22 INTRODUCTION 



15. Histidin/ ~ 



/* 

 -CH C 



C CH 2 -CH COOH. 



The members of this important group differ from all the 

 bodies previously described in having more than one NH 2 

 radical. Kossel termed them the hexone bases because each has 

 six carbon atoms, but the more descriptive term, diamino-acids, 

 is now more generally used. On account of their wide occurrence, 

 (no proteid has yet been found free from arginin) their prominent 

 part in the formation of the so-called " simplest proteids " (the 

 protamins), they have been held by some to form the real 

 nucleus of the proteid molecule. 



/NH 2 

 CH 2 CH-COOH 



I 

 16. Cystin, 5 



CH 2 -CH COH. 



Apparently the sulphur in the proteids exists chiefly, if not 

 solely, in this form. Cystin is closely related to the sulphur- free 

 amino-acids, as can be seen by comparing the following formula? : 



CH 3 CH COOH ( Alanin); 



/OH X NH 2 



CH 2 CH COOH (Serin); 



/SH /NH, 



CH 2 CH COOH (Cystein); 



CH 2 CH COOH 



I /NH 2 



CH 2 CH COOH (Cystin). 



If we consider the composition of these substances, we notice 

 that there is one important point that they all have in common : 

 each one is an acid, which has a NH 2 group substituted for a 

 hydrogen atom on the carbon nearest the add radical (the a- 



1 The structure of histidin is not finally determined. The formula given, 

 that of Pauly, is believed to be correct. 



