90 ORGANIC AGRICULTURAL CHEMISTRY 



Chemical Properties of Proteins 



Composition. — When we turn to the chemical properties 

 of proteins we find that here too we have much less to work with 

 than in most cases. Chemically proteins are characterized by 

 their inactivity and by the difficulty with which they are con- 

 verted into other compounds. In their ultimate composition 

 also there is little to aid in their study. The analysis of many 

 proteins has given results as follows : 



Carbon 5i-55 per cent 



Hydrogen 6-7 per cent 



Nitrogen 1 5-1 7.6 per cent 



Oxygen 20-25 per cent 



Sulphur 0.3-2.5 per cent 



Analysis. — Nitrogen, the characteristic element in proteins 

 which distinguishes them from carbohydrates and fats, may be 

 easily detected by test as in the experiment study following. 

 A positive test for nitrogen does not prove an organic substance 

 to be protein, but it does distinguish proteins from the other 

 two organic food constituents. The quantitative determination 

 of nitrogen in materials which contain protein, such as most plant 

 or animal substances, is the only analytical method known for 

 determining the protein itself. The method universally used 

 is known as the Kjeldahl method for nitrogen as described in 

 the experiment study below. 



Qualitative tests are also made for proteins by means of 

 reagents which give color reactions or precipitations. In gen- 

 eral all proteins respond to these tests, while in some cases cer- 

 tain non-protein compounds also respond. While some of the 

 color reactions have been shown to be due to certain organic 

 groups it is, nevertheless, impossible to differentiate proteins by 

 these reactions. 



Molecular Weight. — When an effort has been made, based 

 upon the percentage composition of the proteins, to arrive at 

 some idea as to the size of the molecule of the compounds, 



