ioo THE CHEMICAL CONSTITUTION OF THE PROTEINS 



The work of Baumann in 1879 upon the decomposition of tyrosine 

 by putrefaction showed that hydroparacumaric acid, or p-oxyphenyl- 

 propionic acid, was the first product and that p-oxyphenylacetic and 

 p-oxybenzoic acids were derived from this body. Bacterial decom- 

 position can be carried as far as benzoic acid and benzene (Traetta- 

 Mosca) . 



Tyrosine therefore must be p-oxyphenylaminopropionic acid. It 

 only remained to determine the position of the NH 2 group, whether it 

 was in the a- or /3-position. 



This was decided in 1882 by Erlenmeyer and Lipp who synthesised 

 tyrosine from phenylalanine. Their first method, of preparing p-sul- 

 phonyl phenyl-a-aminopropionic acid and to exchange the sulphonic 

 acid group for the hydroxyl group, was not successful, as in the fusion 

 with potash the side chain also became oxidised and no tyrosine re- 

 sulted. They then prepared p-nitrophenylalanine, and converted it 

 into p-amidophenylalanine ; on treating this latter compound with the 

 calculated quantity of sodium nitrite and warming, they obtained 

 p-oxyphenylalanine, thus, 



C 8 H 5 C 8 H 4 .N0 3 C 6 H 4 .NH a C 6 H 4 .OH 



CH 2 CH 3 CH 2 CH 2 



CH.NH 2 CH.NH 2 CH.NH 2 CH.NHj, 



COOH COOH COOH COOH 



This compound had the same properties as the natural tyrosine, which 

 was thus proved to be p-oxyphenyl-a-aminopropionic acid. 



Erlenmeyer jun., and Halsey, in 1899, synthesised tyrosine by the 

 condensation of hippuric acid with p-oxybenzaldehyde in the presence 

 of acetic anhydride. The reactions are the same as those described by 

 Erlenmeyer for the synthesis of phenylalanine, except that the hydroxyl 

 group of the p-oxybenzaldehyde becomes acetylated in the process : 



/OH /NHCOC 6 H 8 



C 6 H 4 / +CH 2 ( + (CH 3 CO) 2 = 



\CHO \COOH 



/O . COCH 3 

 C 6 H/ ,NH . COC 6 H 5 



\CH:=C\ + 2 H 2 + CH 3 COOH 



\COOH 



The azlactone is again formed, but, on hydrolysis by alkali, the 

 acetyl group is removed and p-oxy-a-benzoylaminocinnamic acid is 

 obtained. On reduction it yields benzoyltyrosine, from which tyrosine 

 is formed by hydrolysis : 



