PROTEINS. 633 



(c) Peptides. Definitely characterized combinations of two or more 

 ami no -acids, the carboxyl group of one being united with the amino 

 group of the other with the elimination of a molecule of water. The 

 peptones are peptides or mixtures of peptides, the latter term being 

 at present used to designate those of definite structure, such as poly- 

 peptides, dipeptides, etc. 



Products of proteolysis. 



Proteolysis is the change effected in proteins during their diges- 

 tion, and is brought about by the action of bodies termed proteolytic 

 agents, or enzymes. The products formed vary in quantity and com- 

 position with the nature of the proteins and enzymes, and depend 

 also on the condition under which the changes take place. While 

 the compounds grouped together as proteins differ widely in their 

 properties, yet the end-products of any proteolysis will be comprised 

 in a few amino-acids and nitrogenous bases. This would indicate that 

 there exists a close relation between the different proteins, the differ- 

 ence being due more to the atomic arrangement within the molecule 

 than to the quality and quantity of the elements present in protein 

 molecules. 



These decomposition products of proteins are : 



(1) Monamino-acids : 



Glycocoll (amino-acetic acid), CH 2 .NH 2 .CO 2 H. 



Alanine (aminopropionic acid), C 2 H 4 .NH 2 .CO 2 H. 



Valine (amino-iso-valeric acid), C 4 H 8 .NH 2 .CO 2 H. 



Aspartic acid (aminosuccinic acid), C 2 H 3 .NH 2 .(CO 2 H) 2 . 



Glutamic acid (amino-pyrotartaric acid), C 3 H 5 .NH 2 .(CO 2 H) 2 . 



Phenylalanine (phenyl-amino-propionic acid), C 6 H 5 .C 2 H 3 .NH 2 .CO 2 H. 



Iso-leucine (amino-methyl-ethyl-propionic acid), C 6 H 10 .NH 2 .CO 2 H. 



Leucine (amino-caproic acid), C 5 H, .NH 2 .CO 2 H. 



Serine (amino-oxypropionic acid), C 2 H 3 (OH).NH 2 .CO 2 H. 



Proline (pyrrolidine-carboxylic acid), C 4 H 7 .NH.CO 2 H. 



Oxyproline (oxypyrrolidine-carboxylic acid), C 4 H 7 O.NH.CO 2 H. 



Tryptophane (indol-amino-propionic acid), C U .H ]2 N 2 O 2 . 



Cystine (disulphide of aminothio-propionic acid (cysteine), (C 2 H 3 S.NH 3 .- 



C0 2 H) 2 . 

 Tyrosine (oxyphenyl-amino-propionic acid), C 8 H 8 O.NH 2 .CO 2 H. 



(2) Diamino-acids (hexone bases) : 



Arginine (guanidine-aminovaleric acid), C 6 H, 4 N 4 O 2 . 

 Histidine (amino-iimdazol-propionic acid), C 6 H 9 N 3 O 2 . 

 Lysine (diamino-caproic acid), C 6 H 14 N 2 O 2 . 



An enormous amount of work is being done in the attempt to solve 

 the protein molecule by studying these substances (proteoses, peptones, 



