POLYPEPTIDE STRUCTURE OF PROTEINS 155 



By this method it may be shown that in the presence of an excess 

 of alkali, casein combines with a maximal proportion of 180X10" 5 

 equivalents of base per gram. The combining-capacity of casein for 

 alkalies does not exceed this figure no matter what excess of alkali 

 we may employ. We have seen that the minimal combining-capacity 

 of casein for bases is 11.4 equivalents of base per gram. Hence, reason- 

 ing as we did in the case of the compounds of edestin with hydrochloric 

 acid, if the minimal proportion of alkali which just suffices to carry 

 casein into solution corresponds to the union of one molecule of alkali 

 with one molecule of casein, the maximal proportion of alkali which 

 may be bound by casein must correspond to the union of at least sixteen 

 molecules of base with one molecule of protein. If these molecules of 

 alkali were united to the protein through COOH-groups there must 

 be sixteen of them, or over one-fourth of all of the carboxyl-groups in 

 the protein must exist therein in the free, uncombined condition. We 

 have seen that this would be impossible excepting in the case of the 

 second carboxyl in the dicarboxylic acid radicals and of these there are 

 only sufficient in casein to supply one-half of the carboxyl-groups 

 required. Evidently the union of bases with casein is accomplished 

 through some agency other than free carboxyl-groups. 



The nature of the radicals which accomplish the union of protein 

 with alkalies is indicated by the experiments of H. M. Vernon, who has 

 compared the power of proteins and of their hydrolytic decomposition 

 products to neutralize bases. Although the Hydrolytic Decomposition- 

 products of a protein will neutralize more alkali than the undecomposed 

 protein, yet the gain in power to neutralize bases is very much less 

 than one would anticipate in view of the large number of carboxyl- 

 groups which are set free by hydrolysis. In fact the alkali-neutralizing 

 power of the hydrolyzed protein is only slightly greater than the 

 alkali-neutralizing power of the native, undecomposed protein. Now 

 in the process of hydrolysis the COHN groups of the protein are 

 split into NH 2 and COOH groups. The inference is that the 



COHN groups within the protein molecule must be nearly as 



efficient in accomplishing the neutralization of bases as the COOH 

 groups of the constituent amino-acids out of which the protein is 

 built up. 



More direct evidence that the COHN groups in the protein mole- 

 cule are responsible for the neutralization of bases by proteins is afforded 

 by the investigations of Osborne and Leavenworth, who have shown 

 that Edestin, for example, combines with and holds in solution 34.67 

 per cent, of its weight of copper in the form of the otherwise insoluble 

 cupric hydroxide. This, if we assume that each copper atom unites 

 with one nitrogen atom, involves the union of cupric hydroxide with 

 ten out of every sixteen atoms of nitrogen in the edestin molecule. 

 Now this is exactly the proportion of nitrogen which edestin yields in 

 the form of amino-nitrogen after complete hydrolysis. In other words, 



