THE CARBOHYDRATES 189 



Galactose. 



Galactose occurs in combination with glucose in milk-sugar, or lactose, 

 in some gums and seaweeds as the polysaccharide galactan and in some 

 glucosides, e g. xanthorhamnin and saponin of plants, the cerebrosides of the 

 brain and nervous ti.ss-.ue of animals. 



It is prepared from lactose by boiling it with 4 times its weight of 2 per 

 cent, sulphuric acid for 6 hours. The solution is concentrated and allowed 

 to crystallise. The crude ga'actose is recrystallised by dissolving it in four- 

 fifths of its weight of water and adding 2 volumes of 93 per cent, alcohol. 

 Galactose consists of very small hexagonal crystals which melt at 168. 



d-Glucosamine. 



The hydrochloride of glucosamine or aminoglucose was obtained in 1878 

 from chitin, the organic constituent of the shells of the lobster. It has since 

 been obtained from the organic material of the shells of other arthropods. 

 It is a constituent of fungus cellulose and has been prepared from various 

 glucoproteins (p. 468). These conjugated proteins contain glucosamine or a 

 polysaccharide composed of glucosamine as their carbohydrate moiety. 



d-Glucos imine was synthesised by Fischer and Leuchs from d-arabinose 

 and was shown to have the formula, 



H 

 CH 9 OH C C - CI10H 



Preparation. 



Chitin (p. 203), or lobster or crab shells, which have been treated with dilute 

 hydrochloric acid to decompose the calcium carbonate and washed with 

 water, are gently boiled with concentrated hydrochloric acid for 3-4 hours. 

 The solution is evaporated until it crystallises and allowed to cool. The dark 

 brown crystals of glucosamine hydrochloride are filtered off, dissolved in water 

 and the solution evaporated until it crystallises. Glucosamine may be ob- 

 tained from the hydrochloride by the "action of diethylamine or sodium 

 methylate. 



Properties. 



Glucosamine is a very unstable base and is known chiefly in the form of 

 its hydrochloride. 



Glucosamine hydrochloride forms large colourless glistening crystals which 

 are soluble in water. 



It cannot be converted directly into glucose; by the action of nitrous 

 acid it is converted into chitose which Fischer and Andreae have shown to be 

 a furfurane derivative of the formula, 



HO . CH CH . OH 



CH 2 OH . CH CH . OH 



\/ 

 O 



Glucosamine is hence frequently termed chitosamine. 



It resembles glucose closely in its properties : it forms a pentacetyl de- 

 rivative, an oxime and a phenyl-hydrazone and yie'ds an osazone identical 

 with glucosazone. 



its solutions behave like those of glucose as a reducing agent to Fehling's 

 solution, etc. 



