CONSEQUENCES OF THE POLYPEPTID STRUCTURE 21 



proportion of the nitrogen in proteins is present within their 

 molecules in the form of — NH2 groups. Thus in the case of 

 edestin, as we have seen, only 1.8 per cent of the total nitrogen 

 is present in the form of free — NH2 groups. 



Now edestin, as Osborne (47) has shown, is insoluble, when 

 in the free condition, in water. It forms an insoluble hydro- 

 chloride containing 14 X 10~^ equivalents of HCl per gram 

 and, on further addition of acid, passes into solution. Its com- 

 bining-capacity for acids does not remain constant, however, for 

 at neutrality to tropseolin, which corresponds to a free acidity 

 of from 0.01 to 0.001 H+ (60) it neutralizes 127 X 10-^ equiva- 

 lents of acid per gram. Hence, if the acid is neutralized by 



— NH2 groups of edestin, the number of these groups in each 

 molecule of edestin must be at least W" = 9-* From the former 

 determination it would appear that the molecular weight of 

 edestin is 7000, and this corresponds with the molecular weight 

 indicated by its tyrosin and glutamic acid content (40) (1 mol. 

 tyrosin + 3 mols. glutamic acid + • • • )• Nine — NH2 groups 

 in this molecule would correspond to over ten per cent of the 

 total nitrogen, or almost exactly to the whole — NH2 content 

 of the arginin, calculated as the free diamino-acid, which the 

 edestin molecule contains. Since only 1.8 per cent of the total 

 nitrogen of the edestin molecule is present therein in the form 

 of — NII2 groups no less than eighty per cent of the neutralizing 

 power of the edestin for acids must be accounted for in some 

 other fashion than by the assumption of a union of the acid with 

 free — NH2 groups. 



From the investigations of Erb (14), although the exact in- 

 terpretation of his results is in some respects open to question 

 (Chap. IV), it would appear that the combining- weight of egg 

 albumin for acids may be as low as 152, while its molecular weight 

 is, according to Hofmeister, 5400 or some multiple of this. Hence 

 upon the assumption that terminal — NH2 groups bind these 

 acids, there must be at least 35 of them in egg albumin (45), 

 which would correspond to no less than 69 per cent of the total 

 nitrogen in this protein. 



* Osborne believes that an insoluble " monochlorhydrate " is also formed, 

 containing 7 X 10~^ equivalents per gram which would raise the number of 



— NH2 groups to 18, but would, at the same time, double the estimate of the 

 molecular weight. 



