AMINO-ACIDS, PROTEINS, UREA AND URIC ACID 93 



characters very similar to those of proteins. By hydrolysis 

 they yield the amino-acids from which they have been prepared. 

 They give color reactions and precipitation tests similar to 

 those given by proteins. Finally they have been obtained as 

 intermediate cleavage products between the proteins themselves 

 and the amino-acids. Absolute identity of a poly-peptide made 

 synthetically and one obtained as a cleavage product of a 

 protein has not yet been established, but the evidence points 

 strongly to the conclusion that proteins are poly-peptides of the 

 various amino-acids obtained as cleavage products. 



EXPERIMENT STUDY XYHI 



Proteins 



(i) Solubility. Test solubility of egg albumin, wheat gluten and 

 milk casein in water. Compare also Experiment Study XXIX on 

 the separation and isolation of milk proteins. 



(2) Proof of Nitrogen in Proteins, (a) Place about a gram or two 

 of protein (egg albumin) in a dry test tube. Add about an equal 

 volume of fine soda lime. Heat. Hold a piece of red litmus paper at 

 mouth of tube. Also notice odor. Ammonia is liberated, proving 

 nitrogen in the original protein. 



(b) Place a small amount of protein (egg albumin) in a small piece 

 of glass tubing closed at one end. Add a piece of metallic sodium. 

 Heat red hot for some time. Place end of hot tube in a little water 

 to crack it and let fused mass into the water. Boil. Add a drop of 

 HCl and 5 c.c. of FeCla. Now add a little FeS04. A blue precipitate 

 proves presence of CN group as Na3Fe(CN)6. The N came from the 

 protein. 



(3) Quantitative Determination of Protein, (a) Place 0.5 g. of 

 pure protein, e.g. egg albumin, casein or wheat gluten, or 1.0-5.0 g. 

 of a protein-containing plant or animal substance, e.g. wheat, cotton- 

 seed meal, meat, or milk, in a long-necked, round-bottom flask 

 (Kjeldahl flask). Now add 25-30 c.c. concentrated pure sulphuric 

 acid and 10 g. crystallized potassium sulphate. Heat gradually in a 

 hood until the acid boils, being careful to avoid frothing. Continue 

 heating until the organic substance is all oxidized and a light straw- 



