94 PHYSIOLOGY 



benzene derivatives in which one atom of hydrogen in the ring is 

 replaced by one OH group. This is known as Hoffmann's test, but 

 is identical with Millon's reaction, which is given by all proteins con- 

 taining tyrosine in their molecules. 



Closely allied to the foregoing compound is another aromatic 

 ammo-acid, namely, phenyl a-alanine : 



|C 6 H 5 | 



\/ 

 CH 2 CH.NH 2 COOH 



It is an almost constant constituent of proteins. 



TRYPTOPHANE was known long before it had been isolated, 

 owing to the fact that with bromine water it gives a rose-red colour. 

 It had long been observed that this substance was to be obtained at 

 a certain stage in the digestion of proteins by pancreatic juice, but 

 nothing was known about its constitution until Hopkins succeeded 

 in isolating it by precipitation with mercuric sulphate dissolved 

 in 5 per cent, sulphuric acid. Cystine is also precipitated by this 

 reagent, but comes down with a less concentration of the salt than 

 tryptophane, so that it is possible to separate the two substances 

 by a species of fractional precipitation. Tryptophane can be isolated 

 by decomposing the mercury salt with sulphuretted hydrogen, and 

 is obtained in a crystallised form. On distillation it gives an abundant 

 yield of indol and skatol, bodies also obtained during the putrefaction 

 of proteins. Tryptophane itself is indol amino-propionic acid : 



.CH 2 CHNH 2 .COOH 

 CH 



NH 



C. AMINO-ACIDS OF HETEROCYCLIC COMPOUNDS 

 Three of the disintegration products of proteins can be grouped in 

 this class. Two of them contain the pyrrol ring, namely, proline and 

 oxyproline. 



PROLINE, which was first isolated by Fischer, is a-pyrrolidin 

 carboxylic acid and has the formula 



CH 2 CH 2 



CH 2 CH.COOH 



\/ 

 NH 



