60 GENERAL BIOCHEMISTRY 



Written in tlic chain form ^vith the aldehyde gronp ai the top, tlie 

 highest-numbered asymmetric (arljon aioni is in the n eonfiguration 

 when the liychoxyl group is written to tiie right ol the chain. Tluis 

 the monosaccharide is a d sugar regardless ot the configurations about 

 other asymmetric carbons and regardless of the actual optical rotation. 

 In the case of D-glucose both the « and ^ forms have positive optical 

 rotations in ordinary aqueous solutions, but this is not true for other 

 sugars, D-fructose, for instance. The actual optical rotation depends 

 upon all the structural groups present, the solvent, the temperature, 

 etc., and is of little direct value in classifications on structural grounds. 



In the pyranose form written like the chain, the oxygen bridge in- 

 volves carbon atom five. The bridge connects to the atom from the 

 right in the d isomer. Written in this way, the bonds of the oxygen in 

 the ring have no correlation with reality except the points of con- 

 nection. Hence the right-hand formula on page 59 is preferred as a bet- 

 ter representation of the actual relative positions of the groups. Even 

 this portrayal has limitations due to the difficulties of projection from 

 three to two dimensions. Sets of atomic models are very useful in 

 visualizing carbohydrate structures and actually aid in the solution of 

 structural problems. 



In addition to the foregoing classifications, monosaccharides are 

 subdivided into ketoses and aldoses when ketone or aldehyde groups, 

 respectively, are present in either free or semiacetal form. Moreover, 

 the monosaccharides are subdivided according to size, depending on the 

 number of oxygen or of carbon atoms. Some systems use one element, 

 some the other, and none are free of difficulties. When oxygen atoms 

 are counted, perhaps the commonest method, a diose might be con- 

 sidered the smallest carbohydrate. The compound is optically inactive, 



CH2OH— CHO 



glycolicaldehyde 



having no center of asymmetry. The next higher members are the 

 three oxygen compounds shown and are trioses. The ketose has no 



CHO CHO CH2OH 



I I I 



HCOH HOCH CO 



I . I I 



CH2OH CH2OH CH2OH 



D-glycerose L-glycerose dihydroxyacetone 



center of asymmetry, and internal semiacetal formation does not occur 

 because of the strain in a three-membered ring, so this first ketose is 

 optically inactive. 



