142 



PROTEINS 



If all the nitrogen of a protein is accounted for in the amino acid 

 analysis, the number of amino acid residues can be calculated, and the 

 sum of the molecular weights of the residues probably equals the molecular 

 weight of the protein. The molecular weight of ^-lactoglobulin obtained 

 in this way is 42,020, and that given by the ultracentrifuge method is 

 41,500. 



Protamines are small proteins, m.w. 3000 to 5000, and hemocyanins 

 are very large, m.w. several million. Many viruses have apparent 

 molecular weights up to several hundred miUion. Home authorities 

 prefer to call these values particle weights rather than molecular weights, 

 giving proteins probable molecular weights approximating 17,600, or a 

 multiple thereof; that is, 2, 4, 6, 8, 16, 24 ... , 384 times 17,600. For 

 example, zein and hemoglobin are reported to have molecular weights 

 of about 35,000 (2 units) and 68,000 (4 units), respectively. The 

 molecular weight of the proteoses is supposed to range from 4000 to 

 5000 and that of the peptones from 800 to 1000. 



Color reactions 



Many color tests have been proposed by different investigators to 

 detect the presence of proteins. The most common are given below. 



The Xanthoproteic Test. With concentrated nitric acid, most proteins 

 give a yellow color that becomes more pronounced if the solution is made 

 alkaline. The familiar stain that is formed if nitric acid comes in 

 contact with the skin is due to the action of this acid upon the proteins 

 of the skin. The cause of the test is the formation of a nitro-phenyl- 

 derivative somewhat similar, perhaps, to picric acid. A modified phenyl 

 grouping such as is contained in tyrosine and tryptophan seems to be 

 necessary to the test. Tryptophan gives a better test than tyrosine, 

 whereas phenylalanine, although it contains the phenyl group (CeHs), 

 does not give the test at all. All the common proteins give the test, 

 but considerable quantities of protein are required. Although the test 

 is general, it is not very sensitive. 



The Millon Test. A brick-red color is developed when some proteins 

 are heated with the Millon reagent (mercury dissolved in nitric acid). 

 The reaction is due to the presence of the phenol group (C6H4OH), 

 which is contained in the amino acid, tyrosine. Proteins that contain 

 no tyrosine will therefore fail to give a Millon test. Gelatin gives a faint 

 Millon test either because it contains a minute quantity of tyrosine or 

 because it has not yet been freed of other tyrosine-containing proteins. 

 Carbolic acid and salicylic acid, which are not amino acids, likewise 

 give this test because they contain the phenol group. 



The Biuret Test. A pink to purple color is obtained when proteins 

 are treated with alkali and minute quantities of copper sulfate. The 



