98 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



Tyrosine. 



By fusing cheese with caustic potash Liebig, in 1 846, obtained a new 

 compound, consisting of a mass of fine silky needles, soluble with diffi- 

 culty in water ; he named it tyrosine from rvpos, cheese. The same 

 substance was isolated by Warren de la Rue from cochenille in 1848, 

 and a year later Hinterberger obtained it by the hydrolysis of horn. 

 Its presence in albumin, fibrin and caseinogen was demonstrated by 

 Bopp. 



The results of numerous investigations were published in 1860 by 

 Stadeler, who found tyrosine in silk-fibroin, mucin and various other 

 proteins, and who also noted its occurrence in the free state in various 

 animal organs, generally in conjunction with leucine. In plant seed- 

 lings it is also found in the free state. Since 1 860, tyrosine has been 

 constantly obtained from proteins by hydrolysis with acids and by the 

 action of trypsin, and has long been regarded as a constituent of the 

 protein molecule. 



Its formula C 9 H n NO 3 was determined by "Warren de la Rue and 

 by Hinterberger. Strecker, in 1 850, showed that it behaved like leucine 

 and glycine, but pointed out that it did not belong to this series ; and 

 Wicke, in 1857, suggested that it stood in the same relation to the series 

 of aromatic acids as glycine and leucine did to the fatty acids. Stadeler 

 was really the first to show that tyrosine was an aromatic compound, 

 when he obtained chloranil (tetrachloroquinone) from it by the action 

 of chlorine ; he also found that it had a constitution similar to that of 

 glycine and leucine. Frohde also held this view, but Thudichum and 

 Wanklyn, as they could not obtain picric acid from tyrosine by the 

 action of nitric acid, considered that it was not an aromatic compound. 



Stadeler's discovery of the formation of chloranil from tyrosine led 

 to the supposition that tyrosine was a derivative of salicylic acid, and 

 on this assumption Schmidt and Nasse attempted to synthesise tyrosine 

 from ethylamine and iodosalicylic acid, and from amidosalicylic and 

 ethyl iodide, but did not succeed. On heating tyrosine they obtained 

 a base C 8 H n NO 2 , which they thought analogous to the one Schmidt 

 had obtained by heating amidosalicylic acid ; on this account they 

 adhered to the theory that tyrosine was ethylamidosalicylic acid. 



A great advance was made by Barth in 1865, who showed that 

 tyrosine was not ethylamidosalicylic acid. As yet salicylic acid had 



