19191 , ft «; 



DODGE TYROSINASE REACTION OO 



Bovie ('15) presents a series of such graphs of the titra- 

 tion of several well-known acids and bases. At the 1916 meet- 

 ing of the Botanical Society of America he suggested that a 

 study of such titration graphs would enable one to titrate 

 more than one substance in a solution, as the hydrogen ions 

 of phosphoric acid may be titrated. 



A study of the tables of dissociation constants by Born- 

 stein and Roth ('12) also gives information useful in inter- 

 preting such graphs. For example, it will be noted that all of 

 the phenol groups dissociate between P H 9.0 and 11.0, mostly 

 between 9.0 and 10.0. Carboxyl groups give up their ions at 

 different points, depending upon the groups to which they 

 are attached. Where they are attached to the same atom as 

 an amino group, as in most of the amino acids, they dissociate 

 between P H 7.0 and 9.0. In acids, like aspartic and glutamic, 

 the carboxyl at the end of the chain farthest from the amino 

 group dissociates on the acid side of neutrality. It will be 

 noted in the case of the few polypeptids given that poly- 

 peptid formation shifts the dissociation point toward the 

 neutral point, i. e., toward acidity, possibly by separating the 



amino and carboxyl groups. 



Bearing these facts in mind, let us turn our attention to 

 tyrosin. The dissociation constants of tyrosin, as given by 

 Kanitz ('07), show the carboxyl dissociating at P H 8.4, the 

 phenol group at 9.4, and the amino group at 11.8. Hence in 

 plotting its curve, we should expect the curve to rise rapidly 



until 8.4 is reached, slightly less rapidly until 9.4, become 



horizontal until well beyond 10.0, then rise again until 11.8, 

 finally becoming horizontal as soon as the amino group has 

 been bound. A glance at the curve B, plotted from an aver- 

 age of all determinations, will show that this is essentially 

 the case. Another phenomenon is also apparent. The steep- 

 ness of the curve reveals to a certain extent the dissociation 

 of the salt formed. A weak acid will form a salt which will 

 add a larger proportion of negative ions to the positive than 

 a strong acid which gives off hydrogen ions more rapidly, so 

 that the resulting curve is steep. Therefore it will be seen 

 that the acid is much weaker than the phenol group. 



