146 THE CARBOHYDRATES 



Comparatively little is known about the chemistry of these 

 substances ; at one time there was even some doubt as to 

 whether they were really carbohydrates, since the ratio of 

 hydrogen to oxygen seemed to be less than that required for 

 compounds belonging to this group. Analyses by Tromp de 

 Haas and Tollens,* however, agreed fairly well either for the 

 formula (C 6 H ]0 O 5 ) n or 2C 6 H 10 O 5 . H 2 O. On the other hand, 

 Schryver and Haynes find that the analysis agrees with the 

 formula C 17 H 24 O 16 . 



Pectinogen distilled with hydrochloric acid yields furfural 

 in a quantity which indicates that one pentose group is con- 

 tained in each complex of 17 carbon atoms. 



There is evidence in these substances of acid groups com- 

 bined with metallic elements as in gum arabic ; f by boiling 

 pectose with dilute acids or caustic alkalis, different substances 

 are produced, such as pectin, parapectin, metapectin, pectic 

 acid which is combined with bases, such as calcium, and 

 forms the middle lamella of cell walls parapectic acid and 

 parapectosic acid, some of which are soluble in water, while 

 others, such as pectin, swell up in water and gelatinize. The 

 final product of these changes, namely, metapectic acid, is 

 readily soluble in water ; it appears to be closely related to, 

 or identical with, arabic acid, and on hydrolysis with dilute 

 sulphuric acid gives arabinose. 



This view receives confirmation from the work of Bour- 

 quelot and Herissey, who discovered an enzyme occurring 

 in malt, which is not identical with diastase, and which is 

 capable of hydrolysing pectose to a reducing sugar, namely, 

 arabinose. This enzyme, to which they gave the name pec- 

 tinase, acts both on unaltered and on coagulated pectic bodies, 

 but, conversely, the coagulating enzyme pectase is without 

 action on pectic bodies which have been previously hydrolysed 

 by pectinase. 



Collectively, the pectic substances are not reducing bodies, 

 and are insoluble in cuprammonia. They give precipitates 

 with metallic salts, and are likewise thrown down by the sul- 

 phates of magnesium, ammonium, or sodium, or by alcohol. 



* Tromp de Haas and Tollens: " Annalen," 1895, 286, 278. 

 fEhrlich: " Chem. Zeit.," 1917, 41, 197. 



