizes that one cannot claim to understand the nature of a chemical 

 compound until he has a knowledge of its structural formula, one can 

 readily appreciate how far from our goal we are. One can then also 

 realize how crude is our classification of proteins that has been given 

 above. Under the heading of what we call albumins we may have billions 

 of different proteins, resembling one another in some respects, and differ- 

 ing in others. 



The Ami no Acid Content of Different Proteins 



Until the technique of the quantitative determination of the amino 

 acids reaches the point where it will be possible to recover 100 per cent of 

 amino acids from a known mixture, ah exact answer to the problem of 

 the amino acid content cannot be given. The figures we can gather to-day 

 are therefore more of relative value than of absolute. 



Not all proteins contain all the amino acids. We shall learn later 

 that from the nutritional point of view proteins are divided into "com- 

 plete" and "incomplete" and that under the latter we include those pro- 

 teins which lack some of the amino. acids which are essential for the 

 maintenance of proper nutritional .conditions of animals, like tryptophan, 

 tyrosin, lysin or cystin. 



Reactions of Proteins 



Color Reaction. The proteins give a number of color and precipitating 

 reactions, which are characteristic of a group, though not specific. 



The Millon's Reaction. When a protein is boiled in Millon's reagent, 

 which consists of a mercury solution in nitric acid and to which a small 

 amount of nitrous acid is added, the solution will turn rose colored to 

 dark red. This reaction is given by all substances having an oxyphenyl 

 radical. In the proteins it is the tyrosin radical which gives this reaction. 

 Proteins, like gelatin, which do not contain tyrosin, do not give this re- 

 action. 



