POLYPEPTIDE STRUCTURE OF PROTEINS 153 



Supposing the insoluble compound to have been formed by the union 

 of only one molecule of acid with one molecule of edestin, since 1 gram 

 of the protein is, in this compound, equivalent to only 14X10" 5 mole- 

 cule of hydrochloric acid, about 7000 grams of protein would be 

 neutralized by one gram-molecule of hydrochloric acid. The molecular 

 weight of edestin, if the insoluble compound is formed by the union of 

 one molecule of hydrochloric acid with one molecule of protein, must 

 therefore, be about 7000. Nine NH 2 groups in this molecule would 

 correspond to ten per cent, of the total which edestin contains. If we 

 were to assume that the insoluble compound contains two molecules of 

 hydrochloric acid per molecule of protein, then our estimate of the 

 molecular weight of edestin would have to be doubled, but as the 

 estimated number of free NH 2 groups would also be doubled, this 

 would leave us still the same proportion, ten per cent., of the total 

 nitrogen in the molecule in the form of free NH 2 groups. 



Now the measurements of Van Slyke and Birchard have shown that 

 only 1.8 per cent, of the total nitrogen of the edestin molecule is 

 present therein, in the form of free NH 2 , groups, so that no less than 

 8.2 tenths or eighty-two per cent, of the neutralizing-power of edestin 

 for acids remains to be accounted for in some other fashion than by 

 the assumption of a union of the acid with free NH 2 groups. 



The same or similar measurements have been made, employing a 

 diversity of proteins, arid always with the same disparity between the 

 actual proportion of free amind-groups in the molecule of the protein 

 and the proportion which would be required to accomplish the neutral- 

 ization of all the acid which the protein is capable of binding. Thus, 

 it has been pointed out by Kossel and Cameron that the acid-combining 

 capacity of the protamine, Salmine is equal to the combining-capacity 

 of all of the guanidine-groups of the arginine radicals which this protein 

 contains. Yet salmine yields no nitrogen whatever on treatment with 

 nitrous acid. Sturine, which is another protamine, contains 67 per cent, 

 of its nitrogen in the^form of histidine and 6 to 7 percent, in the form 

 of lysine. It yields nitrogen on treatment with nitrous acid corre- 

 sponding to the co-amino-group of the lysine. Only about three or four 

 out of every hundred nitrogen atoms in sturine are therefore present 

 in the form of free amino-groups. Yet one hundred nitrogen atoms in 

 sturine will neutralize no less than twenty-four equivalent gram-mole- 

 cules of acid. Evidently at least twenty of these acid molecules must 

 attach themselves to the molecule of protein at some other points than 

 those provided by free NH 2 -groups. 



The number of Free Carboxyl-groups in any protein cannot be much 

 in excess of the number of free amino-groups, for otherwise the protein 

 would be overwhelmingly acid in its character, and behavior, and 

 besides, since relatively few of the amino-acid radicals in most proteins 

 are dicarboxylic acid radicals, if a great excess of free carboxyl-groups 

 were present in the molecule, the combined amino-groups could not 

 all be attached to carboxyl-groups as they are in the polypeptides, and 



