82 



Miss D. J. Lloyd and Mr. C. Mayes. 



This group does not give off nitrogen gas under the action of nitrous acid 

 (Van Slyke, 23). Kossel (10) states that the acid binding power of salmin is 

 exactly that of its guanidin groups, and Kossel and Cameron (11) also 

 consider that the free amino-groups of clupein must be those of its guanidin 

 groups. Bracewell (2) considers that in all proteins the mechanism of acid 

 fixation is by means of free amino-groups, and that in proteins such as 

 gelatine, which contain both lysin and arginin, the acid-binding power should 

 be given by the sum of half the lysin nitrogen plus one-quarter of the 

 arginin nitrogen, each nitrogen atom binding one equivalent of acid. From 

 this he calculates that since lysin contains 6 - 32 per cent, of the total nitrogen, 

 and arginin 14 - 70, the maximum binding power of gelatine for acids should 

 be 0*00085 equivalents per gramme, i.e., - 008o equivalents for 10 grm. By a 

 titrimetric method he finds that 1 grm. of undissolved gelatine powder can 

 remove 0'00070 equivalents of acid from a supernatant solution. 



By the method which we have employed in this work the acid-binding power 

 of gelatine, calculated on its lysin and arginin-content (which we find equal to 

 0'0086 equivalents for 10 grm. of gelatine), lies not at the true maximum of 

 the N' : [HJ curve (fig. 4), but close to the first apparent maximum. It is 

 possible, then, that in solutions of hydrochloric acid less than 0'02 N gelatine 

 binds hydrochloric acid by means of its free amino-groups, and that it is the 

 average ionisation constants of these basic groups that is given by the value 

 4 - 8 x 10 -12 . But with increasing concentration of acid, more acid is bound 

 than can be accounted for on this hypothesis, and it is therefore necessary to 

 consider what part the imino- nitrogen of the peptide linkage ( — COHN — ) 

 could play. Bobertson (22) states that the acid-binding power of proteins is 

 not much increased by hydrolysis, and we have found that the reaction of a 

 1 per cent, solution of gelatine, which was found to be Ph = 1'13, had only 

 changed to Pn = 1*12 after 7 hours at 100° C. This change is of the same 

 order as the experimental error of the method, nevertheless hydrolysis of the 

 gelatine had occurred during the heating in the strong acid solution, as was 

 shown by the fact that the gelling power had been destroyed. It seems, 

 therefore, that the peptide linkage can function as an acid-binding group. 



Calculating again from Van Slyke's figures, if every amino-nitrogen atom 

 of the free ammo-acids of gelatine (i.e., the amino-nitrogen from lysin and 

 arginin + the imino-nitrogen from the peptide linkages) can act as a point for 

 the fixation of an equivalent of acid, then 10 grm. of gelatine should be able 

 to combine with - 092 equivalents of hydrochloric acid. If only the di-amino 

 acids (arginin and lysin, together with histidin) can do so, then the maximum 

 value for acid fixed in a 1 per cent, solution of gelatine would be 020 equi- 

 valents. It can be seen from fig. 3 that the value 0*020 for N — corresponds to 



