42 THE CHEMICAL CONSTITUTION OF THE PROTEINS 

 He concluded that tyrosine was related to paracumaric acid, 



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



in the same way as 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 here garded as 



OH 

 C 6 H 4 X 



\C 2 H 4 NH 2 



and tyrosine as oxyphenylamidopropionic acid, the nitrogen being 

 attached to the side chain and not to the benzene ring as supposed by 

 Schmidt and Nasse. Earth's attempt to synthesise tyrosine from para- 

 cumaric acid by the following reactions 



/OH /OH /OH 



C 6 H 4 ( - C 6 H 4 / - C 6 H / 



\C 2 H 2 COOH \C 9 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 



\CO 2 H 



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

 Earth's formula was therefore correct. 



The work of Baumann in 1879 u P n the decomposition of tyrosine 

 by putrefaction showed that hydroparacu marie acid or p-oxyphenyl- 

 propionic acid was the first product and that tyrosine must be p-oxy- 

 phenylaminopropionic acid. It only remained to determine the posi- 

 tion of the NH 2 group, whether it was in the a- or ^-position. 



This was decided in 1 882 by Erlenmeyer and Lipp who synthesised 

 tyrosine from phenylalanine. Their first method to prepare p-sulpho- 



1 Until 1900 phloretic acid had the constitution OH . C 6 H 4 . CH . COOH, but in that 



CH 3 

 year Bougault showed it to be 0-oxyphenylpropionic acid OH . C 6 H 4 . CH 2 . CH 2 . COOH. 



