106 PHYSIOLOGICAL CHEMISTRY 



molecule are put together. On hydrolysis, proteins yield a mix- 

 ture of ammo acids. These, then, must be the building stones 

 out of which the proteins are constructed. How are these 

 building stones put together? Much evidence has accumulated 

 to show that the amino acids are joined together in long chains, 

 the different units being linked by what is known as the amid or 

 "peptid" linking, the union, with loss of water, of the car- 

 boxyl group of one acid with the amino group of another. 



NH 2 CH 2 COOH + NH 2 CH 2 COOH - 

 NH 2 CH 2 CO NH CH 2 COOH + H 2 



A compound of this type is called a peptid, if made from two 

 amino acids, a dipeptid, if from three amino acids, a tripeptid, 

 if from many amino acids, a polypeptid. On partial hydrolysis 

 of proteins, such compounds actually have been found in the 

 resulting mixture, and have been shown to be identical with 

 compounds of known structure built up in the laboratory. The 

 most complex compound of this nature yet synthesized was 

 prepared by Emil Fischer. He prepared a substance having 

 eighteen amino acids in the chain, thus an octadecapeptid. This 

 compound had a molecular weight of 1213, and from its prop- 

 erties, was still much simpler than a protein. The synthesis of 

 such compounds is extremely laborious and expensive, so that 

 no attempt has been made to carry the process further. One of 

 the factors tending to increase the labor of synthesis is the fact 

 that the amino acids used are optically active. As ordi- 

 narily obtained, they consist of equal portions of the dextro- 

 and levorotatory forms, and must be separated into the two 

 varieties. The method most used* to accomplish this consists 

 in preparing the salts of the two forms with some optically ac- 

 tive base, such as brucine, cinchonine, etc. The brucine com- 

 pounds of the d- and 1-forms may be separated since they usu- 

 ally vary considerably in solubility, and on concentration one or 

 the other will crystallize out first. A second method consists 

 in allowing various microorganisms to act on the mixture of 

 the two optical isomers. Usually one form will be destroyed 



