HEXOSES 



95 



The following formulae illustrate the relationship between 

 the five naturally occurring hexoses :■ — 



CHO 



HCOH 



I 

 HOCH 



HCOH 



HCOH 



I 

 HCOH 



H 



Glucose 



CHO 



in 



HO' 



I 

 HOCH 



HCOH 



HCOH 



I 

 HCOH 



H 

 Mannose 



CHO 



HCOH 



I 

 HOCH 



I 

 HOCH 



HCOH 



I 

 HCOH 



H 

 Galactose 



H 

 HCOH 



io 



I 

 HOCH 



HCOH 



HCOH 



I 

 HCOH 



H 



Fructose 



H 

 HCOH 



io 



I 

 HCOH 



HOCH 



I 

 HCOH 



I 

 HCOH 



H 



Sorbose 



From these formulae it will be seen that the spatial arrange- 

 ment of the third, fourth, fifth, and sixth carbon atoms in 

 glucose, mannose, and fructose is identical ; for this reason 

 they all give the same osazone when once the first and second 

 carbons have been condensed with phenylhydrazine ; on the 

 other hand, they all give different hydrazones in which only 

 the terminal carbon atom i is involved, leaving the rest of the 

 chain from carbon atoms 2-6 different in each case. 



Further evidence for the close relationship existing 

 between the three sugars, glucose, fructose, and mannose is 

 furnished by the fact that if a 5 per cent solution of any one 

 of these three sugars is treated with one-tenth of its volume of 

 10 per cent caustic potash and left in an incubator for twelve 

 to twenty-four hours at 37° C, the solution will be found to 

 contain all three sugars. This may be accounted for by 

 assuming that the two terminal links in the six carbon chain 

 of all three sugars can undergo molecular rearrangement to 

 the so-called enolic form as under — ■ 



CHO 



H— C— OH 



Glucose 



CHO 



HOCH 



1 

 Mannose 



CH2OH 



io 



Fructose 



CHOH 



II 

 COH 



I 

 Enolic modi- 

 fication 



in which form they all become identical, and the change back 

 from the enol form may give rise to any or all of the three. 

 The significance of this lies in the explanation it offers for the 

 possible interconversion of these sugars in the plant. 



