18 UNITY AND DIVERSITY IN BIOCHEMISTRY 



Table V {continued) 



CHoOH CH2OH CH2OH CH2OH CH2OH 



I ' I I I I 



C=0 C=0 C=:0 c=o c=o 



I I I I i 



HO— C— H HO— C— H HO— C— H H— C— OH HO— C— H 



I I I I I 



H-C— OH HO— C— H H— C— OH HO— C— H H— C-OH 



i I I I ! 



H— C— OH K— C— OH H— C— OH H— C— OH HO— C— H 



I I I I I 



H— C— OH H— C— OH HO— C— H H— C— OH HO— C— H 



I i I I ! 



CH2OH CH2OH CHoOH CH2OH CHoOH 



D-Sedoheptose D-Manno- L-Perseulose D-Gluco- L-GIuco- 



D-Sedoheptulose ketoheptose L-Galaheptulose ketoheptose ketoheptose 



D-Manno- 



heptulose 



The direction of rotation is indicated by (+) or (— ), as in the following 

 examples : d(— ) ribose, d(+) glucose, l(— ) glucose, d(— ) arabinose. 



From the four tetroses, by further syntheses it is possible to obtain eight 

 pentoses, which will be aldopentoses, and from these, sixteen aldohexoses 

 (Table IV). The configuration of the ketoses can be deduced from that of 

 the aldoses, according to whether they derive from D-glyceraldehyde or 

 from L-glyceraldehyde. Table V shows a series of known ketoses, both 

 natural and synthetic. 



(b) Cyclic Formulae 



In the above, the sugars have been considered as straight chain molecules, 

 but it has been established that in the crystals of sugars containing more 

 than 4 carbon atoms, cyclic tautomeric forms are present. In solution these 

 sugars exist almost completely as cyclic molecules in equilibrium with small 

 amounts of the straight chain form. 



The fact that aldoses having more than 4 carbon atoms do not give certain 

 reactions given by aldehydes is due to their being predominantly in the 

 cyclic form. On the other hand, when a solution of glucose is prepared, the 

 rotatory power of the solution progressively decreases until it reaches a 

 certain value at which it remains constant (mutarotation). This is due to 

 the fact that glucose exists in the form of two cyclic isomers, and that an 

 equilibrium is set up between these two isomers and small amounts of the 

 straight chain form, as shown in the formulae below : 



