2 ANIMAL PROTEINS 



compounds attached. In these chains an acid radical may 

 combine with the amido group of another amido acid, the 

 acid group of the latter combining with an amido group of 

 another amido acid, and so on. Hydrogen may be substituted 

 in these chains by alkyl or aromatic groups. There is 

 obviously infinite possibility of variation in constitution 

 for compounds of this character, the general nature of which 

 varies very little. Practically all of the proteins are found 

 in the colloid state, and this makes them very difficult to 

 purify and renders the ultimate analysis in many cases 

 doubtful. It is, for example, often difficult to ascertain 

 their moisture content, for many are easily hydrolyzed with 

 water only, and many part easily with the elements of water, 

 whilst on the other hand many are lyophile colloids and 

 practically cannot be dehydrated or dried. A few, such 

 as gelatin and some albumins, have been crystallized. 



The constituent groups have been investigated chiefly 

 by hydrolytic methods. The chains of amido acids are 

 split up during hydrolysis, and individual amido acids may 

 thus be separated. The hydrolysis may be assisted either 

 by acids, alkalies or ferments, but follows a different course 

 according to the nature of the assistant. Under approxi- 

 mately constant conditions of hydrolysis, the products 

 obtained are in approximately constant proportions, and 

 this fact has been utilized by Van Slyke in devising a method 

 of proximate analysis. It is not possible in this volume to 

 enter deeply into the constitution of the different proteids. 

 Reference must be made to works on pure chemistry, 

 especially to those on advanced organic chemistry. It will 

 be interesting, however, to mention some of the amido 

 acids and groups commonly occurring in proteids. These 

 comprise ornithine (i : 4 diamido valeric acid), lysine (i : 5 

 diamido-caproic acid), arginine (i amido, 4 guanidine valeric 

 acid), histidine, glycine (amidoacetic acid), alanine (amido 

 propionic acid), amido- valeric acid (amido-iso-caproic acid), 

 liacine, pyrollidine carboxylic acid, aspartic acid, glutamic 

 acid (amido-glutaric acid), phenyl-alanine, serine (hydroxy- 

 amido propionic acid), purine derivatives (e.g. guanine), 



