THE CHEMICAL CONSTITUTION OF THE UNITS 99 



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



He concluded that tyrosine was related to paracumaric acid, 



(OH)C 6 H 4 . CH = CH . COOH, 



in the same way that alanine was related to acrylic acid. Ost confirmed 

 this result of Earth's several years later, when he obtained p-oxybenzoic 

 acid by fusing tyrosine with caustic soda. 



Tyrosine was now regarded as ethylamidopara-oxybenzoic acid ; on 

 reduction, therefore, it should yield ethylamine, but instead of this 

 Hiifner, in 1868, obtained ammonia, and he supposed tyrosine to be 

 amidophloretic acid. 1 This view was strengthened when Barth, in the 

 following year, obtained p-oxybenzoic acid from phloretic acid and 

 also from Schmidt and Nasse's base. This he regarded as 



XDH 



^CoH.NHa 



and tyrosine as oxyphenylamidopropionic acid, the nitrogen being 

 attached to the side chain and not to the benzene ring as was sup- 

 posed by Schmidt and Nasse. Earth's attempt to synthesise tyrosine 

 from paracumaric acid by the following reactions, 



/OH /OH /OH 



C 6 H / - C 6 H 4 <; -> C 6 H / 



\C 8 H 2 COOH \C,H 3 C1COOH \C 2 H 3 NH 2 COOH, 



which were also put forward by Beilstein and Kuhlberg, was not suffi- 

 ciently successful to prove that tyrosine had this formula, so that 

 Ladenburg, who stated that the reactions of tyrosine could be just as 

 well explained by the formula, 



.NHOC 2 H 6 



4 \co 2 H, 



synthesised this compound. It was quite different to tyrosine, and 

 Earth's formula was therefore correct. 



1 Until 1900 phloretic acid was supposed to have the constitution, 



OH . C 6 H 4 . CH(CH 3 ) . COOH, 



but in that year Bougault showed it to be 0-oxyphenylpropionic acid, 

 OH . C 6 H 4 . CH 2 . CH 2 . COOH. 



