198 THE CARBOHYDRATES 



As regards the configuration of the pectic acid molecule, 

 it has been suggested by Nanji, Paton, and Ling * that it 

 contains a six-sided ring, each side of which is occupied by an 

 appropriate carbohydrate residue as under : — 



COOH 



Ga Ga 

 / \ 



l\Ga g/ 

 HOOC '^^ y' 



\ 

 COOH 



G = Galactose. 

 A = Arabinose. 

 Ga = Galacturonic acid. 



a formula f which bears a striking resemblance to the basal 

 nucleus suggested for starch (p. 157). The empirical formula 

 of this acid is C35H50O33, i.e. approximately double that pro- 

 posed by Schryver and Haynes, namely C17H24O1B. 



Properties of Pectins. 



1. Neutral pectins are soluble in warm water, without 

 boiling, up to 2 per cent, yielding a viscous solution ; such 

 solutions do not give a jelly unless boiled with sugar and 

 tartaric acid. 



2. A 2 per cent solution of pectin is mixed with one-tenth 

 its volume of a freshly prepared solution of pectase and a pinch 

 of calcium carbonate ; in from one to two hours the reaction 

 will have become acid and the solution should have set to 

 a gel. 



3. Treated with caustic soda, pectins are saponified with 

 formation of sodium pectate which, on addition of acetic acid 

 and calcium chloride, gives a precipitate of calcium pectate. 



4. About 0*5 gram of pectin is placed in a 150 c.c. distilla- 

 tion flask with 20 c.c. of water ; as soon as solution is effected, 



* Nanji, Paton, and I-ing : " J. Soc. Chem. Ird.," 1925, 44, 253 T. 

 t See also Norris and Schryver : " Biochem. Journ.," 1925, 19, 685. 



