22 CHEMICAL STATICS 



It has been pointed out by Kossel and Cameron (37) that the 

 acid-combining capacity of the protamin, salmin, is equal to the 

 combining capacity of the guanidin groups of the arginin radicals 

 which it contains, yet salmin yields no nitrogen on treatment 

 with nitrous acid. Sturin (41) contains 67 per cent of its nitro- 

 gen in the form of arginin, 10 per cent in the form of histidin 

 and 6 to 7 per cent in the form of lysin. It yields nitrogen on 

 treatment with nitrous acid corresponding to the co-amino group 

 of the lysin. Only about three out of every hundred nitrogen 

 atoms in sturin are therefore present in the form of free — NII2 

 groups. Yet one hundred nitrogen atoms in sturin will neu- 

 tralize no less than 24 equivalents of acid. All of these pro- 

 tamins, moreover, possess a certain, although relatively inferior 

 power of neutralizing bases. 



The number of terminal — COOH groups in any protein can- 

 not be much in excess of the terminal — NH2 groups, as the f ormol- 

 titration and behavior towards cupric hydroxide (32) (33) (66) 

 (67) show, and also because the protein would otherwise be 

 overwhelmingly acid in character (30) and the majority of the 

 proteins possess a distinct capacity for neutralizing acids, even 

 when they are themselves predominantly acid. Now free casein * 

 is insoluble in water, but when combined with acids or with 

 bases it is soluble. To carry one gram of casein into solution 

 11.4 X 10~^ equivalents of base just suffice, indicating a com- 

 bining- weight for casein of about 8800. The tyrosin, glutamic 

 acid and sulphur contents of casein indicate a minimal molecu- 

 lar weight of from 4000 to 4400. 



In the presence of excess of base, however, casein attains a 

 maximal combining capacity (measured by the gas-chain) of 

 180 X 10~^ equivalents per gram, so that it behaves like a 16- 

 basic acid, and if —COOH groups bind the base there must 

 be 16 of them in the molecule, corresponding to 25 per cent of 

 the total oxygen, or, almost exactly, to the percentage of the total 

 oxygen which is contained in the —COOH groups of the glutamic 

 acid in the casein molecule, calculated as the free acid. In order 

 to provide so many free carboxyls, the form of the casein mole- 

 cule would necessarily be that of a branched chain, or the radi- 

 ating spokes of a wheel, at the centre of which must exist unions 

 of the type: _C-C- 



* For references to the sources of these data Cf. Chap. IV. 



