PROTEINS 



Albuminoids. (Scleroproteins) 



Though the organic framework of plants consists of cellulose, 

 that of animals is formed of nitrogenous substances classified as 

 albuminoids. Like cellulose, they are very resistant chemically to 

 foreign influences, water, salt solutions, acids and bases. 



The most important of the albuminoids is collagen, derived from 

 bone, cartilage and the fibrils of connective tissue. On boiling with 

 water, it swells and gradually dissolves, undergoing hydrolytic cleav- 

 age and forming glue or gelatin. Gelatin, which has been the subject 

 of the most important investigations concerning hydrophile gels and 

 from which the whole class of gels take their name, does not occur in 

 the organism at all. The most important data concerning it have 

 been given on page 68, et seq. What has been said, especially in 

 reference to the preparation of a solution of agar (p. 137) holds for 

 gelatin as well. It should be recalled that acids and alkalis greatly 

 increase the swelling of gelatin. The swelling capacity reaches a 

 maximum with increasing concentration of HC1 (0.025 n) and KOH 

 (0.028 n) (Wo. OSTWALD). We thus find an absolute parallelism 

 between the swelling of gelatin and the ionizatitfn of albumin (see 

 pp. 152 to 156). In excellent agreement with this is the fact that 

 the minimal swelling occurs at the isoelectric point of gelatin, namely, 

 with an H ion concentration of 2.10~ 5 (L. MICHAELIS, R. CHIARI). It 

 must be emphasized especially, that a very dilute solution of gela- 

 tin depresses (according to G. QUINCKE) the surface tension of water 

 12 per cent. The solubility of CO 2 is very considerably greater in 

 gelatin sols than in water (in contrast to other hydrophile sols). 



Compared with other colloids (serum albumin), gelatin lowers the 

 solubility of easily soluble electrolytes and increases that of those 

 soluble with difficulty. The following are the figures from the in- 

 vestigations of Wo. PAULI and M. SAMEC:* 



There dissolves in 100 gm water -f 4 per cent gelatin + 10 per cent gelatin 



Ammonium chlorid 28 . 49 27 . 55 26 . 48 



Magnesium chlorid 35. 94 35. 22 35. 13 



Ammonium sulphocyanate 62 . 46 61 . 46 58 92 



1.5 per cent gelatin 



Calcium sulphate 0. 223 0. 295 



Tertiary calcium phosphate 



Ca 3 (P0 4 )2, 0.011 0.018 



Calcium carbonate . 004 . 015 



Silicic acid 0.023 0.027 



The solidification and the melting points depend greatly upon the 

 previous history of the gelatin; the longer gelatin is warmed the 

 less it tends to solidify. Upon heating a 2 per cent gelatin solution 



