THE MONOSACCHARIDES 37i 



of the direction of their rotatory power, but because of their structural rela- 

 tionships to d and /-glucose respectively. 



4. Ring Structure. — Although it has long been customary to write the 

 formulas of monosaccharides as straight carbon chains, in recent years it has 

 become evident that the structure of such sugars is best represented by a ring 

 type of formula (Haworth, 1929). The 1-5 ring form of ^-glucose exists 

 in a and /3 forms depending upon the position of attachment of the H and OH 

 groups to the aldehydic carbon atom, which has now also become asymmetric: 



H OH H OH 



I I ' i 



HO C C OH HO C C Tj 



1/iH k\ l/iH A\i 



l\H /I l\? /I 



I I 



CH20H CH20H 



a d glucose ^ d glucose 



The a and {3 ring forms of glucose are stable compounds, but other ring 

 forms of these compounds, known as gamma sugars, also exist.. These are 

 unstable, highly reactive compounds and apparently have only a transitory 

 existence, but because of their lability probably play an important role in 

 metabolic processes. 



The Monosaccharides. — The pentoses are five carbon atom sugars 

 (CsHioOs)- Eight isomeric aldoses and four isomeric ketoses having this 

 formula are known to be possible, but only three of these — ^-xylose, /-arabi- 

 nose, and ^-ribose are definitely known to occur in plants. Although the 

 molecules of the pentose sugars apparently also have a ring structure, only 

 the more familiar straight-chain formulas are given here for these three 



sugars; 



H H H 



I I I 



c=o c=o c=o 



I I I 



H— C-OH H— C— OH H— C— OH 



HO— C— H H— C— OH HO— C— H 



HO— C — H H— C —OH H— C -OH 



H— C— OH H— C— OH H— C— OH 



k i k 



d-arabinose d-ribose d-xylose 



