74 



CARBOHYDRATES MONOSACCHARIDES 



the solution the rotatory power rapidly rises to the equlibrium-value 

 of the rotatory power of ordinary glucose. 



From these observations it appears that the true formula for d- 

 glucose is either: 



HO C H H C OH 



HCOH 



\0 



HOCH 



HC/ 



HCOH 



CH 2 OH 



a-d-glucose. 



HCOH \ 

 HOCH / 

 HC/ 

 HCOH 



CH 2 OH 



p-d-glucose. 



of which the former is the a (highly rotating) form, and the latter the 

 )8 form of low rotatory power. In solution, an equilibrium is finally 

 attained between the two forms, and the attainment of this equilibrium 

 is much accelerated by an alkaline reaction. The rotatory power of 

 the pure a form is ()D + 110; that of the pure (3 form () D + 19. 

 The rotatory power of an equilibrated solution of the mixed glucoses 

 is (a) D + 52.5. From these figures it is a simple sum in proportion 

 to calculate that in a ten per cent, solution of glucose, about thirty- 

 seven per cent, is of the a form and about sixty-three of the form at 

 equilibrium. 



We see that glucose contains, therefore, not four but five asymmetri- 

 cal carbon atoms, a fact which is not revealed by a study of long- 

 standing or equilibrated solutions and was therefore very naturally 

 overlooked in the first attempts to attach a structural formula to 

 individual hexoses. If this be true of the other hexoses as well, however, 

 then there must exist not 2 4 = 16 stereo-isomers of glucose, but 2 s = 32. 

 As a matter of fact, we find that many of the sugars exhibit mutaro- 

 tation, for instance d-glucose, d-galactose, d-mannose, d-fructose, 

 1-arabinose, l-xylose, and some of the disaccharides. There is little 

 room for doubt that the structural formulae of each of these sugars 

 are analogous to the formulae for glucose which are depicted above. 



Since the hexaldoses all give the aldehyde reactions, that is, reduce 

 metallic oxides in alkaline solution, and unite with phenylhydrazine 

 by means of an aldehyde group, we must suppose that in the presence 

 of these reagents the oxide grouping is broken down and the aldehyde 

 group regained. This fact is very readily understood if we suppose 

 that every solution of glucose contains a trace of the aldehyde form, in 

 equilibrium with the oxide forms. A reagent such as a metallic oxide 

 or phenylhydrazine reacts with the trace of aldehyde form and thus 



