The Titration Curve of Gelatine. 



81 



If j^—, = ft • [OH], 



or [OH]/K 2 , where K 2 = K w /K a = 0'86 x 10- 14 /3-5 x 1CT 7 = 2 5 x 10"S. 



Then C = (K 2 + [OH])n7[OH], and when we substitute the above value for 

 K 2 we see that n should be nearly equal to C at extremely small concen- 

 trations of hydioxyl ion and should then remain appreciably constant. This 

 is not the case in fig. 5 ; here n appears to rise abruptly to about O005 and 

 there seeks a maximum only to commence rising again to give a very steep 

 curve. 



Hence it is obvious that in alkaline solution gelatine does not behave simply 

 as a weak acid dissociating in accordance with the law of mass action. It is 

 possible that this abrupt rise accompanies some structural change of the 

 protein molecule such as Dakin had shown to occur in strong alkaline 

 solution (4). It must always be borne in mind that the hydrolysis of the 

 — C(OH):ISr — groupings with the formation of free carboxylic and amino-groups 

 occurs very rapidly in alkaline solution as measured by formaldehyde titration. 

 Now the hydrolytic breakdown of the gelatine is not accompanied by a greatly 

 increased basic binding power. in the system, for a 1 per cent, solution of 

 gelatine in sodium hydroxide having a reaction of Pn = 12 - 97 was found after 

 3 hours at 100° C. to have changed to a reaction of 1291. Further standing 

 for 48 hours at room temperature was accompanied by a change of reaction to 

 12 - 90. This change in reaction corresponds to an increased combination of 

 gelatine and base to the extent of only - 011 equivalents of sodium hydroxide 

 to 10 grms. of gelatine. 



V. Mechanism of Fixation of the Hydrochloric Acid. 



The most obvious points of attachment for acids in the gelatine molecule 

 are the free amino-groups, and if hydrochloric acid forms salts with gelatine 

 by addition of these groups, the salts should be regarded as hydrochlorides. 

 Gelatine contains 18'0 per cent, of nitrogen in its total dry weight. Accord- 

 ing to Van Slyke and Birchard (25) 3 - 16 per cent, of this 18 per cent, (equal 

 to half the lysin) can be removed as nitrogen gas by the action of nitrous 

 acid, and can therefore be regarded as existing in the molecule in the form of 

 free amino-groups. If these groups are the only ones in the molecule which 

 can bind hydrochloric acid, then the maximum combining power of 10 grains of 

 gelatine should be - 0039 equivalents. Kossel and Cameron (11) and Kossel 

 and Kellaway (12) have also brought forward evidence to show that proteins 

 such as clupein and salmin, which contain no lysin, but which contain arginin, 

 have a free amino-group in the guanidin nucleus of the arginin they contain. 



