92 ORGANIC AGRICULTURAL CHEMISTRY 



Poly-peptides 



The question then arises, how may these amino-acids be 

 joined together to form the proteins? In our discussion of 

 amino-acids it was emphasized that the pecuUar characteristic 

 of them is that they are both acid and basic compounds in the 

 same molecule. The amino group, acting as ammonia, forms 

 amide compounds with organic acids. This is illustrated by 

 the formation of the benzoyl derivative of amino-acetic acid in 

 which the radical of benzoic acid replaces one of the hydrogens 

 of the amino group. The product, benzoyl-amino-acetic acid, or 

 benzoyl- glycine, is hippuric acid. 



. CH2NH(H) - COOH CHsCNH) - COOH 



Amino-acetic acid (glycine) 



+ - H2O -> 



CeHs - CO(OH) Cells - CO 



Benzoic acid Benzoyl-glycine 



(hippuric acid) 



Now it has been found that by certain reactions two molecules 

 of amino-acetic acid react with each other, the amino group in 

 one reacting with the carboxyl group of the other just as above, 

 and a compound is obtained as follows : 



CH2NH(H) - COOH CH2(NH) - COOH 



+ ^ _H20-> I 



CHsCNHs) - CO(OH) CH2(NH2) - CO 



Glycyl-glycine 



The compound formed is known as glycyl-glycine, exactly analo- 

 gous to benzoyl-glycine, and is it called a di-peptide. 



In a similar way some 30 compounds have been prepared by 

 joining together certain amino-acids. Not only, however, may 

 two amino-acids be thus united (the amino-acids being the same 

 or different), but 3, 4, 5, 6, 8, 10, 14 and finally 18. These are 

 known as tri-peptides, tetra-peptides, deca-peptides, octa-deca- 

 peptides, etc., depending upon the number of amino-acids united. 

 As a class they are known as poly-peptides. 



Now the important fact is that these poly-peptides exhibit 



