\ 



THE CARBOHYDRATES 65 



It is obtained from chitin, which forms the exoskeleton of large numbers of the inver- 

 tebrata, by boiling this with concentrated hydrochloric acid. It is stated to have 

 been obtained as a, decomposition product of certain proteins and their derivatives, 

 such as the mucins. It is of special interest as affording an intermediate product 

 between the carbohydrates and the oxy-amino acids which can be obtained by the 

 disintegration of proteins. In solution it is dextro-rotatory, reduces Fehling's solu- 

 tion, and gives an osazone resembling that derived from glucose. n 



GLYCURONIC ACID. C 6 H 10 O 7 , may be regarded as one of the first results of 

 oxidation of the glucose molecule. The group which has undergone oxidation is not the 

 readily oxidisable CHO group, but the CH 2 OH group at the other end of the molecule. 

 The formula of this acid is therefore : 



COOH 



I 

 (CH.OH) 4 



I 

 CHO. 



In the free state it does not occur in the animal body. It is constantly found in the 

 urine after administration of certain drugs such as phenol, camphor, or chloral, and 

 then occurs as a conjugated acid with these substances. These conjugated acids are 

 lee vo -rotatory, though the free acid is dextro-rotatory. In the free state it reduces 

 Fehling's solution and gives an osazone which is not sufficiently characteristic to dis- 

 tinguish from glucosazone. It does not undergo fermentation with yeast. This test 

 is therefore the best means of distinguishing the acid in urine from glucose. 



THE FORMATION OF GLUCOSIDES 



The graphic formulae given on p. 61 do not explain all the possible modes of arrange- 

 ment of the groups of the sugar molecules. Many of these sugars, when dissolved in 

 water, present the phenomenon known as multi-rotation. If their rotatory power 

 be taken immediately after solution, it is found to be greater or less than the rotatory 

 power taken some hours or days later. Glucose, for instance, immediately after solu- 

 tion, has a high specific rotatory power, which diminishes rapidly if the solution be 

 boiled, and more slowly if it be allowed to stand. Finally, the specific rotatory power 

 becomes constant at + 52-5 D. This change in rotatory power seems to be associated 

 with a change in the arrangement of the groups, the aldose for example assuming, 

 y the shifting of a mobile oxygen atom, what is known as a lactone arrangement. 



Thus glucose COH(CHOH) 2 CHOH.CHOH.CH 2 OH becomes 

 CHOH.(CHOH) 2 .CH.CHOH.CH 2 OH 



This change in the arrangement of the molecule renders a further stereoisomerism 

 jible, owing to the fact that now the end group which was formerly COH becomes 



H 



I 

 O C OH 



I 

 C 



that now there are five instead of four asymmetric carbon atoms. The two isomers 



5 



