THE PROTEINS 95 



OXYPROLINE is the oxy- derivative of this body and has the formula 

 C 5 H 9 N0 3 , the exact position of the oxy- group having not yet been 

 determined. Doubts have been expressed whether the pyrrol group is 

 present as such in the protein molecule, or whether proline, for example, 

 is not formed by the closing of an open chain of a compound belonging 

 to the amino- acids in the fatty series. Thus from an oxy-amino- 

 valerianic acid CH 2 OH . CH 2 . CH 2 . CH . NH 2 . COOH we can by dehy- 

 dration make the compound CH 2 CH 2 . C.H 2 . CH . COOH, which will be 



Ntf 



seen to be identical with that given for proline. 



The third member of this group contains the iminazol ring : 



CH NH 



II >H 



C W 



and is known as HISTIDINE. Its structural formula is as follows : 



CH NH 



II / CH 



C -- W 



CH 2 .CH.NH 2 .COOH 



i.e. it is iminazol a-amino-propionic acid or iminazol alanine. Since 

 it occurs in the phosphotungstic precipitate from the products of acid 

 disintegration of proteins and contains six carbon atoms, it was 

 formerly classified with lysine and arginine as a hexone base. 



D. SULPHUR-CONTAINING AMINO-ACIDS 



Sulphur forms an integral part of the molecule of all classes of 

 proteins except protamines. In some substances allied to proteins, 

 such as keratin, it may occur to the extent of 3 per cent. On boil- 

 ing proteins with caustic potash or soda, a portion of the sulphur 

 is split off to form a sulphide, which gives a black precipitate on 

 addition of copper salts. On this account it was formerly thought 

 that the sulphur must be present in two forms, the oxidised and the 

 unoxidised, in the protein molecule. Recent investigation has shown, 

 however, that practically the whole of the sulphur is present in the 

 form of CYSTINE, and that this body on boiling with alkaline solutions 

 gives up only a little more than half its content in sulphur. 



This substance, which has been known for many years as the 

 chief constituent of a rare form of urinary calculus and as occurring in 

 the urine in certain cases of disordered metabolism, is again a deriva- 



