84 



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



at some of the hydroxy-groups of the substituted amino-acids serine and 

 hydroxyproline, is not to be ignored. .Hydrolysis of gelatine by caustic 

 soda has been shown to increase slightly its basic binding power, a fact 

 which suggests that not all the — COHN — linkages are as potent as base 

 fixers as tbe free — COOH — groups. Loeb (14, 15) has shown that bases 

 react with gelatine at the same hydroxyl ion concentrations in equivalent 

 proportions. This fact shows that the reaction is ionic, and that the com- 

 pounds formed are of the nature of ionisable salts. Loeb only worked with 

 solutions whose alkalinity was less than Pn = 9. His experimental values 

 correspond very closely to our values over the same range. 



The long slow rise of n with increasing alkalinity is very striking. A 

 feature in which the alkali-gelatine system differs markedly from the acid- 

 gelatine system is illustrated by a different property of the gelatine, namely, 

 the turbidity of the gel. Sols of 1 per cent, gelatine in distilled water set 

 in a few hours to white, turbid gels. In the presence of O'OOl N hydro- 

 chloric acid the gel is not turbid, but quite clear and glassy after 24 hours' 

 standing at 15° C. In the presence of caustic soda, however, the turbidity 

 of the gel 24 hours old persists up to a concentration of - 002 N soda. 

 Thus there is both quantitative and qualitative evidence to show that in 

 the same protein the mechanism of fixing acids is different from that of 

 fixing bases. 



VI. Summary. 



1. Hydrochloric acid combines with gelatine in solutions whose acid 

 concentrations are less than - 04 normal, according to the law of mass action. 

 Kb for gelatine is 4"8 x 10 -12 at 20° C. if 839 be taken as the reacting weight 

 of gelatine. 



2. The theory is put forward that over this range of the curve of com- 

 bination of gelatine with hydrochloric acid, the combination occurs at the 

 free — NH 2 groups. These groups are present in the lysin, arginin, and 

 possibly some other of the amino-acids of the gelatine. The ionisation 

 constants at these groups are taken as approximately equal. The salts 

 formed are hydrochlorides. 



3. In concentrations of hydrochloric acid greater than 0'04 normal, the 

 proportion of acid fixed is greater than would follow from the combination 

 of hydrochloric acid with a weak base, with an ionisation constant of 

 48 x 10~ 12 . This is not due to the hydrolytic decomposition of the gelatine 

 and release of further free — NH 2 groups. It seems possible therefore that 

 combination is also occurring at the nitrogen of the peptide linkages. 



4. In dilute sodium hydroxide of a concentration less than O'Ol normal, 

 the gelatine combines with the base less rapidly than would follow by a 



