472 PRINCIPLES OF SOIL MICROBIOLOGY 



/COOH 



the general formula of which is H 2 NR X , like aspartic acid 



\COOH 

 (COOH-CH 2 -CHNH 2 -COOH) and glutamic acid, possess properties 

 of stronger acids than the mono-carboxylic acids. One of the carboxyls 

 in the dibasic acids is relatively strong, the other is of about the same 

 strength as the carboxyl groups of the ordinary mono amino acids. 

 The three basic amino acids, arginine, histidine and lysine, are fairly 

 strong bases and only show acid properties at extremely small hydro- 

 gen-ion concentrations, i.e., pH 12.0 to 14.0. When the hydroxyl 

 group of a carboxylic acid is replaced by an amino group, an acid- 

 amide, like CH3CONH2 (acetamide) is formed. 



A mixture of amino acids obtained by the hydrolysis of proteins 

 can be analyzed according to Fischer's method. The diamino-acids 

 are first separated from the mono-amino acids, by precipitation with 

 phosphotungstic acid. Excess of HC1 is added and the mixture 

 of mono-amino acids is then evaporated to a sirup in vacuo and dis- 

 solved in ethyl alcohol; dry hydrochloric acid gas is passed into the 

 solution to form the chlorides of the amino-acid esters. The esters 

 must then be set free by neutralization of the HC1, and extracted with 

 ether or chloroform. The esters are separated by fractional distillation 

 in vacuo. When a knowledge of the total proportion of the various 

 groups of amino acids is sufficient, the method of Van Slyke affords 

 a good procedure. 



Fischer 2 succeeded in combining amino acids into complex groups 

 known as peptides, thus obtaining dipeptides (CH 2 NH 2 CO • NHCH 2 

 COOH or glycyl-glycine) and other polypeptides, the more complex 

 ones approaching native proteins, in their general properties. The 

 investigations of Fischer and associates gave strong evidence support- 

 ing the view that the protein molecule is built up of amino acids 

 according to the following structure: NH 2 • CHR • CO - (NH • CHR • 

 CO) X -NH-CHR-COOH. The albumins, globulins, glutelins and 

 gliadins (or prolamins) are the most important vegetable proteins. 3 

 The chemical nature of the different proteins is determined by the 

 quantitative relationship of the various amino acids and their arrange- 

 ment in the molecule. 



2 Fischer, E. Untersuchungen fiber Aminosiiuren, Polypeptide unci Proteine. 

 Berlin. 1906. 



3 Osborne, T. B. The vegetable proteins. Longmans Green & Co. 2nd 

 ed. 1924. 



