CARBOHYDRATES 



25 



l)uint to the absence of aldehyde groups {e.g., failure to bind bisulfite or 

 give the usual Schiff test^). The reason for this apparent contradiction 

 is that most of the sugar at any one time exists in a cyclic or "oxide-ring" 

 structure, which is derived by interaction of the aldehyde group with 

 one of the — OH groups, usually at carbon 5: 



HCOH 



,0H 



O 



IIOCH o 



(3) -^c- 



(2) /C = 



HCOH OH 



1(5) (6) 



HC— CHoOH 



HO^(i)/H 



(2) /C^ 



HCOH ^O 



(4) 



H 



II 1(5) (G) 



HOCH o HC— CH2OH 



H 



(S) (6) 



Alpha ring form 



HOCH o HC-CH,OH 



(3) \p^ 



Beta ring form 



Free aldehyde or 

 oiDcn chain form 



Open-chain and ring formulas of D- glucose 



Note that carbon 1 has now become asymmetric so that there are two 

 stereoisomers of the ring structure. The alpha-forms of D-sugars have 

 the — OH of carbon 1 on the right when the formulas are written as shown 

 with carbon 1 on top. For sugars belonging to the L-series (see diagrams, 

 pp. 23, 24) the alpha-ring forms have this OH on the left. 



The oxide ring forms of the sugars are often represented diagrammati- 

 cally. For example, for the above alpha-ring form: 



H-^OH 



H 



(2) 



HO 



(3) 



OH O 



-H H H , 



/(5) 



(6) 

 CH2OH 



(4) 



H 



A B 



Diagrams representing a-D-glucose 



In A the diagram is drawn with carbon 1 at the top, but in B the molecule 

 has been moved into a different position. The student should realize 

 that the essential features of such formulas lie in the number and kind 

 of atoms present, and in what order they are linked together. Whether 

 the formulas are written with a particular part {e.g., carbon 1 in the 

 formulas above) at the top, side, or bottom, is incidental and merely 

 a matter of convenience. 



In the diagram B above, — OH groups shown pointing downward have 



^ Restoration of the pink color to Schiff's reagent, a dilute solution of rosaniline 

 which has been decolorized with sulfurous acid. 



