The Titration Curve of Gelatine. 



83 



the maximum level of the second branch of the curve. The curve, however, 

 contiuues to rise to a height of 0"030, though later it falls again. The experi- 

 mental error in determining Ph becomes enormously magnified on the third 

 limb of the [H] : N curve, and therefore too much stress cannot be laid on its 

 smoothed form. However, it seems clear that some of the — COHN — groups 

 can act as basic groups combining with acids. What role, if any, other groups 

 (such as the hydroxyl groups of the hydroxy-acids) in the molecule play in 

 acid fixation is still unknown. It will be necessary to follow experimentally 

 the fate of the chlorine ion before final decisions are possible. At present 

 our calculations of W involve the assumption that the gelatine hydrochlorides 

 are present as completely ionised salts, an assumption that is liable to lead 

 to an increasing error with increasing values of [H]. 



The Mechanism for the Fixation of Sodium Hydroxide. 



The theory that proteins fix bases by means of their free carboxyl groups 

 has given way on the accumulation of evidence that there are not enough 

 of the latter to explain the quantitative relations 



Brailsford Eobertson (p. 236) suggests that the enolic imino-linkages 

 — C = N— 



, are responsible and " that the neutralisation of bases by the 



OH 



proteins is accomplished, at any rate for the greater part, by the dicarboxylic 

 radicles which they contain." He gives the formula for potassium protein 

 compounds as 



OK H OH 



C = 

 X C = N — 



OK H OH 



According to the most recent analysis of gelatine (Dakin, 1920), the only 

 di-carboxy acids present are aspartic acid (3'4 per cent.) and glutamic acid 

 (5"8 per cent.). Calculating from these figures, tbe maximum combining 

 capacity of a 1 per cent, solution of gelatine should be 0168 equivalents. 

 On examining the curve of base fixed in fig. 4, it can be seen that this 

 value for n is by no means a maximum, but that instead the curve inflects 

 and rises with increasing gradient. If every — COHN — group in the 

 molecule is considered capable of acting as a point of attachment for bases, 

 the maximum value for n' should be - 09 ; n' however rises considerably 

 above this figure. There must therefore be other means by which the 

 gelatine molecule can fix sodium hydroxide. The possibility of linkage 



