126 THE CARBOHYDRATES 



means of dilute acids breaks it up into one molecule of 

 fructose and one of gentiobiose (see p. 119), while Aspergillus 

 niger resolves it into one molecule of glucose and one of 

 sucrose. Gentianose does not reduce Fehling's solution. 



SUGARS OF UNKNOWN MOLECULAR WEIGHT OR SUGAR-LIKE 



POLYSACCHARIDES. 



Of these sugars lupeose and agavose are examples. The 

 former, which occurs in lupin seeds, does not reduce Fehling's 

 solution, and on hydrolysis yields galactose, fructose, and 

 glucose. It is supposed to be a tetrasaccharide.* 



Agavose, obtained from Agave americana, is an optically 



inactive sugar of unknown constitution which reduces Fehling's 



solution. f 



ABNORMAL OR ILL-DEFINED SUGARS. 



Buston and Schryver J have isolated from cabbage leaves 

 a substance whose formula is C3H8O4 and to which they assign 

 the constitution CHgOH . CHOH— 0— CH^OH. They suggest 

 that this sugar may be produced by the condensation of 

 formaldehyde with glycolHc aldehyde and thus may be re- 

 garded as a simple disaccharide. It does not reduce Fehling's 

 solution, is not hydrolysed by acids, and does not react with 

 phenylhydrazine to form an osazone. 



ESTIMATION OF SUGARS. 



A. VOLUMETRIC METHODS. 

 I. ESTIMATION BY MEANS OF FEHLING'S SOLUTION. 



The principle of this method lies in the fact that certain 

 sugars are capable of reducing copper sulphate in hot alkahne 

 solutions to cuprous oxide, the presence of which is indicated 

 by a yellow-red precipitate. 



Fehling's solution is made up in two solutions : — 



A, containing 69-28 grams of pure crystallized copper sul- 



phate in I litre of distilled water. 



B, containing 350 grams of Rochelle salt and 100 grams 



of caustic soda in i litre of distilled water. 



* Schulze : " Ber. deut. chem. Gesells.," 1910, 43, 2233. 

 t Michaud and Tristan : " Amer. Chem. J.," 1892, 14, 548. 

 X Buston and Schryver : " Biochem. Journ.," 1923, 17, 470. 



