RESULTS OF ANALYSIS 65 



compounds. Some of the simpler polypeptides are not precipitated 

 by phosphotungstic acid. 



Sulphur-containing substances and sometimes sulphur have been 

 found in the reflux condenser, and the smell of iodoform has been 

 noticed in the hydrolysis of spongin. 



2. Formation of Humin. Nearly all proteins on hydrolysis yield an 

 insoluble brownish-black residue, humin or melanin. Humins are 

 formed by boiling carbohydrates with concentrated mineral acids, and 

 if nitrogenous matter be present, the humin contains nitrogen. 

 Samuely [1902] suggested that the humin formed from proteins was 

 due to a secondary reaction between amino acids and carbohydrates, 

 and obtained melanins containing nitrogen on boiling various amino 

 acids with hydrochloric acid in the presence of carbohydrate. Maillard 

 [1912] similarly obtained melanin-like substances on heating glucose and 

 other carbohydrates with glycine and alanine ; the reaction was most 

 rapid with xylose and arabinose, and alanine was the most reactive of 

 the amino acids. These products were shown by Maillard [1913] to 

 yield cyclic bases on heating, and he suggested that cellulose and pro- 

 tein were the origin of the pyridine and allied bases found in coal-tar. 

 Pyridine and other bases were obtained by Pictet and Chou [1916] by 

 hydrolysing caseinogen in the presence of formaldehyde, and Maillard 

 [1916] maintained that his results were substantially in agreement 

 with those of Pictet and Chou. Gortner and Blish [1915], knowing 

 that zein, which contains neither tryptophan nor carbohydrate and only 

 a small quantity of histidine, gave very little humin, heated zein with 

 acid in the presence of tryptophan and carbohydrate and obtained 

 86 -6 per cent, of the nitrogen of the tryptophan in the form of humin ; 

 with histidine in the place of tryptophan 0*5 per cent, of its nitrogen was 

 contained in the humin. Tryptophan thus is largely concerned in the 

 formation of humin. Grindley and Slater [1915] found that other 

 amino acids were concerned in the formation of humin. A detailed 

 investigation upon the amino acids taking part in the formation of 

 humin was made by Roxas [1916]. Alanine, leucine, phenylalanine 

 and glutamic acid are not factors in humin formation ; proline may be 

 a factor under certain conditions in the formation of humin. Trypto- 

 phan gave up 71 per cent, of its nitrogen, tyrosine 15, cystine 3*1, 

 arginine 2*3, lysine 2'6, histidine 1*8. Generally, fructose and xylose 

 were more reactive than glucose, and the three hexone bases reacted 

 more readily in weak acid solutions than in strong. The amino nitro- 

 gen of arginine, histidine, and tryptophan was lost in the formation of 

 humin ; tyrosine and cystine did not react with loss of nitrogen, 

 PT. I. 5 



