THE CHEMICAL CONSTITUTION OF ITS UNITS 41 



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, and a year 

 later, in 1849, 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 

 organs, generally in conjunction with leucine. Since then, 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 1850, 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 held 

 to the accuracy of the theory that tyrosine was ethylamidos-alicylic acid. 



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

 tyrosine was not ethylamidosalicylic acid. As yet salicylic acid had 

 never been obtained from tyrosine, and Barth, in his attempt to prepare 

 this compound from tyrosine by oxidation, by fusion with potash, ob- 

 tained para-oxybenzoic acid and acetic acid, the decomposition taking 

 place as follows : 



C 9 H n N0 3 + H 2 + O = C 7 H 6 3 + C 2 H 4 O 2 + NH 3 . 



