GLYCOGEN 171 



4. It does not reduce Fehling's solution. 



5. On boiling with mineral acids, it is converted into 

 dextrose. 



Estimation. 



This is best effected by heating the aqueous solution for 

 three hours in a boiling water bath with about 2-2 per cent 

 HCl, and then neutralizing and estimating the resulting 

 glucose by means of Fehling's solution ; the amount multi- 

 plied by 0-9 gives the weight of glycogen. 



According to Ling, results so obtained are vitiated by the 

 presence of mannose which is produced by the partial hydrolysis 

 of the mannan. He recommends hydrolysis by boiling for 

 three hours with 8 per cent sulphuric acid and estimating 

 iodometrically * the glucose and mannose produced before 

 and after removal of the mannan. 



LICHENIN AND ISO-LICHENIN. 



Lichenin is the name given to a water-soluble polysaccharide 

 extracted from " Iceland moss " — Cetraria islandica — and other 

 lichens. When an aqueous extract of Iceland moss is concen- 

 trated, a gelatinous precipitate of lichenin is formed while 

 the solution contains a substance known as iso-lichenin. The 

 latter substance, also known as lichen starch, is dextro-rotatory 

 and gives a blue colour with iodine, and is said by Pringsheim 

 and others f to be identical with amylose of the starch grain. 

 Lichenin, on the other hand, is optically inactive and gives 

 no blue colour with iodine ; according to Pringsheim it owes 

 its gelatinizing properties to the fact that it is a carbohydrate 

 ester of silicic acid. J 



On the other hand, Hess§ claims to have prepared highly 

 purified ash-free samples of lichenin possessing unimpaired 

 gelatinizing properties as compared with the less purified 

 material, from which he concludes that the gelatinizing power 



* See Baker and Hulton : " Biochem. Journ.," 1920, 14, 754. 

 t Pringsheim and Kusenack : " Zeit. physiol. Chem.," 1924, 137, 265 ; 

 1925, 144, 241. Pringsheim and others : " Ber. deut. chem. Gesells.," 



1924. 57. 1581- 



J Cf. p. 154, under Amylo-hemicellulose. 



§ Hess : " Zeit. angew. Chem.," 1924, 37, 993, 



